Mathilde Royer
email: mathilde.royer15@gmail.compasdeconflit

J Lefebvre

J Breton

A Boursier

M Compagnon

D Pons

Incidence des neutropénies et agranulocytoses sous clozapine nécessitant un arrêt ou une surveillance particulière du traitement : étude rétrospective dans un établissement psychiatrique sur une période de 14 ans

Keywords: Agranulocytose, Clozapine, Effet indésirable, Neutropénie, Schizophrénie 4 Agranulocytosis, Clozapine, Adverse effect, Neutropenia, Schizophrenia

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Résumé :

Contexte. -La clozapine est un neuroleptique qui a un rôle important dans la prise en charge de la schizophrénie et des psychoses réfractaires. Puisqu'elle induit des neutropénies sévères, l'Agence nationale de sécurité du médicament (ANSM) a créé un cahier de surveillance à renseigner pour tous les patients concernés.

Objectifs. -L'objectif principal de notre étude est d'évaluer l'incidence des neutropénies et des agranulocytoses sous clozapine ayant nécessité l'arrêt définitif du traitement ou nécessité la mise en place d'un suivi biologique plus régulier. L'objectif secondaire est de déterminer les paramètres pouvant influencer la survenue des neutropénies.

Méthodes. -Etude rétrospective réalisée à partir des prescriptions de clozapine dans un établissement psychiatrique de 543 lits entre 2003 et 2016. L'analyse des paramètres biologiques a permis de déterminer l'incidence des neutropénies ainsi que celle des agranulocytoses. Les paramètres ayant pu influencer la survenue de la neutropénie légère, modérée et sévère et de l'agranulocytose ont ensuite été analysés : sexe, âge, posologie, traitements concomitants. Pour les patients présentant une neutropénie < 2000/mm 3 nous avons recueilli des données supplémentaires : l'âge lors de la survenue de la neutropénie, le délai de survenue post-instauration du traitement, la posologie de la clozapine, et traitements concomitants.

Résultats. -97 patients sous clozapine ont été recensés sur cette période. Le taux d'incidence global des neutropénies est de 15,6%. Aucune agranulocytose n'est survenue au cours de ces 13 ans. Pour un tiers des patients, le score des neutrophiles a diminué à 1500/mm 3 et la clozapine a été arrêtée. La fréquence de la neutropénie est plus élevée chez la femme et chez les patients pour lesquels le traitement n'a pas été instauré en monothérapie. (La clozapine a

Introduction:

La clozapine est un neuroleptique (NL) atypique appartenant à la famille des dibenzodiazépines, elle possède une action à la fois dopaminergique et sérotoninergique [START_REF] Byerly | Pharmacokinetics of clozapine and risperidone: a review of recent literature[END_REF].

Elle est commercialisée pour la première fois en France en 1971 mais suite à la survenue de L'objectif principal de notre étude est d'évaluer l'incidence des neutropénies et des agranulocytoses sous clozapine ayant nécessité l'arrêt définitif du traitement ainsi que celles pour lesquelles une surveillance rigoureuse a permis de le poursuivre. L'objectif secondaire est de déterminer les paramètres pouvant avoir une influence sur la survenue des neutropénies.

Matériel et méthodes :

Recueil des informations

Une étude rétrospective monocentrique a été réalisée à partir des prescriptions effectuées Pour chaque patient, le sexe, l'âge au moment de l'introduction du traitement, l'association ou non d'autres thérapies antipsychotiques lors de l'instauration, les NL prescrits en plus de la clozapine, la NFS et plus particulièrement les PNN ont été relevés. La neutropénie légère est définie par une valeur de PNN < 2000/mm 3 . On parle de neutropénie modérée quand les PNN sont inférieurs à 1500/mm 3 et de neutropénie sévère quand ils sont inférieurs à 500/mm 3 . L'agranulocytose, qui engage le pronostic vital, est définie par un score de PNN < 500/mm 3 en présence de signes infectieux ou par une valeur isolée de PNN < 100/mm 3 [START_REF] Andrès | Neutropénie de l'adulte et du sujet âgé[END_REF].

Pour les patients présentant une numération de PNN < 2000/mm 3 nous avons recueilli des données supplémentaires : l'âge lors de la survenue de la neutropénie, le délai de survenue post-instauration du traitement (jours), la posologie de la clozapine, le nombre et le type de traitements concomitants au moment de la neutropénie (classe pharmaceutique et caractère neutropéniant selon les données disponibles dans la base de données publique des médicaments [7]) et la présence ou non d'une monothérapie antipsychotique. Quand cela était possible, nous nous sommes intéressés au suivi biologique de la neutropénie en étudiant le délai de retour à la normale des PNN. Par ailleurs, pour les cas de neutropénies sévères et/ou d'agranulocytoses, l'attitude thérapeutique a été détaillée.

Evaluation statistique des données

Les résultats sont présentés sous forme de moyenne avec écart-type, valeurs minimales et maximales pour les valeurs quantitatives et sous forme de fréquence pour celles qualitatives.

Les taux d'incidence de neutropénie ont été comparés en utilisant le test de Pearson (Khi2) (ddl=1). Dans certains cas le risque relatif (RR) est calculé. Les moyennes ont été comparées en utilisant le test de Wilcoxon. Pour l'ensemble des tests, une valeur P<0,05 a été considérée comme significative.

Résultats :

Caractéristiques de la population étudiée

Cette analyse rétrospective monocentrique réalisée sur une période de 14 ans a permis de mettre en évidence 97 patients pour lesquels la clozapine a été prescrite. Nous avons exclu de l'étude 16 patients pour lesquels la date d'instauration était inconnue, car non effectuée au CH Philippe Pinel. De plus, 4 patients ont aussi été exclus car le traitement n'a pas été débuté (deux d'entre eux avaient un score initial de PNN trop bas ; deux autres pour une raison indéterminée).

La population était donc constituée de 77 patients avec un sex-ratio d'environ 2 : 1 (53 hommes vs 24 femmes).

Nous avons suivi l'évolution du nombre d'instaurations de la clozapine au cours de la période étudiée, soit 14 ans. Nous avons observé deux périodes au cours desquelles une augmentation du nombre d'instaurations de la clozapine a eu lieu : 2011 et 2015 (Figure 1). La moyenne d'âge au début du traitement était de 41,8 ± 14,2 ans (min : 20 ans ; max : 77 ans). Pour 48 d'entre eux (62,3%), l'instauration s'est faite en monothérapie. Pour les 29 autres patients (37,7%), elle s'est faite en bi, tri ou quadrithérapie (Tableau I). Dans ce second cas, le neuroleptique retrouvé le plus fréquemment associé était la loxapine (72,4%, soit 21 patients sur les 29 patients ayant au moins un autre neuroleptique associé) (Figure 2).

Incidence des neutropénies

Parmi les 77 patients ayant reçu de la clozapine, 12 d'entre eux (15,6%) ont présenté une neutropénie au cours de leur traitement. La neutropénie était modérée pour 2 patients (2,6%) et sévère pour 2 autres (2,6%), la clozapine a donc dû être arrêtée pour ces 4 patients (Figure 3). Chez les 8 patients (10,4%) qui présentaient un score de PNN compris entre 1500 et 2000 PNN/mm 3 , le traitement par clozapine a été poursuivi et un suivi renforcé de la NFS a eu lieu.

Paramètres pouvant influencer la survenue de NP

Nous nous sommes intéressés à l'étude d'une éventuelle corrélation entre le sexe du patient et la survenue de la neutropénie. Ainsi, 6 femmes et 6 hommes ont présenté une neutropénie ce qui correspond à un taux de neutropénie de 25% (6 femmes sur 24) chez la femme contre 11,3% (6 hommes sur 53) chez l'homme (P=0,125). Dans notre échantillon, la fréquence de survenue d'une neutropénie était deux fois plus importante chez les femmes que chez les hommes.

Nous avons ensuite étudié l'impact de l'instauration du traitement en monothérapie ou en association sur l'apparition d'une neutropénie. Pour cela, nous avons comparé la fréquence des neutropénies en fonction de l'instauration de la clozapine en monothérapie ou en association. Nous avons observé une incidence des neutropénies 1,6 fois moins importante lorsque la clozapine était instaurée en monothérapie (6 cas de neutropénie dans chaque groupe soit 12,5% vs 20,7% pour les instaurations en monothérapie et en association respectivement (P=0,337)).

Dans un second temps, nous nous sommes intéressés au groupe de 12 patients ayant présenté une neutropénie. L'âge moyen lors de la survenue de la neutropénie était de 43,6 ± 15,1 ans avec un âge minimum de 20 ans et un âge maximum de 69 ans. Cette moyenne était plus élevée chez les femmes que chez les hommes mais la différence n'est pas significative. (45,3 ± 17,3 ans vs 42,0 ± 11,7 ans ; P=0,377) Nous avons ensuite étudié le délai moyen de survenue de la neutropénie par rapport au début du traitement. Celui-ci était de 92 ± 80,9 jours (min : 6 j / max : 266 j). 83,3% d'entre elles sont apparues pendant les 18 premières semaines de traitement.

Par ailleurs, nous nous sommes intéressés aux traitements concomitants retrouvés lors de la neutropénie. En moyenne 5,3 ± 2,5 traitements par patient étaient associés à la clozapine.

Pour 10 patients (83,3%) au moins un traitement neutropéniant était associé, principalement des NL (61,5% ; Figure 4) et majoritairement la loxapine (retrouvé chez 5 patients ; Figure5). Ainsi, seuls 4 patients sur les 12, soit 33,3%, recevaient de la clozapine en monothérapie au moment de la survenue de l'effet indésirable (EI). Enfin, dans notre population, la posologie moyenne de clozapine était de 220,8 ± 130,2 mg/j (min : 25 mg /max : 400 mg) au moment de la survenue de la neutropénie.

Suivi biologique

Parmi les 12 patients ayant présenté une neutropénie au cours de cette période, les bilans biologiques de suivi étaient disponibles pour 9 d'entre eux. Le délai moyen de retour à la normale des PNN était de 9,7 ± 11,0 j. Par ailleurs, deux patients ont présenté une neutropénie sévère ayant nécessité, comme explicité plus bas, l'administration de facteurs de croissance.

En excluant ces 2 patients, le délai de retour à la normale est de 8,9 ±12,6 j.

Prise en charge des neutropénies sévères

La prise en charge des 2 patients ayant présenté une neutropénie sévère s'est faite de la façon

Incidence des neutropénies

Notre analyse rapporte une incidence globale de neutropénie de 15,6%. Elle est difficilement comparable aux données de la littérature car, au contraire de celles-ci, elle inclut les patients ayant des PNN compris entre 1500 et 2000/mm 3 pour lesquels l'arrêt du traitement n'est pas nécessaire. Néanmoins une étude thaïlandaise avait mis en évidence une incidence globale de 6,3% de NP en prenant en compte ces critères [START_REF] Ratanajamit | Incidence and risk for neutropenia/agranulocytosis among clozapine users: A retrospective cohort study[END_REF]. Le phénomène de margination excessif des PNN est souvent responsable de pseudo-neutropénie et nécessite un second bilan de contrôle.

Ces données n'ont pas été quantifiées dans notre étude, entrainant un biais avec une probable surestimation des neutropénies. Cependant, l'expérience dans notre centre démontre qu'une seconde analyse de la NFS, après mobilisation du patient, soulève dans certains cas de fausses neutropénies. Il serait intéressant de mettre en place une formation permettant de sensibiliser les infirmiers aux modalités de prélèvements pour les NFS afin de diminuer le taux de fausse neutropénie.

Par ailleurs, le taux de neutropénies modérées et sévères, impliquant l'arrêt immédiat du traitement, est de 5,2%. Ce taux d'incidence est comparable à celui retrouvé dans la littérature. En effet, l'équipe de Levoyer et al à Rennes retrouvait un taux d'incidence des neutropénies modérées et sévères de 4,1% [START_REF] Levoyer | Dix ans d'expérience clinique de la clozapine portant sur 170 patients[END_REF]. De plus, nous observons une incidence des neutropénies sévères de 2,6%, celle-ci est légèrement supérieure à celle rapportée par Lieberman et al, comprise entre 1 et 2% [START_REF] Lieberman | Maximizing clozapine therapy: Managing side effects[END_REF]. Il est en outre important de noter qu'aucun décès sous clozapine n'a été rapporté dans notre établissement. Les neuroleptiques ont un pouvoir neutropéniant plus ou moins important qui peut potentialiser le risque de neutropénie et d'agranulocytose [15]. Dans notre étude, nous constatons que la fréquence de neutropénie est plus élevée chez les patients pour lesquels ces recommandations n'ont pas été respectées. Cette observation est comparable à celle faite par K. Lau et al en Chine qui observaient une augmentation de l'incidence de neutropénies et d'agranulocytoses lors de la prescription d'un antipsychotique en plus de la clozapine [START_REF] Lau | Neutropenia and agranulocytosis in Chinese patients prescribed clozapine[END_REF].

Dans leur étude, 88% des patients ayant présenté une neutropénie recevaient un traitement de la classe des psychotropes en plus de la clozapine. Si l'on s'intéresse uniquement à l'association avec un autre NL afin de pouvoir comparer nos résultats, ils retrouvaient 8 prescriptions sur 16 (50%) pour lesquelles une association clozapine/NL était présente.

Malheureusement la liste des NL concernés n'est pas citée [START_REF] Lieberman | Maximizing clozapine therapy: Managing side effects[END_REF].

Comme le montre la littérature, l'association de la clozapine à d'autres antipsychotiques semble une pratique courante pour des patients non stabilisés [START_REF] Levoyer | Dix ans d'expérience clinique de la clozapine portant sur 170 patients[END_REF][START_REF] Mercier | Enquête observationnelle de prescription de la clozapine au centre hospitalier Charles-Perrens à Bordeaux, plus de 15 ans après l'AMM en France[END_REF]. Dans l'enquête observationnelle de Mercier, la clozapine est associée à un autre antipsychotique dans 41,9 % des cas [START_REF] Mercier | Enquête observationnelle de prescription de la clozapine au centre hospitalier Charles-Perrens à Bordeaux, plus de 15 ans après l'AMM en France[END_REF]. Par conséquent l'incidence élevée de neutropénie sous l'association clozapine/loxapine ne doit pas remettre en cause la sécurité clinique de la loxapine mais plutôt être considérée comme un biais dans notre étude liée à une pratique locale adaptée au recrutement de notre hôpital.

Suivi biologique

La surveillance hématologique imposée depuis la réintroduction de la clozapine dans les années 90 a permis de faire diminuer le nombre de décès sous clozapine par agranulocytose (diminution du risque infectieux principalement) : 50% dans les années 70 à 3% actuellement [START_REF] Miller | Review and management of clozapine side effects[END_REF]. Ceci s'explique par la baisse de l'incidence des agranulocytoses qui est passée de 2 à 0,38%.

Dans notre population, il n'y a eu aucun cas d'agranulocytose en 14 ans. Deux patients (2,6%) ont présenté une neutropénie sévère mais n'ont pas présenté de signes cliniques associés.

Notre étude confirme l'importance de la surveillance de la NFS une fois par semaine au cours des 18 premières semaines de traitement. En effet, nos données soutiennent le fait que le risque de développer une agranulocytose est plus important au cours du premier trimestre [START_REF] Alvir | Clozapineinduced agranulocytosis: Incidence and risk factors in the United States[END_REF]. Cette surveillance permet de déceler au plus tôt les neutropénies et dans certains cas une surveillance renforcée permet de poursuivre le traitement.

Cependant, les résultats de notre travail ne sont pas toujours facilement interprétables de par le faible effectif, constituant le principal biais de notre étude. De plus, le fait de ne pas connaître la date et les modalités d'instauration de la clozapine dans le groupe de 16 patients pour lesquels elle a eu lieu en dehors de l'établissement réduit encore plus cet effectif. Ces 16 patients, n'ayant pas présenté de neutropénie, ne sont pas inclus dans les tests statistiques.

Conclusion :

Cette étude montre un taux d'incidence global des neutropénies de 15,6%. Ce taux est élevé mais prend en compte les neutropénies légères pour lesquelles une surveillance rigoureuse de la NFS permet la poursuite du traitement par clozapine. Dans deux tiers des cas, le traitement a pu être poursuivi ce qui montre l'intérêt de la surveillance régulière de la NFS des patients sous clozapine instaurée par l'ANSM.

  plusieurs cas d'agranulocytose, elle est rapidement retirée du marché. Vingt ans plus tard, en 1991, elle revient avec une autorisation de mise sur le marché restrictive : utilisation en monothérapie, prescription initiale hospitalière et réservée aux spécialistes en psychiatrie, neurologie et gériatrie. Il en est de même pour le renouvellement [2]. Elle est alors réservée aux schizophrénies réfractaires et aux psychoses au cours de l'évolution de la maladie de Parkinson. Elle est utilisée après échec d'autres thérapeutiques car son fort pouvoir leucopéniant et neutropéniant implique une surveillance étroite. Afin de faciliter le suivi, un carnet de surveillance a été mis en place par l'Agence nationale de sécurité du médicament et des produits de santé (ANSM) [3]. Il est obligatoire pour chaque patient et permet un suivi rigoureux de la numération formule sanguine (NFS) grâce à la réalisation d'un hémogramme initial puis hebdomadaire durant les 18 premières semaines et enfin mensuel pendant toute la durée du traitement. Lors de chaque réalisation de la NFS, la date, la posologie de clozapine, la numération des leucocytes (GB) et des granulocytes neutrophiles (PNN) sont indiqués afin de détecter une éventuelle leucopénie ou neutropénie. Ce carnet doit obligatoirement être présenté à la pharmacie, rempli et signé par le médecin, pour que la dispensation puisse avoir lieu. Le pharmacien indique la date et le nombre de comprimés délivrés attestant ainsi de la dispensation. La perturbation de la NFS de type GB < 3000/mm 3 ou PNN < 1500/mm 3 entraine un arrêt immédiat du traitement et sa contre-indication à vie. Si la NFS met en évidence des GB compris entre 3000 et 3500/mm 3 et/ou des PNN compris entre 1500 et 2000/mm 3 , la surveillance hématologique doit être renforcée par un contrôle bihebdomadaire de la NFS. L'état des lieux du taux d'incidence des neutropénies sous clozapine dans la bibliographie internationale révèle que ce dernier varie de 1,3% [4] à 10,28% [5] selon les études pour une neutropénie définie par un score de PNN < 1500/mm 3 . Les patients ayant une numération des PNN inférieure à 2000/mm 3 nécessitent une attention particulière puisque la mise en place d'un suivi rapproché permet dans certains cas de ne pas arrêter le traitement. Néanmoins, dans ces études, il n'est pas précisé si ce suivi a été réalisé et les données de la littérature ne précisent pas le taux d'incidence des patients ayant des PNN compris entre 1500/mm 3 et 2000/mm 3 et pour lesquels la mise en place d'une surveillance étroite de la NFS a permis de ne pas arrêter le traitement.

  entre 2003 et 2016 au sein de notre établissement psychiatrique comportant 543 lits et places. Une extraction informatique, à partir de notre logiciel de prescription (Genois®), a permis de recueillir la liste des patients ayant eu une prescription de clozapine au cours de ces 14 années. Les patients pour lesquels la date d'instauration du traitement n'était pas connue, car non entreprise dans notre établissement, et ceux n'ayant pas débuté le traitement par clozapine (score initial de PNN trop bas ou pour raison indéterminée) ont été exclus. Cette extraction a aussi permis d'observer l'évolution du nombre d'instaurations de traitement par clozapine durant cette période.

  suivante : le premier a reçu du lénograstim pendant 7 jours et une antibiothérapie prophylactique par ceftriaxone (10 jours) et amikacine (4 jours). Le second a reçu du lénograstim pendant 8 jours et de la ceftriaxone pendant 6 jours. Il n'y a eu aucun signe clinique ni aucune complication chez ces patients et la récupération biologique s'est faite en moins de 15 jours. En parallèle, une déclaration de pharmacovigilance était envoyée au centre de pharmacovigilance de référence. Discussion : Caractéristiques de la population étudiée Notre étude nous a permis de recenser 93 patients ayant reçu de la clozapine à plus ou moins long terme sur une période de 14 ans. Pour une partie d'entre eux, l'instauration s'est faite en dehors de l'établissement et nous n'avons pas pu retrouver la date d'instauration. Ils ont donc été exclus de l'étude. Parmi les 77 instaurations de traitement ayant eu lieu dans notre établissement, nous avons constaté une nette augmentation de la prescription de clozapine au cours des 7 dernières années. Ceci s'explique en partie par le recrutement de nouveaux prescripteurs, généralement plus jeunes, ayant plus recours à la clozapine dans leurs stratégies thérapeutiques. Néanmoins, le nombre d'instaurations reste faible par rapport à d'autres établissements. Ceci pourrait être expliqué, d'une part par la crainte de nos psychiatres à utiliser ce médicament ou d'autre part, par les contraintes imposées par la surveillance. Il pourrait être intéressant de communiquer davantage sur les recommandations de prescription de la clozapine au sein de notre établissement au travers de revues de cas ou d'audits de prescription par exemple. Par ailleurs, dans cette étude nous retrouvons une prédominance masculine (68,8%) et une moyenne d'âge de 41,8 ans lors de l'instauration de la clozapine. Les caractéristiques démographiques retrouvées dans notre échantillon sont proches de celles rapportées par Levoyer et al qui retrouvaient 62,9% d'hommes et une moyenne d'âge de 43,1 ans [8].

D

  'autre part, le taux d'incidence des patients ayant présenté une neutropénie avec des PNN compris entre 1500 et 2000 PNN/mm 3 est de 10,4%. Ainsi, 66% des patients ayant présenté une neutropénie ont pu poursuivre le traitement par clozapine grâce à une surveillance rapprochée de la NFS. Au cours de cette période le contrôle biologique se faisait au minimum 2 fois par semaine jusqu'à la normalisation des PNN et la dispensation pharmaceutique était adaptée : délivrance de la quantité nécessaire de comprimés de clozapine jusqu'à la prochaine NFS. Paramètres pouvant influencer la survenue de NP Nous nous sommes intéressés aux facteurs susceptibles d'influencer la survenue de la neutropénie. Ainsi, l'incidence semble varier en fonction du sexe. En effet, le risque relatif de survenue d'une neutropénie est plus important chez la femme que chez l'homme (RR=2,21). Cette observation avait déjà été faite par Gareri et al. [11]. Nous avons ensuite recherché une éventuelle corrélation entre la posologie de clozapine administrée et la survenue de l'EI. La posologie de clozapine retrouvée chez nos patients était très variable mais pour tous les patients elle était bien inférieure à la posologie maximale recommandée dans le résumé des caractéristiques du produit (RCP) (900 mg/j) [2]. L'absence d'influence de la posologie sur la survenue de la neutropénie est liée au mécanisme physiopathologique à l'origine de l'EI. En effet, les données de la littérature semblent être en faveur d'un mécanisme immunoallergique. La clozapine agirait comme un haptène : en se fixant à la surface des PNN au niveau médullaire, elle induirait l'apparition de nouveaux sites antigéniques entrainant une réponse immunitaire et la destruction des PNN. La survenue de l'EI n'est donc pas liée à un effet directement toxique de la clozapine et n'est pas corrélée à la dose de clozapine reçue par le patient [12]. Il est de plus intéressant de noter que certains allèles rares du complexe majeur d'histocompatibilité (CMH ; HLA-B et HLA-DQB1) semblent entrainer une prédisposition aux neutropénies sous clozapine [13]. Par ailleurs, les recommandations actuelles attestent que la clozapine, lors de son instauration, doit être prescrite en monothérapie [2]. Cependant, nous avons constaté un taux d'instauration en monothérapie de seulement 62,3%, soit 48 patients dans notre échantillon de 77 patients. Ce taux est inférieur à celui retrouvé par Levoyer et al en 2004 lors de leur étude [8] qui portait sur 170 patients hospitalisés dans un centre médico-psychologique de Rennes. Les recommandations ne sont donc pas appliquées pour encore plus d'un tiers des patients sous clozapine et comme le soulignaient Péré et al dès 1993, la clozapine est trop souvent instaurée et/ou utilisée avec d'autres neuroleptiques [14]. Cette pratique pourrait être en partie expliquée par la crainte des psychiatres du washout de l'antipsychotique, c'est-à-dire une période pendant laquelle le patient ne serait sous aucun traitement pharmacologiquement actif, entrainant une recrudescence des signes aigus pendant la période nécessaire pour atteindre l'état d'équilibre de la clozapine.

  Pour améliorer la sécurité et l'efficience des prescriptions de clozapine, une évaluation des pratiques professionnelles (EPP) avec un audit observationnel portant sur les prescriptions de clozapine dans notre centre hospitalier pourrait servir à faire un état des lieux des pratiques tout en impliquant les prescripteurs. Cela pourrait favoriser les échanges et permettre une modification des pratiques afin d'améliorer la prise en charge médicamenteuse des patients atteints de schizophrénie réfractaire et l'adhésion aux recommandations. La prévision d'une nouvelle réévaluation pourrait être envisagée dans les 6 mois suivant l'EPP. De plus, il serait intéressant d'avoir un classement des antipsychotiques en fonction de leur imputabilité dans le risque d'apparition de neutropénie ou d'agranulocytose qui pourrait servir d'outil supplémentaire d'aide à la validation pour le pharmacien. Au-delà de l'association à d'autres antipsychotiques, il est important de rappeler que l'association avec tout médicament neutropéniant doit être évitée avec la clozapine (antithyroïdiens de synthèse, carbamazepine, phénothiazines, deferiprone, captopril, sulfamides antidiabétiques) pour éviter une potentialisation du risque. Au CH Philippe Pinel, la bithérapie clozapine/loxapine est fréquemment rencontrée pour la prise en charge des patients pour lesquels la clozapine en monothérapie n'est pas suffisante. L'acte de validation pharmaceutique au cours duquel le pharmacien de notre établissement autorise régulièrement l'association avec la loxapine peut expliquer cette observation et la diminution du taux d'instauration en monothérapie par rapport à celui obtenu par Levoyer et al. En effet, nos recommandations locales proposent aux cliniciens, en cas de tableau clinique nécessitant l'association de deux NL, d'utiliser la loxapine (antipsychotique à profil sédatif) pour le traitement des symptômes aigus et transitoires chez les patients agités ou agressifs. Cette dérive des pratiques est liée à la complexité de la prise en charge de la schizophrénie dans nos établissements spécialisés où l'on rencontre des cas de schizophrénie réfractaire ne répondant pas à la monothérapie par clozapine. Comme tous les NL présentent un pouvoir neutropéniant plus ou moins important, la loxapine est privilégiée car le risque est relativement faible comparé à d'autres neuroleptiques. Dans le RCP il est spécifié que la loxapine présente parmi ses EI un risque exceptionnel d'agranulocytose [15] et un seul cas d'agranulocytose sous loxapine chez une patiente de 88 ans a été publié dans la littérature à notre connaissance [17].

  Figure I : Evolution du nombre d'instaurations de traitement par clozapine dans notre

Figure

  Figure IIs: Neuroleptiques associés à la clozapine lors de l'instauration du traitement.

Figure III :

 III Figure III : Diagramme de flux de patients.

Figure IV :

 IV Figure IV : Classe thérapeutique des traitements concomitants retrouvés au moment de la

Figure V :

 V Figure V : Classification des médicaments co-prescrits avec la clozapine au moment de la


Franck Di Rienzo
email: franck.di-rienzo@univ-lyon1.fr

Pierric Joassy

Thiago Kanthack

Eoghan Tadhg

Ursula Macintyre

Yoann Debarnot

Christophe Blache

Christian Hautier

Aymeric Collet

Guillot

Tadhg Eoghan Macintyre

Ursula Debarnot

Yoann Blache

Christophe Hautier

Christian Collet

Aymeric Guillot

Effects of Action Observation and Action Observation Combined with Motor Imagery on Maximal Isometric Strength

Keywords: neural plasticity, neural priming, muscle function, motor cognition, conditioning

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Dose-response relationships in the development of muscle strength have been the focus of a large body of research since the middle of the 20 th century [START_REF] Rhea | A meta-analysis to determine the dose response for strength development[END_REF][START_REF] Peterson | Maximizing strength development in athletes: a meta-analysis to determine the dose-response relationship[END_REF]. Strength development originates from both structural and functional adaptations [START_REF] Komi | Training of muscle strength and power: interaction of neuromotoric, hypertrophic, and mechanical factors[END_REF]. Strength development is classically associated with hypertrophy, i.e. an increase in muscle mass [START_REF] Damas | A review of resistance traininginduced changes in skeletal muscle protein synthesis and their contribution to hypertrophy[END_REF]. Muscle hypertrophy occurs as a long-term result of resistance training (programs scheduled within a span of several weeks), and stems from an adaptive response to the mechanical and metabolic demands elicited by training [START_REF] Schoenfeld | The mechanisms of muscle hypertrophy and their application to resistance training[END_REF]. Functional adaptations also play a key role in strength development, particularly during the early stages where short-term improvements occur in the absence of muscle hypertrophy [START_REF] Sale | Neural adaptation to resistance training[END_REF][START_REF] Moritani | Neuromuscular adaptations during the acquisition of muscle strength, power and motor tasks[END_REF]. Indeed, strength produced during maximal voluntary contractions mirrors the capacity of the central nervous system to synchronise the recruitment of motor units [START_REF] Champely | The power of the mind: the cortex as a critical determinant of muscle strength/weakness[END_REF]. Weighted resistance training above 90% of the maximal voluntary contraction threshold emphasises neurophysiological adaptations yielding improved motor units recruitment [START_REF] Kraemer | Strength and power training: physiological mechanisms of adaptation[END_REF].

Resistance training programs typically consist of physical exercise with progressive poundage.

Nonetheless, the potential relevance of training with mental practice is gaining increased attention [START_REF] Tod | Psyching-up" and muscular force production[END_REF][START_REF] Tod | A Systematic Review of the Effect of Cognitive Strategies on Strength Performance[END_REF][START_REF] Paravlic | Effects and Dose-Response Relationships of Motor Imagery Practice on Strength Development in Healthy Adult Populations: a Systematic Review and Meta-analysis[END_REF]. Recent evidence supports the positive effects of motor imagery (MI), i.e. the voluntary process of mentally simulating a movement without engaging in its physical execution, on strength and its inclusion within resistance training programs [START_REF] Tod | A Systematic Review of the Effect of Cognitive Strategies on Strength Performance[END_REF][START_REF] Paravlic | Effects and Dose-Response Relationships of Motor Imagery Practice on Strength Development in Healthy Adult Populations: a Systematic Review and Meta-analysis[END_REF]. Functional brain imaging findings support the hypothesis of a functional equivalence between MI and the physical execution of the same action. MI activates brain regions controlling the actual motor preparation and execution, albeit with reduced magnitude [START_REF] Lotze | Motor imagery[END_REF][START_REF] Munzert | Motor imagery and its implications for understanding the motor system[END_REF][START_REF] Hétu | The neural network of motor imagery: an ALE meta-analysis[END_REF]. There is evidence of premotor, parietal and primary sensorimotor cortex activation during MI [START_REF] Gerardin | Partially overlapping neural networks for real and imagined hand movements[END_REF][START_REF] Ehrsson | Imagery of voluntary movement of fingers, toes, and tongue activates corresponding body-part-specific motor representations[END_REF][START_REF] Solodkin | Fine modulation in network activation during motor execution and motor imagery[END_REF][START_REF] Gao | Evaluation of effective connectivity of motor areas during motor imagery and execution using conditional Granger causality[END_REF]. MI also recruits subcortical regions such as the basal ganglia and the cerebellum [START_REF] Hanakawa | Functional properties of brain areas associated with motor execution and imagery[END_REF][START_REF] Szameitat | Motor imagery of complex everyday movements. An fMRI study[END_REF][START_REF] Wagner | Mind the bend: cerebral activations associated with mental imagery of walking along a curved path[END_REF][START_REF] Burianová | Multimodal functional imaging of motor imagery using a novel paradigm[END_REF]. At the peripheral level, the effector-specific facilitation of corticospinal pathways during MI is well-established [START_REF] Stinear | Corticospinal facilitation during motor imagery[END_REF]Grosprêtre et al., 2016, for reviews).

Furthermore, both [START_REF] Jowdy | Muscular responses during mental imagery as a function of motor skill level[END_REF] and [START_REF] Gandevia | Mental rehearsal of motor tasks recruits alpha-motoneurones but fails to recruit human fusimotor neurones selectively[END_REF] reported low-threshold muscle activity during MI. The subliminal muscle activation reproduced the intensity and regimen of the imagined contractions [START_REF] Bakker | Changes in muscular activity while imagining weight lifting using stimulus or response propositions[END_REF][START_REF] Guillot | Muscular responses during motor imagery as a function of muscle contraction types[END_REF][START_REF] Lebon | Modulation of EMG power spectrum frequency during motor imagery[END_REF]. Henceforth, rather than a purely mental state, MI should overall better be considered a "(…) special form of motor behaviour" [START_REF] Stephan | Motor imagery--anatomical representation and electrophysiological characteristics[END_REF].

From an applied viewpoint, MI increases the cognitive demand on brain motor regions, which then could prompt experience-based plasticity and improve motor performance [START_REF] Rienzo | Online and Offline Performance Gains Following Motor Imagery Practice: A Comprehensive Review of Behavioral and Neuroimaging Studies[END_REF]. Pioneering investigations of the effects of MI on strength focused on behavioural measures, sometimes associated with recordings of muscle activation [START_REF] Cornwall | Effect of mental practice on isometric muscular strength[END_REF][START_REF] Yue | Strength increases from the motor program: comparison of training with maximal voluntary and imagined muscle contractions[END_REF]. There is an emerging consensus that embedding MI within resistance training programs of several weeks yields additional strength benefits. Despite methodological differences across studies (e.g. the MI to physical practice ratio administered during the training sessions), strength typically increases from 10 to 30% [START_REF] Reiser | Strength gains by motor imagery with different ratios of physical to mental practice[END_REF]. Improvements occur in the absence of muscle hypertrophy, hence suggesting neural adaptation [START_REF] Yue | Strength increases from the motor program: comparison of training with maximal voluntary and imagined muscle contractions[END_REF]. More recently, we observed strength gains after a single-session of MI [START_REF] Rienzo | Short-term effects of integrated motor imagery practice on muscle activation and force performance[END_REF]. These findings corroborate the proposition of the psycho-neuromuscular theory of mental practice, which posits that motor simulation can be effective to improve the cortical gain over motor units [START_REF] Jacobson | Electrophysiology of mental activities[END_REF]. While short-term changes in the cortical gain over motor units might drive most of the short-term effects [START_REF] Rienzo | Short-term effects of integrated motor imagery practice on muscle activation and force performance[END_REF][START_REF] Grosprêtre | Neural mechanisms of strength increase after one-week motor imagery training[END_REF], Hebbian-type plasticity comparable to that elicited by physical training may be the core process mediating improvements of motor performance through MI (Di Rienzo et al., 2016, for a review). This applies to strength development paradigms, where resistance training programs with MI extend across several weeks. For instance, [START_REF] Ranganathan | From mental power to muscle power--gaining strength by using the mind[END_REF] demonstrated that strength improvements consecutive to MI training were associated with increased amplitudes of the cortical motor potentials generated during high-intensity strength trials (see also [START_REF] Yao | Kinesthetic imagery training of forceful muscle contractions increases brain signal and muscle strength[END_REF][START_REF] Grosprêtre | Neural mechanisms of strength increase after one-week motor imagery training[END_REF].

Action observation (AO) is another cognitive motor process that shares common neural substrates with both MI and the actual motor preparation/execution [START_REF] Tkach | Congruent activity during action and action observation in motor cortex[END_REF][START_REF] Hardwick | Neural correlates of action: Comparing meta-analyses of imagery, observation, and execution[END_REF].

AO and MI remain functionally dissociable and hierarchized with regards to the degree of involvement of the brain motor regions [START_REF] Macuga | Neural representations involved in observed, imagined, and imitated actions are dissociable and hierarchically organized[END_REF]. Nonetheless, AO consistently engages premotor, parietal and cerebellar networks. Brain networks recruited during AO have been described as part of a "mirror neuron" system [START_REF] Rizzolatti | The mirror-neuron system[END_REF][START_REF] Casile | The mirror neuron system: a fresh view[END_REF].

Research in both humans and primates demonstrated that the mirror neuron system encodes relevant behavioural features of the observed actions [START_REF] Agnew | The human mirror system: a motor resonance theory of mind-reading[END_REF][START_REF] Vogt | From visuo-motor interactions to imitation learning: behavioural and brain imaging studies[END_REF][START_REF] Alaerts | Observing how others lift light or heavy objects: time-dependent encoding of grip force in the primary motor cortex[END_REF][START_REF] Catmur | Understanding intentions from actions: Direct perception, inference, and the roles of mirror and mentalizing systems[END_REF]. AO thus involves inputs to the brain motor system under the form of "(…) Externally guided motor simulation" (Vogt et al., 2013, p. 3). AO improves motor learning, specifically when the observer has a previous experience of the task [START_REF] Rizzolatti | Premotor cortex and the recognition of motor actions[END_REF][START_REF] Bird | Sequence learning by action, observation and action observation[END_REF][START_REF] Mattar | Motor learning by observing[END_REF][START_REF] Calvo-Merino | Seeing or doing? Influence of visual and motor familiarity in action observation[END_REF]. However, few studies have investigated the efficacy of AO to improve strength. This is rather surprising considering the currently growing interest in mental training with AO to potentiate the recovery of muscle function [START_REF] Pomeroy | The potential for utilizing the "mirror neurone system" to enhance recovery of the severely affected upper limb early after stroke: a review and hypothesis[END_REF][START_REF] Sale | Action observation and mirror neuron network: a tool for motor stroke rehabilitation[END_REF][START_REF] Buccino | Action observation treatment: a novel tool in neurorehabilitation[END_REF]. [START_REF] Porro | Enhancement of force after action observation: behavioural and neurophysiological studies[END_REF] pioneered that daily training sessions with AO scheduled over the course of two consecutive weeks improved index finger abduction strength by 30%. The gains were specific to the movement observed during training.

Noteworthy, the authors recorded increased excitability of corticospinal pathways targeting the first dorsal interosseous after training completion. These data support earlier findings by [START_REF] Ranganathan | From mental power to muscle power--gaining strength by using the mind[END_REF] suggesting that the statements of the psycho-neuromuscular theory of MI could also apply to training with AO. MI and AO have been broadly conceptualized within the field of motor cognition as motor simulation processes due to their capacity to engage the brain motor system in the absence of physical execution [START_REF] Jeannerod | The representing brain: Neural correlates of motor intention and imagery[END_REF][START_REF] Jeannerod | Neural simulation of action: a unifying mechanism for motor cognition[END_REF]. Previously, [START_REF] Shepard | Ecological constraints on internal representation: resonant kinematics of perceiving, imagining, thinking, and dreaming[END_REF] suggested that MI and AO belonged to a continuum extending from passive/external (bottom-up) to active/internal (top-down) motor simulation [START_REF] Vogt | Multiple roles of motor imagery during action observation[END_REF] for a more recent discussion). Consequently, the overlaps between MI and AO are readily exemplified in sporting situations. For instance, observing a teammate or an opponent's performance concomitantly elicits the simulation of an adapted motor response. An emergent body of research investigates training with AO and MI concomitantly (AO+MI). AO+MI was referred to as "(…) video guided imagery" or "(…) imagining imitation" in the scientific literature. Eaves et al. (2016b) underlined that combining bottom-up and top-down processing through AO+MI yields synchronized visual and kinaesthetic inputs to the brain. Performing MI while observing the same movement on a video might thus facilitate the build-up of accurate motor representations by prompting attentional focus on the relevant spatial and temporal features of the coordination.

Similarly, video cues provide direct inputs to the visual system and reduce the need for top-down processing of MI. The top-down processing of kinaesthetic information might consequently benefit from increased allocation of attentional resources. Transcranial magnetic stimulation provided evidence of higher corticospinal excitability during AO+MI compared to either AO or MI alone [START_REF] Sakamoto | Combining observation and imagery of an action enhances human corticospinal excitability[END_REF][START_REF] Ohno | Excitability changes in the human primary motor cortex during observation with motor imagery of chopstick use[END_REF][START_REF] Wright | Combined action observation and imagery facilitates corticospinal excitability[END_REF][START_REF] Mouthon | Task-dependent changes of corticospinal excitability during observation and motor imagery of balance tasks[END_REF]. Functional brain imaging recordings confirmed that AO+MI was associated with increased activity in brain motor regions compared to AO and MI alone [START_REF] Nedelko | Action imagery combined with action observation activates more corticomotor regions than action observation alone[END_REF]. Due to greater involvement of motor regions in the brain, AO+MI might represent a more efficient way of enhancing the motor performance compared the isolated practice of AO and MI [START_REF] Eaves | Motor imagery during action observation modulates automatic imitation effects in rhythmical actions[END_REF][START_REF] Taube | Non-physical practice improves task performance in an unstable, perturbed environment: motor imagery and observational balance training[END_REF][START_REF] Bek | Enhancing voluntary imitation through attention and motor imagery[END_REF].

Research on the effects of AO+MI on motor performance remains sparse. A limited number of experiments compared the effects of AO+MI to AO or MI alone. This is particularly valid when considering strength development, although recent studies have specifically compared the efficacy of AO+MI with that of MI practice alone. In a seminal study, [START_REF] Scott | Motor imagery during action observation increases eccentric hamstring force: an acute non-physical intervention[END_REF] reported greater effectiveness of AO+MI compared to MI alone to increase hamstring strength. The study design also included a MI-control condition (i.e. MI focused on the upper-limbs). The three-weeks of training involved 3 sessions of 20 min of MI training incorporating the "Physical, Environmental, Task, Timing, Learning, Emotion Perspective" (PETTLEP) guidelines [START_REF] Holmes | The PETTLEP approach to motor imagery: A functional equivalence model for sport psychologists[END_REF]. The AO+MI and MI alone session focused on Nordic hamstring exercises, while the AO component of the AO+MI condition involved an external visual perspective. Eccentric hamstring peak torque increased in the right leg in the AO+MI group (i.e. ~6% of increase compared to the baseline), whereas the ~3.5% increase in the MI alone group did not reach significance. Further analyses (magnitude-based inference) confirmed the potential superior relevance of AO+MI, specifically from a clinical perspective. Similarly, [START_REF] Smith | The Effects of Combining PETTLEP Imagery and Action Observation on Bicep Strength: A Single-Case Design[END_REF] addressed whether the adjunct of AO to MI training improved the maximal strength on a biceps curl machine. The authors implemented a multiple-case counterbalanced design, where AO+MI and MI alone were each administered during four weeks after a three-week baseline period. Surprisingly, the authors found that MI and AO+MI were both equally efficient. Notably, AO did not focus on an external model, but was built from a video capture of participants' own physical performance, filmed from the first person perspective during a set of repetitions to failure during the baseline period. Although their findings conflicted with those reported by [START_REF] Scott | Motor imagery during action observation increases eccentric hamstring force: an acute non-physical intervention[END_REF], the authors did not rule out the hypothesis that AO+MI might outperform MI alone. This is primarily due to the trajectory of performance changes. When AO+MI took place before MI alone the authors systematically observed a performance increase along the course of the four weeks, whereas MI alone appeared to maintain the strength benefits. They also considered the possibility of delayed gains, where AO+MI would ultimately outperform MI alone if the intervention was prolonged up to 15 weeks (e.g. [START_REF] Wakefield | From strength to strength: A single-case design study of PETTLEP imagery frequency[END_REF]. In keeping with these exploratory findings, we found no experiments that compared AO with AO+MI in a strength paradigm.

Considering the limited amount of research on the development of maximal isometric strength and the growing interest in AO+MI, we designed the present study to investigate the short-term effects of AO and AO+MI on maximal isometric strength. Based on the psycho-neuromuscular theory, we hypothesized that AO+MI should outperform AO, while AO would outperform the control condition.

Material and Methods

Participants

Twenty-one participants were recruited from the Faculty of Sports Sciences of the University Claude Bernard Lyon 1 (F-69100, Villeurbanne; 17 males; mean age = 22.25 ± 2.18 years). All were athletes of regional to national level in terrestrial sports (judo, karate, climbing, basket and soccer), and had a background of at least 2 years in resistance training (i.e. >1 year of practice, 2-4 sessions of 45 min per week). Participants were also screened based on their ability to engage in MI practice using a standardized MI questionnaire, i.e. average score >5 on the Movement Imagery Questionnaire-3 [START_REF] Williams | Further validation and development of the movement imagery questionnaire[END_REF]. The study was a relative strength contest with reference to body weight. This was expected to encourage the commitment to a maximal effort throughout experimental sessions. We did not provide any information regarding the purpose of the study until after completion of the design. The local Ethics Committee approved the study. Each participant provided written informed consent according to the statements of the Declaration of Helsinki (2013).

Experimental design

The repeated-measures design comprised three experimental sessions of 30 minutes, separated from each other by 48 hours. We interposed a no-training day between experimental sessions, which were scheduled at 12 am before lunch to control for the potential influence of circadian rhythms. Each experimental session consisted of 10 maximal elbow flexions against a fixated force platform.

Participants attempted to lift the fixated force platform from a seated position with the elbow at 90° and the hand against the platform (Figure 1). For each trial, the voluntary maximal isometric contraction was sustained for 12 s. Each trial was separated from each other by a 60 s period allocated to recovery.

*** Insert Figure 1 about here *** Experimental settings

Participants warmed-up for 10 min in a quiet room. They first completed 6 minutes of body weight exercises (i.e. lunges and squat jumps). Then, they seated on a bench equipped with a reclining seatback. They were instructed to keep permanent contact between the reclining seatback and the back of their head, the posterior apex of the thoracic spine, and the pelvis in order to standardize the trunk position (Figure 1A). Participants constantly fixed a cross mark on the wall at eye level. From this standardized position, each participant performed 5 consecutive isometric contractions of 12 s of incremental intensity against the fixated force platform. They were instructed to achieve a self-paced increase in the intensity of the isometric contractions across the 5 trials, but below their maximal isometric strength threshold. Each trial was separated by recovery periods of 50 s. Participants then finished the warm-up by two maximal isometric contractions sustained for 12 s, separated by 90 s of passive recovery. Finally, the experimenter provided oral instructions corresponding to the upcoming experimental condition. While sitting on the bench, they faced a force platform under which they placed their dominant hand in supine position, with the elbow at 90°, as controlled by goniometers.

We rigorously controlled the hand position at each trial, checking that the wrist, palm and fingers remained in flat contact with the force platform (Figure 1A).

Experimental conditions

Across the repeated measures of the design, we administered the following experimental conditions during the inter-trial recovery periods: i) observation of a bodybuilder athlete performing the task (AO), ii) observation of a bodybuilder athlete performing the task combined with MI of oneself performing the task (AO+MI) and iii) watching passively a video documentary about basketball shooting (CONTROL). Experimental settings were identical to those during the warm-up.

During each 60 s inter-trial recovery period, we administered three blocks (15 s each) of mental practice corresponding to the different experimental conditions. The onset and the offset of each trial and recovery periods was externally cued by audio stimuli (Presentation, Neurobehavioral systems®, 240 Hz, 50 Db). To prevent carryover effects, e.g. residual muscle fatigue from one experimental session to another, experimental conditions were administered in counterbalanced order (block randomization).

During inter-trial recovery periods, participants watched a 15 s video displayed on a screen in front of them. During AO and AO+MI, the video displayed a bodybuilder athlete (79 kg, 7.63% of body fat, 22 years old) completing the maximal isometric strength task in identical experimental settings. The video showed the bodybuilder from a lateral view (Figure 1A). During AO, the experimenter gave participants the following instructions: "Watch carefully the athlete performing the maximal voluntary contraction. Focus on the effort produced by the athlete trying to lift the platform.

Focus your attention on his upper limb and observe the intense muscles contractions all along with the 10 maximal effort intensity". The experimenter explicitly instructed participants not to engage in MI during the AO condition. This method is comparable to that described by Eaves et al. (2016a), where the authors requested: "Please refrain from undertaking any MI during (…) observation in this pure AO condition" (p. 94). During AO+MI, participants performed the same action observation task as during AO while simultaneously engaging in MI. When watching the video, the participants were instructed to: "Imagine yourself in an attempt to lift the platform. Feel the intense contraction of your biceps and your shoulder muscles during the maximal effort. Focus on the total recruitment of muscle fibres throughout the duration of the effort". Participants were prompted by such instructions to engage in the kinaesthetic modality of MI. During CONTROL, participants remained motionless and watched the video documentary for 15 s.

Dependent variables

Psychometric recordings

Functions of Observational Learning Questionnaire

Before the first experimental session, participants completed the Functions of Observational Learning Questionnaire (FOLQ; [START_REF] Cumming | The functions of observational learning questionnaire (FOLQ)[END_REF]. This is a qualitative tool investigating participants' uses of action observation with the aim to improve performance in their sporting activities [START_REF] Cumming | The functions of observational learning questionnaire (FOLQ)[END_REF]. The FOLQ investigates the uses of AO with reference to the cognitive and motivational functions of MI earlier described by [START_REF] Paivio | Cognitive and motivational functions of imagery in human performance[END_REF] to improve sporting performance. Participants reported each item on a Likert-type scale ranging from 1 ("I never use") to 7 ("I often use"). The FOLQ specifically investigates two cognitive (Strategy and Skills) and one motivational (Performance) functions of AO. STRATEGY refers to uses of AO to improve cognitive processing related to tactical aspects mediating performance (e.g. "I use observational learning to determine how a strategy will work in an event/game"). SKILLS refers to uses of AO to improve cognitive processing related to technical execution mediating performance (e.g. "I use observational learning to understand how to perfectly perform a skill"). Finally, PERFORMANCE refers to uses of AO to achieve optimal arousal and anxiety appraisal in sporting situations (e.g. "I use observational learning to know how to respond to the excitement associated with performing well"). The FOLQ showed high internal consistency with Cronbach's alphas ranging from 0.84-0.90 [START_REF] Cumming | The functions of observational learning questionnaire (FOLQ)[END_REF].

Likert ratings

After each experimental session, participants rated their perceived strength throughout the maximal isometric strength trials on a Likert-type scale ranging from 1 ("Very low level of strength output") to 10 ("Very high level of strength output"). Participants also rated their motivation to complete the experimental session on a Likert-type scale ranging from 1 ("Very low level of motivation") to 10 ("Very high level of motivation"). We also evaluated the perceived difficulty to complete the experimental session with self-reports using a Likert-type scale ranging from 1 ("Very low level of difficulty") to 10 ("Very high level of difficulty"). After completion of the AO+MI condition, we finally evaluated MI vividness with self-reports with a Likert-type scale ranging from 1 (i.e. "Absence of visual/kinaesthetic information during imagination") to 10 (i.e. "Comparable visual/kinaesthetic information during imagination as during physical practice").

Behavioural recordings

We measured the elbow flexion strength with a force platform (AMTI, model 0R6-7-2000, Watertown, Massachusetts, USA). Data were continuously recorded and synchronized by LabChart Pro V8 © (ADInstruments Pty Ltd, 2014) at 1000 Hz. Data were smoothed with a zero-lag 4th-order low-pass Butterworth filter with a 20 Hz cut-off frequency. During each trial, the sudden force increase in response to auditory stimulus was detected using a threshold detection function (Matlab®).

We then calculated the total force by integrating the force slope with respect to the duration of each trial (12 s, trapezoid rules). We finally normalized the total force in percentage of the total force value recorded during the maximal isometric force trial of the warm-up: The total force was used as the behavioural performance variable quantifying strength.

Physiological recordings

For each trial, we recorded the surface electromyograms (EMG) of the main agonists. After shaving and cleaning the skin with alcohol, we recorded EMG from the biceps brachii using pairs of surface electrodes (1 cm EMG Triode, nickel-plated brass, inter-electrode distance 2 cm, Thought Technology, Montreal, Canada). We also recorded EMG from the anterior deltoideus, a synergist muscle of the experimental task (Di [START_REF] Rienzo | Short-term effects of integrated motor imagery practice on muscle activation and force performance[END_REF]. Electrodes positioning was determined according to the recommendations of the "Surface Electromyography for the Non-Invasive Assessment of Muscles" (SENIAM) project [START_REF] Hermens | Development of recommendations for SEMG sensors and sensor placement procedures[END_REF]. Electrodes location was marked with a pen and photographed to ensure reproducible positioning across experimental sessions. We continuously recorded and synchronized EMG data by LabChart ProV8 © (ADInstruments Pty Ltd, 2014). We processed the raw signal using the TrignoTM Wireless EMG © system (2014, Delsys Incorporated).

We then rectified and smoothed the raw EMG signal with a 20-500 Hz pass-band filter (Butterworth 4 th ). For each trial, we calculated the integrated EMG (iEMG) with respect to the 12 s of each voluntary contraction (Figure 1B). We considered iEMG a global measurement of motor units activation related to isometric strength output [START_REF] Moritani | Reexamination of the relationship between the surface integrated electromyogram (IEMG) and force of isometric contraction[END_REF]. We finally normalized iEMG data in percentage of the maximal iEMG value recorded during the maximal isometric force trial of the warm-up: We continuously recorded electrodermal activity through skin conductance measures (micro Siemens, μS, i.e. 1.10 -6 m -2 .kg-1.s 3 .A 2 ), using two 50 mm 2 unpolarizable bipolar electrodes placed on the second phalanx of the second and third digits of the non-dominant hand (Figure 1; MLT116f GSR Finger Electrodes, ADInstruments, Dunedin, New Zealand). Electrodermal activity reflects the activity of eccrine sweats glands, which are under the unique control of the sympathetic branch of the autonomic nervous system [START_REF] Shields | Is mediation of sweating cholinergic, adrenergic, or both? A comment on the literature[END_REF]. Increased skin conductance attests increased sympathetic activity, and inversely. After checking that electrodermal activity exhibited a similar pattern across experimental sessions, we collected the electrodermal response (EDR) for each maximal isometric strength trial. We then calculated the EDR amplitude as the delta separating the sudden increase and the onset of the prolonged and regular return to the pre-stimulus baseline (for further development and experimental illustration, see [START_REF] Vernet-Maury | The ohmic perturbation duration, an original temporal index to quantify electrodermal responses[END_REF][START_REF] Kanthack | Neurophysiological insights on flexibility improvements through motor imagery[END_REF].

EDR amplitude is typically associated with the mental workload elicited by cognitive processing during both physical and mental tasks [START_REF] Collet | Autonomic nervous system correlates in movement observation and motor imagery[END_REF]. Accordingly, we used EDR amplitudes as an objective measure of participants' commitment to a maximal effort during the isometric strength trials.

Statistical analysis Linear mixed effects analysis

We used R (2018) and nlme [START_REF] Pinheiro | Linear and nonlinear mixed effects models[END_REF] to run a linear mixed-effects analysis with a bysubject random intercept of the psychometric, behavioural and physiological dependent variables. For FOLQ scores, we entered DIMENSION (i.e. SKILL, STRATEGY and PERFORMANCE) as fixed effect.

We investigated self-reports of motivation and perceived performance, as well as self-reports of perceived difficulty to complete the cognitive tasks administered during the inter-trial recovery periods (i.e. AO, AO+MI), using CONDITION (i.e. CONTROL, AO, AO+MI) as fixed effect. For EDR amplitude, we built a linear mixed effect model with the fixed effect of CONDITION and TRIAL (with interaction term). For total force, we entered the interaction between CONDITION and TRIAL, iEMGBICEPS BRACHII, iEMGANTERIOR DELTOIDEUS as fixed effects. For all analyses, we included TRIAL as numeric regressor (i.e. ranging from 1 to 10). This procedure enabled qualitative investigation of linear trends [START_REF] Rienzo | Short-term effects of integrated motor imagery practice on muscle activation and force performance[END_REF], 2019;[START_REF] Kanthack | Neurophysiological insights on flexibility improvements through motor imagery[END_REF] for comparable statistical analysis frameworks). We implemented a backward stepwise procedure to fit the random-coefficient regression model formulae [START_REF] Hocking | A Biometrics invited paper. The analysis and selection of variables in linear regression[END_REF][START_REF] Draper | Applied regression analysis[END_REF]. Inspection of the residual plots did not reveal any obvious deviation from the hypotheses of homoscedasticity or normality. We set up the statistical significance threshold for a type 1 error rate of α = 5%. As effect sizes, we reported partial coefficients of determination (Rp 2 ), using the procedure for linear mixed effects models implemented in the r2glmm package [START_REF] Edwards | An R2 Statistic for Fixed Effects in the Linear Mixed Model[END_REF][START_REF] Jaeger | An R2 statistic for fixed effects in the generalized linear mixed model[END_REF]. We finally investigated main effects and interactions using general linear hypotheses testing of planned contrasts from the multcomp package [START_REF] Hothorn | Simultaneous inference in general parametric models[END_REF][START_REF] Bretz | Multiple comparisons using R[END_REF]. We applied Holm's sequential corrections to control the false discovery rate [START_REF] Holm | A simple sequentially rejective multiple test procedure[END_REF].

Power analysis

We carried out power calculations using the pwr package implemented in R [START_REF] Champely | The power of the mind: the cortex as a critical determinant of muscle strength/weakness[END_REF]. Due to the nature of the experimental hypotheses, sample size should afford a reliable statistical power to detect a main effect of the EXPERIMENTAL CONDITION on total force. Considering the repeated measures nature of the design, a sample size of n = 21 yielded a statistical power of p(1-β) = 0.80 to detect small effect sizes, i.e. corresponding to 1-5% of explained variance with a type 1 error rate of 5%. The statistical power was superior to 0.90 for medium and large effect sizes, i.e. corresponding to 10% of explained variation and higher.

Results

Monitoring compliance to the experimental design

Psychometric data FOLQ scores

The linear mixed effects analysis revealed a statistically significant effect of the DIMENSION (χ 2 (2) = 85.66, p < 0.001, Rp 2 = 0.47). FOLQ for PERFORMANCE scores (M ± SE; 2.85 ± 0.25) were lower than those recorded for both STRATEGY (4.33 ± 0.33) and SKILLS (5.41 ± 0.31) dimensions (fitted difference: 1.46 ± 0.28, p < 0.001; 2.57 ± 0.28, p < 0.001; respectively). FOLQ scores for SKILL were also higher than those for STRATEGY (fitted difference: 1.08 ± 0.28, p < 0.001).

Self-reports on Likert-type scales

The linear mixed effects analysis on motivation and perceived performance yielded no CONDITION effect (χ 2 (2) = 0.53, p = 0.77; χ 2 (2) = 1.35, p = 0.51, respectively), as shown by Figure 2. The COGNTIVE TASK affected Likert ratings of perceived difficulty (χ 2 (1) = 3.90, p = 0.04, Rp 2 = 0.06 -Figure 2). Participants rated that it was more difficult to complete AO+MI (4.38 ± 0.51) than AO alone (3.23 ± 0.65, fitted difference: 1.14 ± 0.59, p = 0.04). Finally, vividness ratings after AO+MI were 6.66 ± 1.83 on the 10-point Likert scale, i.e. corresponding to the "Quite good (…)" vividness item (Figure 2). *** Insert Figure 2 about here ***

Skin conductance data

The analysis of EDR amplitude revealed that the CONDITION × TRIAL interaction approached the statistical significance threshold (χ 2 (2): 4.56, p = 0.10, Rp 2 = 0.01). As shown in Figure 3, there was a marginally negative TRIAL effect on EDR amplitudes during AO+MI (fitted estimate: -0.05 μS ± 0.06, p = 0.09). By contrast, there was no TRIAL effect on EDR amplitudes recorded during both AO (p = 0.74) and CONTROL conditions (p = 0.73). The linear mixed effects analysis revealed no main effect of CONDITION (χ 2 (2) = 3.79, p = 0.15) or TRIAL (χ 2 (1) = 1.36, p = 0.24) (Figure 3).

*** Insert Figure 3 about here *** Analysis of the maximal isometric strength

Raw group values used for normalization (see Materials and methods) of the total force, iEMGBICEPS BRACHII and iEMGANTERIOR DELTOIDEUS, are provided in Table 1.

The CONDITION × TRIAL and CONDITION × iEMGBICEPS BRACHII interactions were removed during the backward stepwise model selection (χ 2 (2) = 1.91, p = 0.38; χ 2 (2) = 3.85, p = 0.15, respectively). However, the total force was affected by the TRIAL × iEMGDELTOIDEUS interaction (χ 2 (2) = 7.90, p = 0.02, Rp 2 = 0.01). As shown in Figure 4, there was a positive relationship between iEMGDELTOIDEUS and strength during AO (fitted estimate: 0.09% ± 0.02, p < 0.01), but not during CONTROL (fitted estimate: 0.02% ± 0.02, p = 0.66) nor AO+MI (fitted estimate: 0.00% ± 0.02, p =0.99).

*** Insert Table 1 about here ***

The total force analysis revealed a main effect of TRIAL (χ 2 (1) = 90.42, p < 0.001, Rp 2 = 0.10), iEMGBICEPS BRACHII and iEMGANTERIOR DELTOIDEUS (χ 2 (1) = 53.39, p < 0.001, Rp 2 = 0.12; χ 2 (1) = 3.66, p = 0.05, Rp 2 = 0.001; respectively). As shown in Figure 4, there was a negative relationship between TRIAL and strength (fitted estimate: -0.94% ± 0.01, p < 0.001). Conversely, the relationship between iEMGBICEPS BRACHII, iEMGANTERIOR DELTOIDEUS and the total force was positive (fitted estimate: 0.17% ± 0.02, p < 0.001; fitted estimate: 0.04% ± 0.01, p = 0.03, respectively). There was also a main CONDITION effect (χ 2 (2) = 32.43, p < 0.001, Rp 2 = 0.02). Total force recorded during AO+MI (79.07% ± 19.58) outperformed that recorded during CONTROL (76.47% ± 20.59; fitted difference: 3.68% ± 0.69, p < 0.001). Likewise, the total force recorded during AO (78.72% ± 23.17) outperformed that recorded during CONTROL (2.79% ± 0.75 of fitted difference, p < 0.001). There was no difference between AO+MI and AO (p = 0.44, see Figure 4). *** Insert Figure 4 about here ***

Discussion

The present experiment investigated the short-term effects of AO and AO+MI on maximal isometric strength. In a previous study, we demonstrated the efficacy of MI practice during the intertrial recovery periods [START_REF] Rienzo | Short-term effects of integrated motor imagery practice on muscle activation and force performance[END_REF]. The performance analysis first revealed a positive relationship between the maximal isometric strength and the activation of primary and secondary agonists of the elbow flexion. This corroborates the well-established relationship between isometric strength and the integrated surface electromyogram (e.g. [START_REF] Moritani | Reexamination of the relationship between the surface integrated electromyogram (IEMG) and force of isometric contraction[END_REF]. The biceps brachii activation accounted for 12% of the variance of the maximal isometric strength. This was expected considering the nature of the strength task [START_REF] Serrau | Muscle activation of the elbow flexor and extensor muscles during self-resistance exercises: comparison of unilateral maximal cocontraction and bilateral selfresistance[END_REF]. We also found a positive relationship between the anterior deltoideus activation and strength, which also confirmed its assistance function in the present elbow flexion force task [START_REF] Rienzo | Short-term effects of integrated motor imagery practice on muscle activation and force performance[END_REF]. Repeated maximal isometric strength efforts lead to a corollary increase in the difficulty to recruit motor units [START_REF] Kroll | Isometric fatigue curves under varied intertrial recuperation periods[END_REF][START_REF] Babault | Neuromuscular fatigue development during maximal concentric and isometric knee extensions[END_REF][START_REF] Camic | Electromyographic and mechanomyographic responses across repeated maximal isometric and concentric muscle actions of the leg extensors[END_REF]. A decrease in strength occurred along with trials repetition, yielding to a 9.46% decrease in maximal isometric strength at the session level. This measure is congruent with the 10% of maximal isometric strength decrease reported by Di [START_REF] Rienzo | Short-term effects of integrated motor imagery practice on muscle activation and force performance[END_REF] in a similar paradigm, and attests participant's commitment to a maximal isometric effort.

The nature of the experimental condition did not impact the decrease in strength across trials.

Performing AO and AO+MI during inter-trial recovery periods did not attenuate the performance decrease across trials compared to CONTROL. This argues for the relative independence between the mental load associated with mental training and the accumulation of muscle fatigue (Rozand et al., 2014a(Rozand et al., , 2014b)). However, both AO and AO+MI outperformed CONTROL in terms of maximal isometric strength output across all trials of the session. These results confirm a previous report from Di [START_REF] Rienzo | Short-term effects of integrated motor imagery practice on muscle activation and force performance[END_REF], where MI practice yielded no effect on the trial-to-trial decrease in strength but enhanced the overall strength output by 2.1-3.5% at the session level (i.e. irrespective of the trial number). Here, short-term improvements during AO and AO+MI ranged from 2.8-3.7% compared to CONTROL. Albeit modest, these gains are of practical relevance considering ceiling effects associated with the voluntary repetition of maximal isometric contractions. The literature reported 10-30% improvements in strength after training programs scheduled within a span of several weeks including mental training [START_REF] Ranganathan | From mental power to muscle power--gaining strength by using the mind[END_REF][START_REF] Porro | Enhancement of force after action observation: behavioural and neurophysiological studies[END_REF][START_REF] Lebon | Benefits of motor imagery training on muscle strength[END_REF][START_REF] Yao | Kinesthetic imagery training of forceful muscle contractions increases brain signal and muscle strength[END_REF]. The levels of perceived performance and motivation to engage in the strength paradigm were similar across experimental conditions. Hence, the superiority of AO and AO+MI compared to CONTROL may not account for psychological factors. We also ruled out the effects of increased attentional focus on the dominant upper limb during inter-trial recovery periods. Indeed, the CONTROL condition also involved a visual exposure to upper limb movements. Skin conductance measures confirmed that the sympathetic activity during maximal isometric strength trials was comparable across conditions. We therefore postulate that there is neurophysiological origin to the short-term beneficial effects of AO and AO+MI on maximal isometric strength.

The psycho-neuromuscular theory hypothesizes that mental training improves muscle function by prompting neural excitability within the neural pathways mediating the forthcoming physical performance. This was advanced as a primary underlying mechanism to the beneficial effects of training with AO and MI on strength [START_REF] Porro | Enhancement of force after action observation: behavioural and neurophysiological studies[END_REF][START_REF] Rienzo | Short-term effects of integrated motor imagery practice on muscle activation and force performance[END_REF][START_REF] Grosprêtre | Neural mechanisms of strength increase after one-week motor imagery training[END_REF]. Indeed, AO and AO+MI both involve low-threshold facilitation of the corticospinal pathways controlling the corresponding physical performance [START_REF] Naish | Effects of action observation on corticospinal excitability: Muscle specificity, direction, and timing of the mirror response[END_REF][START_REF] Wright | Combined action observation and imagery facilitates corticospinal excitability[END_REF][START_REF] Wright | Viewing Instructions Accompanying Action Observation Modulate Corticospinal Excitability[END_REF]Wright et al., , 2018)). We predicted corticospinal facilitation during the inter-trial recovery periods during AO and AO+MI, but not during CONTROL. In light of the psycho-neuromuscular theory, we suggest that the preliminary activation of the corticospinal pathways controlling the elbow flexion task during AO and AO+MI facilitated the forthcoming maximal voluntary isometric contractions. We thus posit that a priming effect would explain the short-term impact of AO and AO+MI on strength (for illustrations in AO experiments, see [START_REF] Obhi | Incidental action observation modulates muscle activity[END_REF][START_REF] Salama | Automatic priming of grip force following action observation[END_REF]. Priming refers to "[a] change in behaviour based on previous stimuli" (Stoykov and Madhavan, 2015, p. 33). At a neurophysiological level, priming originates from short-term plasticity yielding increased cortical gain over motor units [START_REF] Stoykov | Motor priming in neurorehabilitation[END_REF][START_REF] Stoykov | Movement-Based Priming: Clinical Applications and Neural Mechanisms[END_REF]. Priming-related plasticity may first facilitate intramuscular coordination, i.e. the capacity of the central nervous system to recruit motor units targeting agonist muscles [START_REF] Fallentin | Motor unit recruitment during prolonged isometric contractions[END_REF][START_REF] Farina | Motor unit recruitment strategies investigated by surface EMG variables[END_REF][START_REF] Taylor | Multiple features of motor-unit activity influence force fluctuations during isometric contractions[END_REF]. Primingrelated plasticity can also facilitate intermuscular coordination. It involves muscle synergies, e.g. between the biceps brachii and anterior deltoideus. Reciprocal inhibition represents another central component of intermuscular coordination [START_REF] Baratta | Muscular coactivation. The role of the antagonist musculature in maintaining knee stability[END_REF][START_REF] Hautier | Influence of fatigue on EMG/force ratio and cocontraction in cycling[END_REF][START_REF] Simoneau | Strength training in old age: adaptation of antagonist muscles at the ankle joint[END_REF]. Although necessary for joint stabilization, residual activity in antagonist muscles can affect the development of strength [START_REF] Hautier | Influence of fatigue on EMG/force ratio and cocontraction in cycling[END_REF].

The strength task involved a primarily quantitative effort. There was thus a potentially confounding factor with regards to participants' uses of AO, which rarely involved achieving physiological arousal goals, as reflected by the low FOLQ performance scores. AO of a bodybuilder could have placed too low demand on the motor system to elicit priming effects. The performance analysis did not confirm this statement since AO outperformed CONTROL. It can be objected that characterizing an effect of AO alone would require a control condition without upper-limb movements, e.g. watching a bodybuilder sitting passively in a chair. Such control condition would be relevant although it would not discriminate the reallocation of attentional resources to the upper-limb, which is in itself sufficient to elicit a facilitation of corticospinal pathways [START_REF] Hiraoka | Immediate effect of visual attention on corticospinal excitability in the upper trapezius muscle[END_REF], from task-specific motor simulation. Also, since we did not independently evaluate the effect of AO and MI it may be objected that the experimental design does not make possible to identify what was driving the effects on performance during AO+MI. Considering the recent evidence demonstrating the beneficial effect of MI alone in a comparable design [START_REF] Rienzo | Short-term effects of integrated motor imagery practice on muscle activation and force performance[END_REF], the absence of difference between AO and AO+MI could indicate that MI has in fact no effect on performance. Rigorously disentangling this issue would require including a MI alone condition in the design. Albeit valid, this limitation is grounded in the theoretical claim that AO+MI represents a form of combination of AO and MI in the sense of summed cognitive processes. Theoretical framework rather emphasize a theory of AO+MI where AO+MI should be considered a unitary form of motor cognition that is relatively independent from AO and MI in the sense of: "(…) quasi-encapsulated sensorimotor streams, which could either merge or compete depending on their contents and potential usefulness for on-going action plans" (Eaves et al., 2016b, p. 4).

An important implication is that AO+MI might leverage distinct neurophysiological mechanisms than the practice of AO or MI alone, specifically with regards to priming effects. AO+MI required a voluntary state of attentional focus on the proprioceptive information associated with biceps brachii and anterior deltoideus contractions, while concomitantly receiving external video cues. Participants were explicitly instructed to rehearse proprioceptive information related to the targeted muscles. This is known to elicit an effector-specific facilitation of the corresponding corticospinal pathways [START_REF] Wright | Combined action observation and imagery facilitates corticospinal excitability[END_REF]. In addition, in the video of the bodybuilder, the contraction of the biceps brachii was prevalent (Figure 1). This possibly placed a greater emphasis on this muscle during AO+MI [START_REF] Calatayud | Importance of mind-muscle connection during progressive resistance training[END_REF]Eaves et al., 2016b). Accordingly, priming effects during AO+MI possibly favoured intramuscular rather than intermuscular coordination. By contrast, AO involved an attentional focus on the whole-limb coordination [START_REF] Flanagan | Action plans used in action observation[END_REF][START_REF] Stefan | Concurrent action observation modulates practice-induced motor memory formation[END_REF][START_REF] Richardson | Movement interference during action observation as emergent coordination[END_REF], thus suggesting that AO primarily primed intermuscular coordination, and more specifically the synergy between the biceps brachii and anterior deltoideus. This postulate is congruent with the positive relationship recorded between maximal isometric strength and the integrated electromyograms from both the biceps brachii and anterior deltoideus during AO, whereas only the integrated electromyogram from the biceps brachii predicted the maximal isometric strength during AO+MI.

Contrary to [START_REF] Scott | Motor imagery during action observation increases eccentric hamstring force: an acute non-physical intervention[END_REF], who emphasized the clinical relevance of AO+MI compared to MI, we did not find any superiority of AO+MI compared to the training condition involving a single method of motor simulation (here AO). The present design examined the performance outcome at the single-session level, and involved the dominant upper-limb. Both aspects represent major differences with the methodological framework adopted by [START_REF] Scott | Motor imagery during action observation increases eccentric hamstring force: an acute non-physical intervention[END_REF]. Also, it is possible that differences between AO+MI and AO would emerge in the case of longer intervention periods targeting the lowerlimbs. Another qualitative aspect that might account for differences in the pattern of results is that [START_REF] Scott | Motor imagery during action observation increases eccentric hamstring force: an acute non-physical intervention[END_REF] trained their participants in the mental rehearsal of a functional task involving the targeted effectors (i.e. Nordic hamstring). By contrast, the mental training intervention embedded within the inter-trial recovery periods in the present study focused on the same task from which we measured the behavioural performance. Overall, the present experimental design was closer to the methodological framework adopted by [START_REF] Smith | The Effects of Combining PETTLEP Imagery and Action Observation on Bicep Strength: A Single-Case Design[END_REF]. These authors reported comparable strength improvements on an elbow flexion task after either AO+MI or MI training (administered in a counterbalanced order across several weeks). We observed comparable results at the single-session level, particularly regarding the benefits of AO+MI (albeit we used an external visual perspective for the AO component of the AO+MI intervention). The present findings provide further original insights to [START_REF] Smith | The Effects of Combining PETTLEP Imagery and Action Observation on Bicep Strength: A Single-Case Design[END_REF]'s conclusions with regards to equivalent gains following AO+MI and MI alone.

If AO+MI represent a unitary form of motor cognition, it is plausible that the effects on strength operate through qualitatively distinct neurophysiological processes than AO or MI alone. Albeit somehow speculative, this could explain why strength gains may not be dissociable from a quantitative analysis of performance. Finally, present strength gains after AO+MI training had reduced magnitude compared to the findings by [START_REF] Scott | Motor imagery during action observation increases eccentric hamstring force: an acute non-physical intervention[END_REF] and [START_REF] Smith | The Effects of Combining PETTLEP Imagery and Action Observation on Bicep Strength: A Single-Case Design[END_REF], i.e. 2.5-3.5% increase compared to the control condition versus ~6% and ~15% increase, respectively. Aside from considerations related to the cortical representation of the somatic effectors targeted by mental training (see [START_REF] Yue | Strength increases from the motor program: comparison of training with maximal voluntary and imagined muscle contractions[END_REF] for pioneering insights), we suggest that the magnitude of strength gains could be primarily related to mental training duration, specifically when the sessions are separately administered from physical training across a span of several weeks. Conceptually, AO might involve MI to some degree, hence making difficult to control for a strict practice of AO during the AO condition [START_REF] Vogt | Multiple roles of motor imagery during action observation[END_REF][START_REF] Scott | Motor imagery during action observation increases eccentric hamstring force: an acute non-physical intervention[END_REF]. At this point, no method can provide objective means to control the absence of MI during AO, perhaps with the exception of fMRI. Unfortunately, fMRI is not adapted to ecological situations and revealed largely overlapping neural networks (e.g., [START_REF] Macuga | Neural representations involved in observed, imagined, and imitated actions are dissociable and hierarchically organized[END_REF]. This limitation is inherent to any research on AO. Yet, an interesting methodological approach to address this issue in future designs would be asking participants to rate from a Likert-type scale their degree of MI experience during AO. Obviously, such scores would not represent objective proof that MI did not occur in the AO condition, but would provide some evidence of the amount of MI during AO. Here, the benefits reported for AO are unlikely to represent the concomitant effects of MI practice, for several reasons. First, participants received clear instructions not to engage MI during AO, and just focus attention on the upper limb of the bodybuilder. Therefore, if some MI occurred, it was probably not an explicit process. On the contrary, participants were explicitly instructed to engage in MI during the AO+MI condition. In other words, if some MI occurred during AO, it remains a different form of motor cognition than the deliberate MI practice instructed during AO+MI. In addition, the results advocate for distinct mechanisms underlying the benefits of AO and AO+MI condition, specifically about increased synergies under the AO condition. Eventually, the fact that participants reported that AO+MI was more difficult than AO potentially evidenced a different cognitive strategy under the two conditions.

The priming effects of AO and AO+MI occurred without triggering awareness of increased efficacy compared to CONTROL, as attested by the Likert ratings. This finding somehow challenges data by Di [START_REF] Rienzo | Short-term effects of integrated motor imagery practice on muscle activation and force performance[END_REF], where the participants perceived a greater strength during MI practice. MI involves a voluntary process of mental simulation, whereas AO emulates the motor system by engaging the mirror neuron system [START_REF] Shepard | Ecological constraints on internal representation: resonant kinematics of perceiving, imagining, thinking, and dreaming[END_REF][START_REF] Vogt | Multiple roles of motor imagery during action observation[END_REF]. In AO+MI, the participant primarily engaged in top-down processing of kinesthetic information since the video alleviates for the need of top-down visual processing by providing relevant afferent visual information. Yet, AO+MI required coordinating an internal and an external state of attentional focus. In addition, AO+MI required synchronising MI with the timing of the video. Overall, this could increase the mental load, presumably eliciting a conflict between the top-down and bottom-up processing of motor information yielding to a negative bias in performance perception. This postulate is congruent with participants' higher perceived difficulty to perform AO+MI compared to AO as well as with the marginally negative relationship between the number of trials and the EDR amplitudes present during the AO+MI condition only. EDR amplitudes are strongly linked with attentional processes, and more specifically the allocation of mental resources to face the cognitive workload of the task [START_REF] Collet | Measuring motor imagery using psychometric, behavioral, and psychophysiological tools[END_REF][START_REF] Collet | Autonomic nervous system correlates in movement observation and motor imagery[END_REF].

Overall, the increased perceived difficulty of AO+MI compared to AO and CONTROL was associated with autonomic nervous system response patterns attesting an increased mental fatigue state.

To the best of our knowledge, this is the first study investigating the short-term effectiveness of mental training including AO on maximal isometric strength. Previous research demonstrated the potential relevance of AO+MI to improve strength over the course of several weeks of training [START_REF] Sun | Improving motor imagery practice with synchronous action observation in stroke patients[END_REF][START_REF] Scott | Motor imagery during action observation increases eccentric hamstring force: an acute non-physical intervention[END_REF][START_REF] Smith | The Effects of Combining PETTLEP Imagery and Action Observation on Bicep Strength: A Single-Case Design[END_REF]. The present study extends these findings and demonstrates that both AO and AO+MI increased strength at the single-session level. Increased training intensity at the session level contributes to enhance the workout load of longer training programs. Considering the importance of training intensity in strength development [START_REF] Kraemer | Strength and power training: physiological mechanisms of adaptation[END_REF][START_REF] Fleck | Designing Resistance Training Programs[END_REF], this may be an underlying process to the benefits of resistance training programs combining physical training with AO and AO+MI [START_REF] Porro | Enhancement of force after action observation: behavioural and neurophysiological studies[END_REF][START_REF] Scott | Motor imagery during action observation increases eccentric hamstring force: an acute non-physical intervention[END_REF]. Interestingly, AO and AO+MI facilitated strength without increasing participants' own performance perception. They even experienced a greater difficulty to perform AO+MI than AO, which was associated with decreased EDR amplitudes across trials. This result has important practical implications. Implementation of AO+MI during training might increase the mental workload for athletes while concomitantly contributing to increase their performances. While we assumed a neurophysiological origin to the beneficial effects of AO and AO+MI on maximal isometric strength through priming effects on corticospinal facilitation, this remains a working hypothesis considering the nature of the study design. Confirming this interpretation of the results would require replicating the design with direct measures of neural excitability within pathways targeting agonist and antagonist muscles. This could be implemented in future designs with transcranial magnetic stimulation and electroencephalographic measures.

(

  Maximal isometric strength trial 1-10) (Best maximal isometric strength during warm-up) × 100

(

  Maximal isometric strength trial 1-10) (Best maximal isometric strength during warm-up) × 100

Figures caption Figure 1 .

 caption1 Figures caption

Figure 2 .

 2 Figure 2. Barplots with 95% confidence intervals (error bars) of self-reports data on the 10-points

Figure 3 .

 3 Figure 3. Linear mixed effects analysis of EDR amplitude. A: EDR amplitude by TRIAL

Figure 4 .

 4 Figure 4. Linear mixed effects analysis of the total force. A: Total force by iEMGANTERIOR

Table 1 .

 1 Group averages and 95 % confidence intervals for raw total force, iEMGBICEPS BRACHII and iEMGANTERIOR DELTOIDEUS corresponding to the best warm-up trial. iEMG: Integrated electromyogram. 95% CI: 95 % confidence interval.

	Total force	iEMGBiceps brachii	iEMGAnterior deltoideus
	Mean	95% CI	Mean	95% CI	Mean	95% CI
	2617.34 mV.s -1	2276.83 -2957.85	7803.91 mV.s -1	6063.21 -9544.61	2548.14 mV.s -1	1546.65 -3549.62

Acknowledgments

This work was conducted within the facilities of the sport sciences of the University Claude Bernard Lyon 1. There was no funding from organizations for this work. The authors declare no competing interests.