Sébastien Villotte
email: sebastien.villotte@cnrs.fr

Activity-Related Skeletal Changes

come

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There is a wide spectrum of opinions regarding the degree of accuracy that can be achieved in studies of past activities

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This entry presents the most commonly studied ARSC, as well as their specific importance and limitations

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2Reconstructing Past Aactivities: AAn Ooverview Brief Hhistorical Bbackground

Pioneer Sstudies

Looking for skeletal changes produced by the repetition of movements made during life in not really a novel approach: it was already a topic in its own right at the end of the 19th century. For example, as early as 1887, Manouvrier suggested that the medio-lateral flattening of the tibia (called platycnemia), regularly observed in prehistoric skeletons, was not an inherited trait (a hypothesis frequently put forward at the time) but more likely an individual variation related to an increased activity of M. tibialis posterior, particularly during running. A few years later, [START_REF] Charles | The influence of function, as exemplified in the morphology of the lower extremity of the Panjabi[END_REF] described in a rather detailed study various appearances of the lower limbs of skeletons from India, in particular the presence of supernumerary articular facets on the talus, and associated them with the squatting position. Similar studies identifying potential ARSC flourished in the late 19th th and early 20th th centuries, and already circumscribed the main behavioral aspects that could be identified with this type of skeletal changes: forceful and/or repetitive movements, frequently adopted positions, and the practice of long-distance running, for example. Studies of ARSC from that time were often anecdotal and contradicted each other. They were also characterized by a direct and rather simplistic association between skeletal changes and specific behaviors, and lacked, for the most part, proper methodology to record the changes. However, those works laid the groundwork for more ""standardized"" studies to emerge.

The Bblooming of Ppaleopathological Sstudies

The revival of ARSC studies, mainly focusing on paleopathological lesions, began in the late 1960s. They were characterized by (1) the analysis of larger samples, (2) more formalized approaches to record the changes, and (3) greater consideration of the archaeological, socio-cultural and environmental contexts of the population studied. These studies also greatly benefited from considerable advances in sports and occupational medicine, as well as from the development of medical imaging. One of the most iconic studies from that period is the work published by Merbs in 1983, entitled ""Patterns of activity-induced pathology in a Canadian Inuit population"". Merbs recorded several degenerative and traumatic lesions (mostly osteoarthritis) in a sample of 91 adult skeletons of Hudson Bay Inuit, the Sadlermiut, a group who remained mostly isolated and kept traditional ways of life until they disappeared due to infectious disease in 1902--1903. The study revealed quite different patterns of lesion distributions and severity in males and females, which were interpreted in the light of archaeological contextual information, including recollections of informants and accounts of meetings with the Sadlermiut prior to their disappearance. Merbs thus linked, in a relatively cautious way, the pattern seen in males to harpoon-throwing and kayak-paddling, and the pattern seen in females with carrying heavy loads and the scraping, softening, cutting and sewing of skins.

Syntheses, Ccriticism, Sseeking Sstandardization and Nnew Rresearch Ddirections

Since the 1980s, many avenues of research have been explored regarding ARSC. Some authors have attempted to classify and/or draw up exhaustive catalogues of the many skeletal changes claimed to be related to past activities (e.g., [START_REF] Capasso | Atlas of Occupational Markers on Human Remains[END_REF]. Other researchers have focused on a particular ARSC in order to define a solid methodological and interpretative framework for study (e.g., [START_REF] Merbs | Spondylolysis and spondylolisthesis: a cost of being an erect biped or a clever adaptation?[END_REF] for spondylolysis or [START_REF] Kennedy | The relationship between auditory exostoses and cold water: a latitudinal analysis[END_REF] for auditory exostoses). On the other hand, a significant number of scholars expressed concerns or sometimes acerbic criticism regarding the reconstruction of past lifestyles from skeletal changes (e.g., [START_REF] Dutour | Activités physiques et squelette humain: le difficile passage de l'actuel au fossile[END_REF][START_REF] Jurmain | Bioarchaeology's holy grail: the reconstruction of activity[END_REF]. These concerns, depending on the author, focused on:

In a more positive way, the last 50 years saw the development of much methodological research as well as the identification of new ARSC. In terms of improvement, the most successful is arguably the study of robusticity of long bones (for a review, see [START_REF] Ruff | Biomechanical analyses of archaeological human skeletons[END_REF]. Many efforts (e.g., [START_REF] Mariotti | Enthesopathies-proposal of a standardized scoring method and applications[END_REF][START_REF] Henderson | Recording specific entheseal changes of fibrocartilaginous entheses: initial tests using the Coimbra method: recording entheseal changes[END_REF] have also been

The low replicability and the anecdotal nature of such studies; -The fact that many changes considered as ARSC can be caused by conditions other than activity (i.e., the lack of specificity); -The influence of many other factors (e.g., age or sex-related hormone levels) that participate, along with biomechanical stresses, in the appearance and development of changes.

made to create reproducible methods to record changes seen at entheses (i.e., tendon and ligament attachment sites).

More recently, new approaches, such as the measurement of the size of entheseal areas (e.g., [START_REF] Nolte | Three-dimensional surface area of the distal biceps enthesis, relationship to body size, sex, age and secular changes in a 20th century American sample: surface area of the biceps enthesis[END_REF][START_REF] Karakostis | New horizons in reconstructing past human behavior: introducing the "Tübingen University Validated Entheses-based Reconstruction of Activity" method[END_REF] or the quantification of trabecular bone quantity and orientation (e.g., [START_REF] Ryan | Unique suites of trabecular bone features characterize locomotor behavior in human and non-human anthropoid primates[END_REF], have been developed. Finally, experimentation on animal models (e.g., [START_REF] Zumwalt | The effect of endurance exercise on the morphology of muscle attachment sites[END_REF][START_REF] Wallace | Experimental evidence that physical activity inhibits osteoarthritis: implications for inferring activity patterns from osteoarthritis in archeological human skeletons[END_REF], studies of identified human skeletal collections (e.g., [START_REF] Villotte | Enthesopathies as occupational stress markers: evidence from the upper limb[END_REF][START_REF] Alves-Cardoso | Exploring "wear and tear" of joints and "muscle function" assumptions in skeletons with known occupation at death[END_REF] and of living humans (e.g., [START_REF] Shaw | Intensity, repetitiveness, and directionality of habitual adolescent mobility patterns influence the tibial diaphysis morphology of athletes[END_REF][START_REF] Macintosh | Prehistoric women's manual labor exceeded that of athletes through the first 5500 years of farming in Central Europe[END_REF] have multiplied in order to carefully test hypothetical skeletal reaction to mechanical stress in controlled settings.

From Pprecise Aactivities to Iintragroup Aactivity Ppatterns

Among researchers who have taken an active interest in ARSC, there is a wide spectrum of opinions regarding the degree of accuracy that can be achieved in such studies of past activities.

Identification of Sspecific Aactivities in a Ggiven Iindividual

At one end of this spectrum are some scientists who consider that it is possible, for a given individual, to identify the practice of a specific activity. This approach is the most spectacular, but in order to remain credible, it must respect a set of strict conditions [START_REF] Dutour | Activités physiques et squelette humain: le difficile passage de l'actuel au fossile[END_REF]: (1) The link between a physical activity and its consequences must be established based on current medical references, (2) the ARSC have to be as specific as possible to this activity, and (3) hypotheses must be compatible with the archaeological context.

While indispensable, the first criterion greatly limits the generalization of this approach. Very few current sports activities can be directly transposed to the past (javelin throwing, horse riding, archery and canoeing being the main examples), and technological changes (e.g., saddles, or bows) have likely generated varying biomechanical stresses over time. Moreover, nowadays very few scholars consider that a given ARSC is specific to only one activity.

Patterns of Llesions and Iidentification of Pprecise Aactivities in a Ggroup

In a variant of the previous approach, specific activities for a given individual are not the focus. Instead, patterns of lesions are associated with specific movements and then to activities known to be practiced in the studied group. This approach was notably applied, among other scholars, by [START_REF] Merbs | Patterns of Activity-Induced Pathology in a Canadian Inuit Population[END_REF] and [START_REF] Hawkey | Activity-induced musculoskeletal stress markers (MSM) and subsistence strategy changes among ancient Hudson Bay Eskimos[END_REF]. According to these authors (1995, p. 325), it requires (1) ""a relatively large sample size with good skeletal preservation"", (2) ""a relatively narrow time span"", (3) ""cultural and genetic isolation"", and (4) ""a limited number of specialized, but known, activities"". Unfortunately, such a sample probably seems more theoretical than realistic for most bioarchaeologists. Moreover, one may question whether it is worthwhile to study ARSC for such well-documented groups.

Gendered Ddivisions of Llabor

Most scholars use ARSC to formulate hypotheses about task division between males and females. They usually do link ARSC to possible activities, but often without putting too much emphasis on this point. The main criticism formulated against this type of study is the potentially narrow and stereotyped view of gender roles in the past. As noted by Robb (1998, p. 363), ""it is common to ascribe hyperdeveloped arm and shoulder muscles in males to weapon use, while the same muscle markings in females are often explained as the result of plant processing or hide preparation." " Geller (2017, p. 162) criticized the strict gender boundaries drawn in such studies, as ""they abide by the law of averages and discount overlap."" Indeed, it is true that authors tend to mention hypothetical gender-related tasks in a somehow simplistic way that suggests hermetic gender roles. However, the main result of these studies is unquestionably the strong evidence of gendered divisions of tasks in many of the analyzed groups. For instance, sexual differences in asymmetrical humeral robusticity are quite clear in many prehistoric groups in Europe, males being on average much more asymmetrical than females. This trend, that indicates a tendency of males (or at least some of them) being more preferentially involved in unimanual tasks than females, is not systematic but remains fairly widespread (e.g., [START_REF] Sparacello | Patterns of humeral asymmetry among Late Pleistocene humans[END_REF][START_REF] Sládek | Past human manipulative behavior in the European holocene as assessed through upper limb asymmetry[END_REF] and supported by the diachronic study of other ARSC [START_REF] Villotte | I sing of arms and of a man…": medial epicondylosis and the sexual division of labour in prehistoric Europe[END_REF].

Activity Ppatterns

Uniquely, the last approach considers that an ARSC can be associated with a wide range of cultural practices and not with a single task. Developed by [START_REF] Robb | The interpretation of skeletal muscle sites: a statistical approach[END_REF], this approach does not consist of identifying any probable or possible activities, but instead seeks to understand the relationships among task divisions within the studied sample. From the study of entheseal changes in an Italian Iron Age sample, [START_REF] Robb | The interpretation of skeletal muscle sites: a statistical approach[END_REF] highlighted relations between the specialization and intensity of activities on one hand, and the age-at-death, sex and associated grave goods of the deceased on the other hand. Remarkably, this analysis shed new light on the social organization in this early urban population, without setting hypotheses about the actual physical activities practiced by the individuals.

3Key Iissues: Types of ARSC --Ppotential and Llimits

Classifications

The ""atlas of occupational markers on human remains"", one of the most complete syntheses in this field of research, reports approximately 140 ARSC [START_REF] Capasso | Atlas of Occupational Markers on Human Remains[END_REF]. In fact, they form an extremely heterogeneous group, both in terms of types of changes and processes behind their development. This great variety implies application of very different methods to record the changes and affects the interpretations and hypotheses formulated, as these are based on very different theoretical frameworks. Consequently, the methodological and interpretative limitations are also very diverse: even if osteoarthritis of the elbow and platycnemia are both potentially relevant to discussions of past activities, the recording of their presence, the statistical analysis of the data and the resulting interpretations are very different, simply because they represent two clearly distinct biological phenomena. Yet scholars in this field tend to downplay or even ignore these fundamental differences, which does not facilitate dialogue dialog between defenders and proponents of ARSC.

Considering this great heterogeneity, it is difficult to offer a classification that encompasses all the variation seen in this group of morphological changes. One can, arbitrarily though, group the ARSC according to:

Based on these criteria, the following sections briefly present some examples of ARSC, as well as their specific importance and limitations.

Skeletal Aadaptation

The ability of an individual skeleton to adapt in response to environmental changes is called plasticity. Since the 1970s, patterns of skeletal changes under various loading regimes have been illustrated by experimental research and studies of athletes (professional or not), and the body of research that supports the concept of "bone functional adaptation" is considerable (for a review, see [START_REF] Ruff | Biomechanical analyses of archaeological human skeletons[END_REF]. Though the processes are much less well-documented, the skeleton can also adapt to stresses by increasing the surfaces of muscular attachment sites, or by the formation of new articular surfaces covered with cartilage, for example.

Cross-Sectional Bone Geometry

Since the origin of the discipline, biological anthropologists have sought quantitative variables that may be considered as direct measures of bone strength, bone rigidity, or muscular activity. Originally, external linear measurements were analyzed, but they were largely replaced by cross-sectional geometric properties. Inspired by beam model theory used by engineers to analyze buildings, this approach consists of analyzing the distribution and quantity of cortical bone at a specific transverse section of a diaphysis in order to characterize long bone diaphysis adaptations to mechanical loading [START_REF] Ruff | Biomechanical analyses of archaeological human skeletons[END_REF]. Many cross-sectional geometric properties were defined (such as the total area or the cortical area) but the polar second moment of area (""J""), considered as an overall measure of robusticity is the main variable analyzed nowadays. The image of a section made perpendicular to the longitudinal axis of the bone can be obtained through several means, the most common one being non-invasive imaging.

Cross-sectional geometric properties of the lower limb bones, especially the tibio-fibular complex, appear highly relevant to address mobility patterns, including the nature of the terrain covered (e.g., [START_REF] Sparacello | Changing mobility patterns at the Pleistocene-Holocene transition: the biomechanics of the lower limb of Italian Gravettian and Mesolithic individuals[END_REF]. Data from the upper limbs, especially the humerus, permit inferences to be made about past workload (by comparing the The type of skeletal response (adaptation versus alteration);

(1) The anatomical area affected (i.e., teeth, joints, muscular attachment sites, bone itself).

(2) robusticity of the lower and the upper limbs, for instance, see Macintoch et al., 2017), and the preferential use of a single upper limb in activities (by studying asymmetry, see for instance [START_REF] Sparacello | Patterns of humeral asymmetry among Late Pleistocene humans[END_REF].

The potential ability to reconstruct behaviors in past human groups based on the analysis of cross-sectional geometric properties is undeniable. The methods are standardized and thus ensure repeatable and reproducible measurements, and the quantitative nature of the variables studied permits use of a large set of statistical tools that considerably facilitate data analyses. However, some limitations need to be mentioned. First of all, diaphyseal adaptions mainly occur during growth (see references in [START_REF] Pearson | The aging of wolff's "law": ontogeny and responses to mechanical loading in cortical bone[END_REF]. In non-human mammals, growing bone responds to low or moderate exercise through significant additions of new bone, whereas this adaptive response is modest in mature bone.

In humans, bone sensitivity to mechanical loading in adults is also lower than in juveniles, though a relatively great sensitivity seems to persist into young adulthood. What is ""recorded"" in bioarchaeological material is thus likely informative about what the individuals did during adolescence --and maybe early adulthood --but not necessarily at their time of death. Cross-sectional geometric properties correlate strongly with the size and shape of the whole body, and their standardization is thus required [START_REF] Ruff | Biomechanical analyses of archaeological human skeletons[END_REF]. Ways of standardizing cross-sectional parameters have a significant impact on the results and need to be considered carefully. Finally, plastic effects do not fully mask neutral information (e.g., ancestry), and this may have an important effect when comparing temporally or geographically dispersed samples.

Kneeling Ffacets

Skeletal remains sometimes display extended surfaces at synovial joints, or supernumerary articular surfaces (e.g., ""squatting facets"" at the ankle) close to them. These features are usually considered as markers of habitual positions.

Facets on the superior surface of the distal end of the metatarsals have been occasionally described, sometimes in association with distal extension of the proximal articular surface on the superior surface of the proximal phalanges, especially for the hallux. These modifications have been considered to be an indicator of skeletal plasticity to repeated kneeling (e.g., [START_REF] Ubelaker | Skeletal evidence for kneeling in prehistoric Ecuador[END_REF][START_REF] Molleson | A method for the study of activity related skeletal morphologies[END_REF]. There is no medical or experimental data to confirm this hypothesis, but these changes seem clearly related to the hyperextension of the toes. Unfortunately, this ARSC can take many forms on the metatarsals (Fig. 1), from a proximal extension of the distal articular surface to a ridge on the superior surface [START_REF] Ubelaker | Skeletal evidence for kneeling in prehistoric Ecuador[END_REF]. This variation in expression, and the fact that these changes have received relatively little attention until now, may explain why there is currently no standardized method to score this ARSC. As for the other markers of skeletal plasticity at synovial joints, further methodological research is required for this -potentially very pertinent --ARSC.

Skeletal Pplasticity at Eentheses: The Eexample of the Hhand

Several authors have looked for a relationship between muscular activity and the size of muscle attachment sites (e.g., [START_REF] Zumwalt | The effect of endurance exercise on the morphology of muscle attachment sites[END_REF][START_REF] Nolte | Three-dimensional surface area of the distal biceps enthesis, relationship to body size, sex, age and secular changes in a 20th century American sample: surface area of the biceps enthesis[END_REF][START_REF] Karakostis | New horizons in reconstructing past human behavior: introducing the "Tübingen University Validated Entheses-based Reconstruction of Activity" method[END_REF], the underlying hypothesis being that surface area and surface complexity should increase through osteogenic activity stimulated by muscular contraction. Most of these studies focused on one or only a few entheses, and they found no clear relation between entheseal size and endurance exercise, muscle mass, or muscle-tendon length. Instead, the size of entheseal surfaces correlates strongly with body size, and moderately with age and birth year [START_REF] Nolte | Three-dimensional surface area of the distal biceps enthesis, relationship to body size, sex, age and secular changes in a 20th century American sample: surface area of the biceps enthesis[END_REF].

Recently, Karakostis and colleagues proposed an original multivariate approach in order to reconstruct muscle synergies (for a review, see [START_REF] Karakostis | New horizons in reconstructing past human behavior: introducing the "Tübingen University Validated Entheses-based Reconstruction of Activity" method[END_REF]. This approach, strongly supported by several animal experimental studies, focuses on the proportions among different entheseal areas of the same skeleton. Their analyses of human remains, which mainly focuses on the entheses of the hand, provided promising results with potential discrimination of individuals with occupations involving high grip force from individuals involved in precise or less strenuous occupational tasks.

The main methodological issue regarding this approach concerns the identification of attachment site boundaries (and thus delineation of entheseal areas) for some individuals and/or some specific attachment sites [START_REF] Nolte | Three-dimensional surface area of the distal biceps enthesis, relationship to body size, sex, age and secular changes in a 20th century American sample: surface area of the biceps enthesis[END_REF][START_REF] Karakostis | New horizons in reconstructing past human behavior: introducing the "Tübingen University Validated Entheses-based Reconstruction of Activity" method[END_REF]. Regarding the interpretation, more data on the physiological processes involved are required, especially in order to better understand when in the lifespan these skeletal adaptations occur. Based on data from non-human mammals this likely happens mostly during skeletal growth, though structures may respond to loads beyond that period (e.g., [START_REF] Gao | Quantitative comparison of soft tissue-bone interface at chondral ligament insertions in the rabbit knee joint[END_REF].

Skeletal Aalterations

Sometimes the tissues cannot adapt to the stresses they have undergone and become altered. When the alteration corresponds to trauma (e.g., bone, articular or dental fracture), the cause is likely a single event and is therefore less relevant for discussing daily life in the past. Microtrauma, produced by repetitive activity over time, is thus considered as more informative [START_REF] Merbs | Patterns of Activity-Induced Pathology in a Canadian Inuit Population[END_REF][START_REF] Dutour | Activités physiques et squelette humain: le difficile passage de l'actuel au fossile[END_REF]. Such skeletal alterations are mainly described in the medical literature at synovial joints and fibrocartilaginous entheses.

Osteoarthritis and Ddegenerative Eenthesopathies

A synovial joint is characterized by a capsule that delimits a space between the bones and which is filled with synovial fluid, and the presence of cartilage (hyaline and calcified) covering the bones. Synovial joints permit free bone movement and greater mobility, with articular cartilage absorbing shock and reducing friction during movement.

Osteoarthritis (OA) is a degenerative condition and the most common joint disease, affecting millions of people worldwide [START_REF] Siôn | [END_REF].

Fibrocartilaginous entheses are, in many ways, very similar to synovial joints. They are mostly found at long bone extremities and are characterized by two mediating layers (uncalcified and calcified fibrocartilage) between tendon/ligament fibers and bone. These two fibrocartilaginous layers are avascular and separated from each other by a regular calcification front, which represents the surface observed on dry bones [START_REF] Benjamin | The skeletal attachment of tendons-tendon "entheses[END_REF]. Age-related degenerative enthesopathies are quite common in healthy individuals (e.g., [START_REF] Bakirci | Entheseal changes in response to age, body mass index, and physical activity: an ultrasound study in healthy people[END_REF].

The main factors evoked for the appearance of OA and degenerative enthesopathy are: (1) aging -that can be defined as the progressive incapacity of the individual to adapt to the variable environmental conditions, and (2) the repetition of minor trauma. The relationship between these factors (and many others, such as hormonal levels or body mass) is not well understood. For a long time the mechanism behind the appearance of such lesions has been ascribed to a ""wear and tear"" hypothesis: skeletal alterations gradually appear through life and become more and more extreme due to the accumulation of wear and tear produced by physical activity. This hypothesis has now been largely abandoned in biomedical science. Heavy work activities and high-impact sports are recognized as risk factors, but many other factors (such as obesity, previous injury, genetic background, etc.) have been identified, age being the most evident. More and more biomedical studies identify at least two phases during adulthood: an early phase where overuse can induce Top: Extension of the distal articular facets of the metatarsals of a right foot. Bottom: Examples of "kneeling facets" on first metatarsals. microtrauma (and minor skeletal changes), and a second phase in adulthood when mechanical factors play only (if any) a secondary role and are marked by a generalized disruption of the soft and hard tissues and the appearance of major changes [START_REF] Siôn | [END_REF][START_REF] Bakirci | Entheseal changes in response to age, body mass index, and physical activity: an ultrasound study in healthy people[END_REF].

There is a plethora of studies of OA and degenerative enthesopathies in the biological anthropological literature, but there is still no clear consensus regarding their reliability as ARSC. One obvious limitation comes from the relationship between this type of ARSC and age, considering the difficulty of assessing accurately (not to mention precisely) age-atdeath in adult skeletal remains. Moreover, these skeletal alterations are generally recorded using qualitative, nonstandardized, methods, that limit the reproducibility of the studies.

Most studies of identified skeletal collections found little evidence that enthesopathies and/or OA occurrences are related to physical activities (e.g., [START_REF] Winburn | Reconsidering osteoarthritis as a skeletal indicator of age at death[END_REF][START_REF] Alves-Cardoso | Exploring "wear and tear" of joints and "muscle function" assumptions in skeletons with known occupation at death[END_REF]. This is likely due to several factors, the main one being that most studies are based on the assumption that major skeletal changes correlate with high biomechanical stresses, whereas these exuberant skeletal alterations are likely ""only"" indicators of an older age (or of a systemic pathological condition, see [START_REF] Villotte | Understanding entheseal changes: definition and life course changes[END_REF]. Another possible limitation may come from the assumption that the presence of OA and degenerative enthesopathies is related to ""normal"" levels of biomechanical stress. This seems improbable and minor changes, in young adult individuals, are more likely indicators of very high strains.

Taking into account these (and many other) limitations, OA and degenerative enthesopathies may nevertheless have the potential to be a good indicator of heavy workload, but more research is required in order to identify the best skeletal locations. Theoretically, skeletal alterations at these sites should follow a pattern (e.g., right side dominance for the upper limb, influence of age) in identified skeletal collections compatible with biomedical data (e.g., [START_REF] Villotte | Enthesopathies as occupational stress markers: evidence from the upper limb[END_REF][START_REF] Villotte | I sing of arms and of a man…": medial epicondylosis and the sexual division of labour in prehistoric Europe[END_REF][START_REF] Alves-Cardoso | Exploring "wear and tear" of joints and "muscle function" assumptions in skeletons with known occupation at death[END_REF]. For the study of archaeological material, it will also be necessary to find alternatives (such as ratios, see [START_REF] Villotte | I sing of arms and of a man…": medial epicondylosis and the sexual division of labour in prehistoric Europe[END_REF] to eliminate the influence of age without having to rely on the estimation of age-at-death.

Stress Ffractures: The Eexample of Sspondylolysis

Stress fractures due to accumulated injuries from repetitive, sub-maximal loads are common overuse injuries in athletes.

Among the many locations where stress fractures can occur, very few are routinely studied in bioarchaeology, the pars interarticularis of vertebral arches being one of them (Fig. 2). Spondylolysis is an osseous defect in this area and typically affects the fourth and fifth lumbar vertebrae [START_REF] Merbs | Spondylolysis and spondylolisthesis: a cost of being an erect biped or a clever adaptation?[END_REF]. The general prevalence in the adult population is estimated between 3% and 6% [START_REF] Fredrickson | The natural history of spondylolysis and spondylolisthesis[END_REF] and, in most cases, a stress fracture etiology seems most likely. Some sports participants (including throwing sports, gymnastics, and rowing) are clearly at risk, with frequencies above 10% in symptomatic and asymptomatic athletes (e.g., [START_REF] Soler | The prevalence of spondylolysis in the Spanish elite athlete[END_REF]. In biological anthropology, high prevalence of spondylolysis has been linked to specific activity patterns and general heavy labor (e.g., [START_REF] Merbs | Patterns of Activity-Induced Pathology in a Canadian Inuit Population[END_REF]. The main limit to determining causation comes from the influence of genetic factors, as inheritance is an identified factor in the development of a pars interarticularis defect [START_REF] Fredrickson | The natural history of spondylolysis and spondylolisthesis[END_REF].

Some family members may have a higher susceptibility to the lesion than others [START_REF] Fredrickson | The natural history of spondylolysis and spondylolisthesis[END_REF], and spondylosis may occur in association with other similar skeletal conditions (i.e., detached pieces of bone), suggesting that those individuals were genetically predisposed [START_REF] Verna | Several infra-cranial non-metric variations in a single neolithic individual: a common genetic underlying predisposition?[END_REF]. One way to avoid this problem would be to compare the frequency of spondylolysis among several sub-samples (according to biological or social factors) from the same population.

Irritation of the Pperiosteum and Aaquatic Aactivities: External Aauditory Eexostoses

External auditory exostosis (EAE) is a progressive pathological condition caused by an irritation of the periosteum in the external auditory canal (Di [START_REF] Bartolomeo | Exostoses of the external auditory canal[END_REF]. Many conditions (e.g., otitis externa, eczema, trauma, infection) can be responsible for this irritation, but contact with cold water is the chief cause. A considerable number of clinical studies demonstrate that individuals regularly exposed to cold water are most predisposed to develop EAE (for a review, see [START_REF] Villotte | External auditory exostoses and prehistoric aquatic resource procurement[END_REF]. Induced reflex vasodilatation occurring in the bony meatus during cold-water activities produces an increase in tension in the periosteum, and repeated stimulation of the periosteum may induce new bone formation [START_REF] Harrison | The relationship of osteomata of the external auditory meatus to swimming[END_REF]. Wind chill appears to be an important factor, mainly on the severity of exostoses.

In biological anthropology EAE are considered to be a reliable marker of aquatic activities [START_REF] Kennedy | The relationship between auditory exostoses and cold water: a latitudinal analysis[END_REF][START_REF] Villotte | External auditory exostoses and prehistoric aquatic resource procurement[END_REF]. Some limitations should be mentioned though. First of all, as for many other ARSC, individual differential susceptibility to stress exists for EAE [START_REF] Harrison | The relationship of osteomata of the external auditory meatus to swimming[END_REF], and the influence of the genetic background has been strongly suggested for some past human groups (namely Neandertals, cf. [START_REF] Trinkaus | External auditory exostoses among western Eurasian late middle and late Pleistocene humans[END_REF]. Between-group comparisons thus should be carried out with caution. Secondly, even if the process behind the appearance of EAE is extremely well-documented, the relative influence of several environmental factors is not fully understood. It has been shown, for instance, that EAE may occur in individuals frequently involved in aquatic activities in relatively warm water, and that exposure to the evaporation of sea mist, without immersion of the head in cold water, may be sufficient to promote EAE (for a review, see [START_REF] Trinkaus | External auditory exostoses among western Eurasian late middle and late Pleistocene humans[END_REF]. Finally, early changes of EAE are sometimes difficult to grade in skeletal remains [START_REF] Villotte | External auditory exostoses and prehistoric aquatic resource procurement[END_REF].

Alteration or Aadaptation? Diffuse Ccortical Iirregularity at Ffibrous Eentheses

In fibrous muscular attachment sites, which occur typically on the diaphysis of long bones, collagen fibers from tendons or ligaments attach to bone directly or via a mediating layer of periosteum [START_REF] Benjamin | The skeletal attachment of tendons-tendon "entheses[END_REF]. The surface of such entheses is usually irregular and poorly delimited on adult skeletal remains. Lesions at fibrous attachment sites are barely mentioned in the medical literature [START_REF] Henderson | Do diseases cause entheseal changes at fibrous entheses?[END_REF], justifying avoidance of the term "enthesopathy" for the entheseal changes frequently seen at these sites.

Diffuse cortical irregularity (sometimes called ""robusticity"") at fibrous entheses has long been considered as the product of bone remodeling stimulated by repeated -normal or vigorous --muscle contractions during adulthood.

Even if this hypothetical relationship seems reasonable, it is based on almost no medical or experimental data (see [START_REF] Villotte | Understanding entheseal changes: definition and life course changes[END_REF]. The correlations between scores of skeletal changes at fibrous entheses and parameters such as age-at-death and body size appear stronger than for fibrocartilaginous attachment sites [START_REF] Weiss | Examining activity patterns and biological confounding factors: differences between fibrocartilaginous and fibrous musculoskeletal stress markers: entheseal changes and activity reconstruction[END_REF][START_REF] Milella | Estimation of adult age-at-death from entheseal robusticity: a test using an identified Italian skeletal collection[END_REF]. There are, however, empirical data suggesting that physical activities may participate in the occurrence of such diffuse cortical irregularity (e.g., [START_REF] Lieverse | Lower limb activity in the Cis-Baikal: entheseal changes among middle holocene Siberian foragers[END_REF][START_REF] Milella | Exploring the relationship between entheseal changes and physical activity: a multivariate study: a Multivariate Study of Entheseal Robusticity[END_REF]. The "wear and tear" hypothesis may be useful for such type of changes, but more histological and experimental analyses are required before considering diffuse cortical irregularity at fibrous entheses as reliable ARSC.

4Summary and Future Ddirections

It seems undeniable that some skeletal morphologies are related to the practice of physical activities. However, the study of these morphologies in past human groups raises many difficulties, in terms of methods, data analysis and interpretation of the results. Moreover, biomechanical stresses are never the unique cause of a given skeletal morphology. They induce changes in interaction with many other factors such as growth and aging processes, hormonal levels, diet, body proportions, and genetic background.

Several avenues of research are nevertheless open. Animal models, dissection of human cadavers and human identified skeletal collections are fundamental sources to establish solid methodological and interpretative bases. Basic anatomical and histological data, too often neglected, must be used wisely to discuss the results obtained. Thus, for instance, the concept of ""bone functional adaptation"" very often evoked for skeletal areas covered with calcified (fibro)cartilage (i.e., synovial articular surfaces and fibrocartilaginous entheses) has to be replaced by new theoretical frameworks, specific to these tissues. As the identification of a precise activity seems particularly difficult (if not impossible) for most ARSC, other approaches should be explored, notably the statistical identification of subgroups of individuals, without seeking to specify which activities were practiced.

Fig. 1

 1 Fig. 1

Fig. 2

 2 Fig. 2

Acknowledgements

I would like to thank Chris Knüsel, Vito Saparacello, Fred Santos, Charlotte Henderson, Val Mariotti, Caroline Polet, Marco Milella, Alexandros Karakostis, Erik Trinkaus, Cynthia Wilczak, France Alves-Cardoso, Doris Pany-Kucera, Sandra Assis, Bob Jurmain, Geneviève Lopreno, Nivien Speith, and Sirpa Niinimäki for useful discussions on this topic. Thanks to Efthymia Nikita for inviting me to write this entry. I am grateful to Christopher Knüsel for editing the English and for the clarification regarding the term "activity-related changes". This work was partly funded by Centerre National de la Recherche Scientifique (International Emerging Actions), IEA00540.


Abdul Kader
email: abdul-kader.el-haj@univ-nantes.fr

El Haj

Abdul-Hamid Soubra
email: abed.soubra@univ-nantes.fr

Improved Kriging-Based Approach for the Probabilistic Analysis of a Large Diameter Monopile in a Spatially Varying Soil

Keywords: Probabilistic analysis, monopile foundation, spatial variability, failure probability, Kriging

In this paper, a probabilistic analysis of an offshore monopile foundation embedded in a spatially varying clayey soil was performed. An efficient Kriging-based probabilistic approach using a multipoint enrichment was adopted. The aim is to compute the failure probability against exceeding a threshold value on the monopile head rotation. The proposed probabilistic approach was shown to significantly reduce the computation time with respect to the classical Kriging-based approach. Some probabilistic numerical results are presented and discussed.

Introduction

Recently, a Kriging-based probabilistic technique was proposed by [START_REF] Al-Bittar | Kriging-based reliability analysis of strip footings resting on spatially varying soils[END_REF] to compute the small values of the failure probability of geotechnical structures involving spatially varying soils. This technique (called AK-MCS) is an Active learning method combining Kriging with Monte Carlo Simulation MCS method. Other active learning methods may be found in literature as the active learning method by [START_REF] Huang | Assessing small failure probabilities by AK-SS: An active learning method combining Kriging and Subset Simulation[END_REF] that combines Kriging and Subset Simulation and the global sensitivity analysis-enhanced surrogate (GSAS) modeling suggested by [START_REF] Hu | Global sensitivity analysis-enhanced surrogate (GSAS) modeling for reliability analysis[END_REF]. Although the AK-MCS method has been proved to require a reduced number of calls to the mechanical model with respect to the variance reduction techniques such as Subset Simulation SS and Asymptotic Sampling AS, it remains time-consuming in the case of costly-to-evaluate mechanical models such as the three-dimensional mechanical model considered in this paper. In order to reduce the computation time, an improved probabilistic model is suggested in this paper. This model is an extension of the AK-MCS approach in which a multipoint enrichment technique is used. It is called AK-MCSm (where m stands for multi-enrichment). A relevant clustering technique [see [START_REF] Lelièvre | AK-MCSi : A Kriging-based method to deal with small failure probabilities and time-consuming models[END_REF]] was adopted. This technique allows one to consider a set of training samples that ensures a suitable coverage of the limit state surface.

In this paper, one performs a probabilistic analysis at the ultimate limit state (ULS) of a monopile foundation embedded in a spatially varying soil making use of the relevant multi-enrichment approach AK-MCSm. The aim is to compute the failure probability against exceeding a threshold value on the monopile head rotation.

Soil-Monopile Mechanical Model

The mechanical model of the 3D soil-monopile system has been carried out based on numerical simulations using the commercial finite element software Abaqus ( 2016). An open-ended steel monopile of diameter D=4m was studied in this paper. The monopile of 0.05 m thickness and an embedment depth L of 24 m was extended of 1.0 m above the seabed to avoid the soil from going over the monopile. The steel monopile has a density of 7840

. It was assumed to behave as a linear elastic material with Young modulus of 210 GPa and Poisson ratio of 0.3.

The soil consists of an undrained normally consolidated clay. It was supposed to follow an elastic-perfectly plastic model based on Tresca criterion. This model is defined by the undrained cohesion , the undrained Young modulus ( ) and the undrained Poisson ratio ( ). In this paper, the soil was assumed to have a submerged unit weight of 7 and an undrained Poisson ratio of 0.495. The undrained cohesion was supposed to linearly vary with depth as follows:

(1) where is the effective vertical overburden stress. This represents an increase in from its initial value at mudline (taken here equal to ) at a rate of about . These adopted values are typical for a normally consolidated offshore clay [cf. [START_REF] Dean | Offshore Geotechnical Engineering: Principles and Practice[END_REF]]. Notice that the soil undrained Young modulus was assumed to be linearly related to the soil undrained cohesion via the relationship where is a correlation factor that is dependent on the clay plasticity index and the over-consolidation ratio OCR [cf. USACE (1990)]. Thus, the soil Young modulus was also supposed to linearly vary with depth. For a plasticity index and an over-consolidation ratio of 1, a value of is adopted in this work [cf. USACE (1990)]. As may be seen from Figure 1(a), the numerical model has a length of 12D, a width of 6D and a height of 1.6 L. Details on the numerical modeling of the soil-monopile system are not provided herein. The reader may refer to El Haj et al. (2019).

In this paper, the monopile was considered to be subjected to a horizontal load acting at a height =38.6m above the seabed level. This results in an additional moment at mudline of . A vertical load of 2 MN that represents the structure weight was also considered in the analysis. The applied loadings adopted in this study induced a head rotation at mudline (as computed by Abaqus software) of nearly .

Concerning the determination of the ultimate rotation at the monopile head, a load-controlled procedure was used. The corresponding moment-rotation curve is shown in Figure 1(b). As it may be seen from this figure, the ultimate rotation was determined by the tangent intersection method (point A) leading to a limit rotation of and a corresponding ultimate moment capacity at mudline of . This corresponds to an ultimate horizontal force of . The obtained ultimate value of the monopile head rotation will be used afterwards in this paper as a threshold value when handling the probabilistic analysis.

Probabilistic Model

The soil undrained cohesion was considered as a log-normal random field with a constant coefficient of variation of 25%. The mean values of the soil undrained cohesion are those of the deterministic analysis provided in the preceding section. Concerning the autocorrelation function, a square exponential function was used in this paper. The soil undrained Young modulus was implicitly considered as a random field having the same distribution as the soil undrained cohesion since it was assumed to be linearly related to the soil cohesion.

Notice that the discretization of the cohesion random field was performed using EOLE method proposed by [START_REF] Li | Optimal discretization of random fields[END_REF]. Notice also that the discretization of a random field by EOLE leads to an expression that provides the value of this random field at each point of the soil mass as a function of standard Gaussian random variables (this number is equal to the number of eigenmodes). Notice finally that the realizations of the Young modulus random field can be easily obtained from the realizations of the cohesion random field by multiplying the values of the soil cohesion by 500.

The probabilistic analysis at ULS aims at computing the failure probability against exceeding a threshold value on the ultimate rotation of the monopile head. The performance function is given by:

(2) where is the ultimate rotation of the monopile head as was determined before using the mean values of and (

) and is the monopile head rotation under the applied loading for typical realizations of and . The computation time of for each simulation via Abaqus software was equal to about one hour.

The AK-MCSm probabilistic method used in this paper is based on Kriging metamodeling. Notice that the Kriging consists in constructing a meta-model based on a few number of samples (called Design of Experiment DoE) computed using the mechanical model. The predicted response at an unknown sample (based on the constructed Kriging meta-model) is a Gaussian random variate characterized by a mean prediction value and a corresponding prediction variance. The construction of a Kriging metamodel can be easily performed using DACE (Design and Analysis of Computer Experiments) toolbox in Matlab. For more details, the reader may refer to [START_REF] Lophaven | DACE: A Matlab Kriging Toolbox Version 2.0[END_REF]. The present probabilistic AK-MCSm procedure consists of two main stages. These two stages may be described as follows:

Construction of a preliminary Kriging metamodel

In this stage, one first generates a MCS population of samples . Each sample is a vector of M standard Gaussian random variables where M is the number of random variables required by EOLE methodology to accurately discretize the random field. Secondly, a small design of experiment DoE (taken equal to 15 samples) is randomly selected from the generated population. Each sample of the DoE is transformed (using EOLE) into a realization of and a corresponding realization of using . These realizations are used as inputs for the mechanical model while computing the sample system response (i.e. monopile head rotation ) and the corresponding performance function value.

By using the DACE toolbox [cf. [START_REF] Lophaven | DACE: A Matlab Kriging Toolbox Version 2.0[END_REF]], an approximate Kriging meta-model may be constructed in the standard space of random variables on the basis of the DoE and the corresponding performance function values. This meta-model may be used to compute the MCS failure probability and its corresponding coefficient of variation . It should be noted that the value of and the corresponding value of computed so far are not sufficiently accurate. This is because of the very small number of samples (i.e. small size of the DoE) used to construct the present preliminary Kriging metamodel. Thus, an enrichment process is needed.

Enrichment process

The enrichment process is done via an active learning technique. The learning phase stops once the metamodel becomes sufficiently accurate for the computation of the failure probability, which is indicated by a stopping criterion. The aim of the next two subsections is to present the way of selection of the new training samples during the enrichment process and the adopted stopping criterion.

Selection of new training samples

The enrichment process of the AK-MCS method is performed using a learning function U defined by the following equation [see [START_REF] Echard | AK-MCS: An active learning reliability method combining Kriging and Monte Carlo Simulation[END_REF]]:

(3) where is the square root of the Kriging prediction variance. The sample that has the minimum value of U is selected for the enrichment since it is considered to have the highest probability of being misclassified (i.e. the highest probability to have a wrong performance function sign) as was stated by [START_REF] Echard | AK-MCS: An active learning reliability method combining Kriging and Monte Carlo Simulation[END_REF]. It should be remembered that AK-MCS method involves a single sample per iteration of the enrichment process. In order to overcome this shortcoming, a multipoint enrichment procedure is adopted in this paper. [START_REF] Lelièvre | AK-MCSi : A Kriging-based method to deal with small failure probabilities and time-consuming models[END_REF] proposed a clustering technique, named K-weighted-means clustering algorithm (K-wmeans), that takes benefit of the information provided by the AK-MCS learning function. The K-w-means technique consists in replacing the mean of each cluster by a weighted one making use of the learning function U. In this way, each sample will be weighted by the corresponding uncertainty of being misclassified (i.e. the uncertainty to have a wrong sign of its performance function) and thus, the obtained centroids of the different clusters will be the optimal ones for the enrichment. In other words, the selected samples will be situated in the highly uncertain zone (i.e. the zone corresponding to a high uncertainty on the sign of the performance function) all along the limit state surface leading to an efficient multipoint enrichment of the Kriging meta-model. This approach is used in this paper. For more details, the reader may refer to El Haj and Soubra (2019).

Stopping condition

In AK-MCS method, the stopping condition is defined from the perspective of individual responses (not the quantity of interest ), which may lead to some unnecessary extra evaluations of the mechanical model. A more relevant stopping condition that is based on the convergence of the quantity of interest (i.e. ) was proposed by [START_REF] Schöbi | Rare event estimation using Polynomial-Chaos Kriging[END_REF]. This criterion was used in this paper in the aim to reduce the computation time. Indeed, the adopted criterion relies on the convergence of the failure probability, which could be attained before reaching the stopping condition indicated by AK-MCS. [START_REF] Schöbi | Rare event estimation using Polynomial-Chaos Kriging[END_REF] define a limit state margin characterized by upper and lower boundaries of the limit state surface that takes into account the prediction uncertainty in the Kriging metamodel. These authors stated that when these boundaries become close to each other, a thin limit state margin is obtained and thus, the estimated failure probability can be considered as accurate. The proposed stopping criterion is given as follows:

(4) where is the original failure probability based on the Kriging prediction values and, and are respectively the upper and lower boundaries of the failure probability defined as follows:

(5) (6) where and are respectively the upper and lower boundaries of the limit state surface defined by , is a constant ( in this paper) that sets the confidence level equal to 2 = and is a given tolerance taken as in this paper.

Probabilistic Numerical Results

A reference case where the vertical autocorrelation distance was taken equal to 2m was considered, the soil being assumed to be homogeneous in the horizontal direction. The number of random variables required by EOLE to accurately discretize the random field with a small variance of error (<5%) was found equal to 20. This number of random variables was adopted in the present paper. 1 presents the results of AK-MCS approach and those of the proposed AK-MCSm method. As may be seen from this table, the two approaches result in the same value of the failure probability. Concerning the computation cost, the proposed method leads to a significant reduction in the computation time as compared to AK-MCS (7.28 days for AK-MCSm instead of 27.89 days for AK-MCS).

4.1 Effect of the autocorrelation distances , and on Figure 2 presents the evolution of the failure probability with , and where and are the horizontal autocorrelation distances in the directions orthogonal and parallel to the loading plane respectively and is the vertical autocorrelation distance. The aim of this figure is to identify (for the present embedded length and diameter of the monopile) the values of the autocorrelation distances for which the soil spatial variability has a significant effect on the computed failure probability.

For all the three curves of Figure 2, the failure probability increases with the different autocorrelation distances , or . The rates of increase of the failure probability largely decrease beyond given values of the autocorrelation distances [i.e. beyond an autocorrelation distance of 24m (»L) for , 10m (»2.5D) for and 6m (»1.5D) for to attain a constant value of the failure probability corresponding to the case of a homogeneous soil (i.e. with no spatial variability). The failure probability corresponding to the homogeneous case was found equal to 0.12. It should be emphasized here that all the three identified distances (i.e., L, 2.5D and 1.5D) are directly related to the soil displacement field obtained due to the ultimate loading (i.e. they are related to the mechanics of the problem and not connected to the input probabilistic data). Indeed, the deformed soil at the ultimate limit state (i.e. at the ultimate rotation) is concentrated along the embedded length L of the monopile in the vertical direction, and it is limited to 2.5D and 1.5D in the y-and x-horizontal directions as it may be seen from Figures 3(a) and 3(b), respectively. These observations explain the reason why the increase in the autocorrelation distances , and beyond the distances 1.5D, 2.5D and L respectively does not significantly change the value of the failure probability. A reduction in the failure probability with respect to the case of a homogeneous soil may be obtained if at least one of the autocorrelation distances , or is smaller than its corresponding limit value (i.e. 1.5D, 2.5D, L) since the monopile head rotation will be affected by the soil spatial variability in this case, thus inducing a change in the failure probability. Figure 2 shows that the rate of increase in the failure probability (for the small and moderate values of the autocorrelation distances) is the highest when considering the horizontal autocorrelation distance . This rate of increase in the failure probability is smaller for the horizontal autocorrelation length and it becomes the smallest one for the vertical autocorrelation distance

These observations indicate that a significant reduction in the failure probability with respect to the one corresponding to the homogeneous case, may be obtained by first considering the spatial variability in the vertical direction; the spatial variability in the horizontal direction(s) although important could be considered in a second stage. This result is interesting especially that the identification of the soil spatial variability in the vertical direction is easier and cheaper (e.g. by using a CPT test).

Conclusions

A probabilistic analysis at the ultimate limit state (ULS) was performed in this paper for an offshore monopile foundation embedded in a spatially varying clayey soil. A Kriging-based probabilistic approach using a multipoint enrichment technique was adopted. An improved clustering technique proposed by [START_REF] Lelièvre | AK-MCSi : A Kriging-based method to deal with small failure probabilities and time-consuming models[END_REF] was used for learning. Also, this paper makes use of a relevant stopping condition recently proposed by [START_REF] Schöbi | Rare event estimation using Polynomial-Chaos Kriging[END_REF]. The method was shown to be efficient in terms of the computation time with respect to AK-MCS approach. A reduction in the failure probability with respect to the case of a homogeneous soil may be obtained if at least one of the autocorrelation distances ax, ay or az is smaller than its corresponding limit value (i.e. 1.5D, 2.5D, L).

Figure 1 .

 1 Figure 1. (a) Three-dimensional soil-monopile numerical model (b) Moment-rotation curve of the monopile at mud-line level.

Figure 2 .

 2 Figure 2. Effect of the autocorrelation distances , and on .

Figure 3 .

 3 Figure 3. Deformed soil at the ultimate limit state (a) in the yz plane (b) in the xy plane.

Table 1 .

 1 Results of AK-MCS and AK-MCSm approaches.

	Method		Number of added samples	Time (days)
	AK-MCS	1.274	447	27.89
	AK-MCSm (4 clusters)	1.274	520	7.28
	Table

Acknowledgments

This work was carried out within the framework of the WEAMEC, WEst Atlantic Marine Energy Community, and with funding from the CARENE, Communauté d'Agglomération de la Région Nazairienne et de l'Estuaire.


Gabriela Herrera Altamira
email: gabriela.herrera-altamira@loria.fr

Nathalie Skiba
email: nathalie@openedge.cc

Anatole Lécuyer

Laurent Bougrain

Stéphanie Fleck

Gabriela Herrera

Multisensory neurofeedback design for KMI embodiment

Keywords: Human-centered computing → User centered design; Haptic devices Post-stroke rehabilitation, Multisensory feedback, Brain-Computer Interaction

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

INTRODUCTION

In the context of motor rehabilitation after a stroke, kinesthetic motor imagery (KMI) can be used to stimulate patients' brain plasticity. KMI consists of imagining a movement without performing it by reactivating the haptic sensations experienced during a real movement (e.g., tactile, proprioceptive, and kinesthetic). Consequently, KMIs are difficult to perform and learn. Indeed, unlike real movement, people do not have any sensory feedback during KMI.

As a result, they cannot know if they have actually performed KMI, know their performance, or find the most efficient way to perform the task. Some brain-computer interfaces (BCI), which allow a user to control a computer using their brain's electrical signals, use KMI as an interaction modality. Therefore, and in line with [START_REF] Cornelio | Multisensory Integration as per Technological Advances: A Review[END_REF][START_REF] Mayer | Cognitive theory of multimedia learning[END_REF], we thought coupling a BCI with sensory interactions could overcome the KMI learning barriers and thus provide an alternative to traditional rehabilitation therapies This paper describes the interrelated phases that led to the design of a BCI solution that provides the user with three sensory neurofeedback (NFB) modalities: visual, kinesthetic, and vibrotactile, in order to support KMI learning and ultimately support brain stimulation. Furthermore, it underlines the importance of the human-centered approach throughout the design process.

DESIGNING MULTIMODAL SENSORY-NEUROFEEDBACK

To develop solutions to real-world problems, we implemented an interdisciplinary Design-Based Research (DBR) approach, a methodology used in educational research and HCI that emphasizes iterative cycles of design, implementation, and evaluation of the interventions. In addition, all along the design, a participatory approach was implemented to define the problems and orient the design choices as closely as possible to human realities.

Problem-setting: the central rule of all users

First, we chose to involve potential users, therapists, and post-stroke patients from the exploratory phases. Most previous studies rarely involve patients in the early design stages. Instead, practitioners are often solicited as intermediaries of the patient's needs. As a result, the reality of patients is not fully accessible, and the needs of professionals are rarely taken into account. In order to know and understand the realities of both sides and to adjust the design choices, we specifically developed and implemented a participatory approach involving ten caregivers from different specialties (physicians, occupational and physical therapists), 12 patients, as well as researchers and engineers. By combining gamified workshops, card-based focus groups, and pre-tests with explanatory supports, we sought to facilitate cross-views and authentic expression of the various participants. These participatory methods allowed the patients to express themselves as individuals, beyond their pathology, and the caregivers to share their feelings and expectations. Priceless information such as the stress generated by sounds for

Design choices: multimodality and multisensoriality

The ongoing second phase of DBR corresponds to the interfaces design cycles that have resulted in a multimodal KMI-based BCI (see Figure 1) within which three sensory NFB modalities: visual, kinesthetic, and vibrotactile, are offered in a modular way.

-Visual stimulations Thanks to the Grasp-IT VR game, the visual stimulation is involved in both input and output interactions. Its design mobilizes three theoretical principles. The first one is based on the low of parsimony (or Occam's razor) to limit the cognitive load. The second one is based on the influence of visual affordances. Thus, the visual scene invites the user to imagine, for example, that he/she is squeezing a ketchup bottle. The hypothesis is that this simple task, evoked by a familiar object, is easier to construct mentally than without it. The third is gamification, which is known to increase emotional reinforcement and motivation. Three forms of performance feedback are visually given: score, gauge, and hand movements and their consequences on the object (e.g., the ketchup flows proportionally).

-Kinesthetic stimulation This second stimulation aims to provide a kinesthetic consequence to an imagined movement. To do this, Sense-IT (see figure 1) simulates the mechanical deformation of an object held by the person to provide a more rewarding and realistic experience. The device is akin to an actuated deformable interface designed to make tangible (i.e., perceptible by senses and carrying meaning) the effect of a specific hand gesture on objects with different mechanical properties and/or shapes. The artifact deformations are created by small balloons placed inside an empty joystick. Pumps are used to inflate and deflate the balloons, inducing the movement of the user's passive fingers.

Figure 1: Grasp-IT Xmod BCI consists of 4 elements: An EEG system to record the electrical brain activity; the Grasp-IT VR game: a gamified virtual environment that affords the tasks to execute and gives visual feedback about the KMI performance; Sense-IT: an actuated, tangible interface designed to render a kinesthetic sense of a hand gesture on virtual objects (e.g., squeezing a ketchup bottle); Feel-IT: A vibrotactile device consisting of 3 vibration motors (VM) that stimulate the skin on top of some of the key muscles involved during hand movement. -Vibrotactile stimulation The third modality aims to provide the patient with a more embodied KMI experience. Thus, we designed a vibrotactile interface called Feel-IT. It consists of three 10mm ERM vibration motors held in dedicated 3D-printed cases and fixed with adjustable bracelets. The vibration motors do not stimulate the muscles but the Pacinian corpuscles on the skin. Two of them are placed on the main forearm muscles involved in a grasping hand movement so that the stimulation coincides with the natural muscular activation. These muscles are the flexor digitorum superficialis and the extensor digitorum. The third motor was placed on the hand, aligned with the third metacarpal.

The challenge of the vibrotactile feedback design

Each solution is designed to be used separately and coupled to increase the possibilities of embodiment and individualization. However, due to the lack of previous studies on vibrotactile NFB, many challenges had to be overcome, which required a new participatory design step.

-Study 1: Definition of the vibrotactile sensory threshold. To ensure a reliable and positive experience, we determined each motor's minimum sensory threshold (MST) and the uncomfortable threshold (UT). We recruited 40 subjects with no neurological disorders and with hand and arm sensibility, 18 women and 22 men within three age groups (under 40 years of age, n=16; over 60, n=12; and intermediate, n=12). Participants were asked to indicate when they felt the vibration by pressing a button under two conditions (1) increasing intensity, (2) decreasing intensity, counterbalanced (one vibration every 2 to 4 seconds, in 2 to 5% increments). The order of the three motors was randomized. For the UT, participants indicated each time the vibration was uncomfortable or annoying. A significant difference (repeated measures ANOVA) of minimum sensory feeling at the forearm level was noted between the participants younger than 40 and the other age groups. This difference is not observed at the hand level. Furthermore, the older the individuals the more heterogeneity in terms of sensitivity level increase (Fig. 2.a). In addition, the forearm and hand sensitivity significantly differed regardless of age (Fig. 2.b). The distribution of mechanoreceptors on the hand may explain these results. We did not observe a significant difference for UT, with most participants reaching the maximum vibration intensity without complaint.

-Study 2: Vibration patterns coherence. The perceptual coherence when coupling vibrotactile and visual stimuli was also assessed. Through a user test (N=18), we questioned (1) which activation sequence to favor, whether the motors vibrate synchronously (Simultaneous Condition) or in a sequence inspired by the natural activation of the muscles during grasping (Sequential Condition), and with a 2 or 3-motor configuration; (2) the consistency between tactile and visual feedback (fidelity, reliability, and synchronicity); [START_REF] Mayer | Cognitive theory of multimedia learning[END_REF] to what extent the stimulation was perceived as comfortable. Participants reported that regardless of the number of motors and activation patterns, the vibration was comfortable and synchronized with the animation on the screen. Furthermore, its intensity was judged proportional to the visual feedback displayed. 11 participants considered the 3-motor configuration more consistent and better synchronized with the visual animation. Thus, we kept this configuration, as well as the sequential pattern because it was preferred in 3 of the 4 items assessed.

FUTURE WORK

Currently, we are performing tests with the functional BCI to evaluate the unimodal (visual or vibrotactile) and bimodal NFB modalities. At the moment of publication of this paper, 38 participants (20 men, mean age: 32.5, s.d.: 10.78) have participated in our study. Preliminary results show that 71% of the participants prefer bimodal feedback. Further analysis of UX questionnaires and electroencephalographic signals will allow us to explain this choice better. Moreover, a third cycle of participatory assessment is currently running for the kinesthetic NFB, posing specific design and use problems. Finally, we will assess the relevance of combining the third NFB.

Figure 2 :

 2 Figure 2: Box plots of the minimum sensory thresholds (a) for each age group and vibration motor, (b) for each vibration motor grouped by age group. (*p<0.05, **p<0.01, ***p<0.001), RPM stands for revolutions per minute.

Table 1 :

 1 Summary results of the exploration phase

	Dimension Needs	Design choices
		Remain close to classical rehabilitation practices	KMI targeted: Grasp, Pinch, Open hand
	Therapeutic	Limit stress and spasticity (abnormal increase in muscle tone)	no sound and soft haptic stimuli
		Stimulate cerebral plasticity	facilitate KMI-based brain-computer interactions
		Limit the mental load	Uncluttered visual interfaces, without verbal instruction & task affordance
	Pedagogical	Support motivation Support engagement	Gamification training program & rounded in daily life activities Making the patient an actor & situational affordance
		Develop KMI skills	Scaffold the information embodiment through multisensorial NFB
	Ergonomic	Increase caregiver efficiency & reduce patient fatigue Maintain the caregiver-patient relationship	Easy-to-use and to-implement solutions Training program customizable by the caregiver and the patient
	some patients, the vigilance concerning, e.g., the patients' hetero-
	geneity (e.g., age), cognitive overload risk, hemineglect syndrome,
	or spasticity, but also the patients' desire to make choices in their
	rehabilitation and to feel capable, and the professionals' desire to
	take better care of the patients individually, oriented our design
	choices (see summary in the Table 1) and complete state of the art
	(see [2].)


1. Understand the brain activity due to visual and VT stimuli before using it as feedback for KMI-based BCI. 2. Define the VT configuration that better corresponds to the virtual environment and offers the best user experience.

• Number of motors • Activation pattern 3. Ensure the vibrotactile stimulation is coherent and synchronized with the visual stimulation. 4. Study the effects of the vibration on the EEG signals.

configurations to select the one that best matched a virtual grasping movement. (COERLE, approval number: 2022-17). Vibration was applied on the left arm and the virtual hand shown was the left one.

User Test and EEG Insights of Bimodal, Visual and Vibrotactile, Stimulation

Gabriela Herrera Altamira1 , Stéphanie Fleck2 , Anatole Lécuyer3 , Laurent Bougrain 1

• Kinesthetic motor imagery (KMI) is a mental task often used in Brain-Computer Interfaces (BCI). • KMI is a hard task lacking kinesthetic feedback. Vibrotactile (VT) feedback may help the user and support MI [1], but the required intensity and activation sequence choices remain vague. • VT stimulation elicits electroencephalography (EEG) modulations [2], yet, it is unclear whether the EEG activity depends on the vibration intensity and activation pattern.

• We assessed the effects on EEG signals of coupling VT and visual stimuli and its associated perceptual coherence.

• We did not observe significant differences in the EEG activity among the VT configurations, thus we may choose the configuration preferred by the users: the 3-motor sequential.

• Contralateral occipital ERDs in alpha may be due to action observation of the left virtual arm.

• Delta activity may be an indicator of attention increase due to the vibration [2]. As we only saw it in Medium and High conditions, attention may depend on the vibration intensity.

• These results may favor giving delayed feedback after KMI so that the BCI may differentiate the KMI-related brain activity from the stimuli-related one.

• The next step is to evaluate visual, VT, and bimodal feedback after the KMI, its effects on the EEG, accuracy, and user experience.

INTRODUCTION OBJECTIVES

METHODS

RESULTS

DISCUSSION

Experimental Procedure Offline Analysis

Information recorded: EEG (64 channels), users evaluations for each configuration.

We used EEGlab v2022.1 to process EEG signals. We used its statistics study tool to perform statistics with FDR correction and p<0.01 for the Event-Related Spectral Perturbations (ERSPs) topographical analyses. We removed one participant due to bad acquisition.

Activity in alpha and beta

Activity in delta

User evaluation

Poster 12

•

  Bilateral ERDs are elicited by visual and VT stimulation in alpha and beta. • Contralateral ERDs in alpha+low beta are present in the 4 levels while the ipsilateral ERD was present only in Medium and High conditions. • Contralateral occipital ERDs are present mainly in alpha+low beta. A transient ERS was present in delta (1-4 Hz) activity during the Medium and High stimuli in central areas only at the beginning of the stimulation. W. Zhang, A. Song, H. Zeng, B. Xu, and M. Miao, "Closed-Loop Phase-Dependent Vibration Stimulation Improves Motor Imagery-Based Brain-Computer Interface Performance," Front Neurosci, vol. 15, no. January, pp. 1-13, 2021, doi: 10.3389/fnins.2021.638638. [2] W. Li, Q. Xu, Y. Li, C. Li, F. Wu, and L. Ji, "EEG characteristics in"eyes-open" versus "eyesclosed" condition during vibrotactile stimulation," Biomedical Signal Processing and Control, vol. 68, p. 102759, 7 2021.

  Fig 2. Run timeline

Fig 4 .

 4 Fig 4. EEG offline processing pipeline

Fig 1 .

 1 Fig 1. Vibrotactile device

NeuroRhythms Team, Université de Lorraine, CNRS, LORIA, F-54000, Nancy, France

PErSEUs Team, Université de Lorraine F-57073, Metz, France

Hybrid Team, Inria, University of Rennes, Irisa, UMR CNRS 6074, Rennes, France

We acknowledge the support of the French National Research Agency (ANR), under grant ANR-19-CE33-0007 (project GRASP-IT). We thank the company Octarina for their contribution to the graphic environment.