J Prud'homm

F Lemoine

M Abbas

G Carrault

D Somme

R Le Bouquin Jeannès
email: regine.le-bouquin-jeannes@univ-rennes1.fr-

A priori acceptability of a multimodal system for the early detection of frailty in older adults

Keywords: Acceptability, frailty, prevention, information and communication technologies, gerontechnology

Frailty would affect 4 out of 5 people aged of 85 and over in France. As part of a global project aimed to develop a multimodal system for the early detection of frailty among older adults living at home or in a senior residence, we reported prospective quantitative study assessing a priori acceptability of this system. This platform is composed of 5 devices: a weight scale, a tensiometer, a wrist-worn step counter, an activity tracker and a tablet to exchange data with the aforementioned sensors over internet. The inclusion criteria are: age ≥ 80 years old, living at home or in senior residence, assessed as 'robust' or 'pre-frail'. The a priori acceptability is assessed through self-evaluation questionnaires. The results of the first 22 volunteers included in this study would be in favor of an a priori intention to use rather favorable to the use of the system, despite an a priori acceptability judged rather moderate.

I. INTRODUCTION

As of 1 January 2020, the French population was estimated at 67,060,703, of which 9.5% were aged 75 and over and 3.4% were aged 85 and over [START_REF]Population totale par sexe et âge au 1er janvier 2020, France -Bilan démographique[END_REF]. The proportion of people aged 75 and over living at home is 90% [2]. Among them, 83.1% are considered independent [3]. The French National Institute for Statistics and Economic Studies estimates that in France, in 2060, the proportion of people aged 75 and over would be 16.2%, i.e. about 12 million individuals [START_REF]Projections de population à l'horizon[END_REF]. Most of French people express their desire to 'age' at home [START_REF]CSA -Le domicile, un enjeu citoyen[END_REF]. However, the risk of falls threatens this lifestyle choice. Indeed, falls are known to be the third most common cause of hospital admissions in acute medicine departments and the leading cause of fatal accidents among older adults [START_REF]Causes de décès des personnes âgées[END_REF][START_REF]Principales évolutions de la mortalité par cause médicale sur la période 2000-2016 en France métropolitaine[END_REF].

One of the main risk factors for falls is the 'frail status' [START_REF] Fried | Frailty in Older Adults: Evidence for a Phenotype[END_REF][START_REF]Activité physique et prévention des chutes chez les personnes âgées[END_REF][START_REF] Béland | Explorer la fragilité : d'un syndrome gériatrique à une notion polymorphe ? Presses de l'EHESP[END_REF][START_REF] Theou | Points de repère sur les deux principaux modèles de fragilité : syndrome ou risque, phénotype ou index de fragilité ?[END_REF][START_REF] Hoogendijk | Frailty: implications for clinical practice and public health[END_REF][START_REF] Clegg | Frailty in elderly people[END_REF]. Among approaches used to diagnose frailty syndrome, the phenotypic-type approach of Fried et al. [START_REF] Fried | Frailty in Older Adults: Evidence for a Phenotype[END_REF] is one of the most frequently cited in international studies devoted to this concept [9-11, 13, 14]. According to Fried's frailty phenotype, people can be classified as 'robust', 'pre-frail' or 'frail', depending on whether they meet respectively none, one or two, or three of the following criteria: unintentional weight loss, self-reported exhaustion, slow walking speed, weakness (grip strength) and low physical activity [8-11, 13, 14]. There is a continuum between the state of 'robustness' and the state of 'frailty', passing through a state known as 'pre-frailty' [8-10, 15, 16]. According to the European Survey of Health, Ageing and Retirement in Europe (SHARE), 9% of people aged 50 and over in France could be considered as 'frail' and 41% as 'pre-frail' [START_REF] Santos-Eggimann | La fragilité en Europe : résultats de l'enquête SHARE[END_REF]. The proportion of people aged 75 and over who are 'frail' and 'prefrail' could be around 15% and 50% respectively. At the age of 85, less than one person out of five could be considered as 'robust'. The state of 'frailty' would increase the risk of falling (Hazard ratio (HR) = 1.23; Confidence interval (CI) 95% [1.50-2.21]), of loss of functional independency (HR = 1.79; 95% CI [1.47-2.17]), hospitalization (HR = 1.27; 95% CI [1.11-1.46]), and death (HR = 1.63; 95% CI [1.27-2.08]) at 2 years compared to people considered as 'robust' [START_REF] Fried | Frailty in Older Adults: Evidence for a Phenotype[END_REF][START_REF]Activité physique et prévention des chutes chez les personnes âgées[END_REF][START_REF] Béland | Explorer la fragilité : d'un syndrome gériatrique à une notion polymorphe ? Presses de l'EHESP[END_REF][START_REF] Theou | Points de repère sur les deux principaux modèles de fragilité : syndrome ou risque, phénotype ou index de fragilité ?[END_REF][START_REF] Vermeiren | Frailty and the Prediction of Negative Health Outcomes: A Meta-Analysis[END_REF].

The World Health Organization recommends a "proactive identification of older people in the community at risk of frailty" to provide "opportunities to intervene and so prevent or delay functional decline". Thus, we recently proposed the use of a multimodal health monitoring system for the early detection of elements linked to Fried's frailty, within a project presented in the next section. One of the key assumptions of this approach is that older citizens will use this information and communication technology (ICT)-enabled device. However, the development of ICTs is not always successful [START_REF] Terrade | L'acceptabilité sociale : la prise en compte des déterminants sociaux dans l'analyse de l'acceptabilité des systèmes technologiques[END_REF][START_REF] Bel | Prédire l'utilisation d'une nouvelle technologie : le cas des Systèmes de Transports Intelligents Coopératifs[END_REF][START_REF] Quiguer | acceptation et appropriation des Systèmes de Transport Intelligents : élaboration d'un canevas de co-conception multidimensionnelle orientée par l'activité[END_REF][START_REF] Reerink | Services technologiques intégrés dans l'habitat des personnes âgées : examen des déterminants individuels, sociaux et organisationnels de leur acceptabilité[END_REF][START_REF] Sekhon | Acceptability of healthcare interventions: an overview of reviews and development of a theoretical framework[END_REF][START_REF] Grates | New Perspectives on User Participation in Technology Design Processes: An Interdisciplinary Approach[END_REF]. From this 'proof of concept' phase, we have developed an ancillary study to assess the acceptability of ICTs among older individuals.

In the present study, we present the results of the a priori acceptability of the system through the prism of Bel's integrative model, evaluated on the day of the inclusion of volunteers in the research protocol.

p. 2

Colloque JETSAN 2021

II. METHODS

A. Bel's Integrative Model

To do so, we used a theoretical framework that proposes to explore acceptability in several sub-dimensions [START_REF] Terrade | L'acceptabilité sociale : la prise en compte des déterminants sociaux dans l'analyse de l'acceptabilité des systèmes technologiques[END_REF][START_REF] Bel | Prédire l'utilisation d'une nouvelle technologie : le cas des Systèmes de Transports Intelligents Coopératifs[END_REF]: -A priori acceptability: this is the set of representations that individuals have of a future or unknown technology. Those representations will influence and participate in the individual's decisions to whether or not use this technology in the future.

-Acceptance: resulting from experience with the technology, the time of acceptance refers to the moment when the individual is first confronted with the technology. Thus, this phase does not refer to the representations of people but to the actual activities carried out with the new system. -Appropriation: this refers to the integration of the new technology into the person's daily activity.

The 2016 Bel's integrative model of the behavioral prediction use of an unknown technology [START_REF] Bel | Prédire l'utilisation d'une nouvelle technologie : le cas des Systèmes de Transports Intelligents Coopératifs[END_REF] is based on traditional models of technology acceptability: the 1980 Ajzen and Fishbein's theory of reasoned action, the 1985 Ajzen's theory of planned behavior, the 1989 Davis' technology acceptance model, the 2003 Rogers' diffusion of innovation theory, and the 2003 Venkatesh et al.'s unified theory of acceptance and use of technology [START_REF] Terrade | L'acceptabilité sociale : la prise en compte des déterminants sociaux dans l'analyse de l'acceptabilité des systèmes technologiques[END_REF][START_REF] Bel | Prédire l'utilisation d'une nouvelle technologie : le cas des Systèmes de Transports Intelligents Coopératifs[END_REF][START_REF] Quiguer | acceptation et appropriation des Systèmes de Transport Intelligents : élaboration d'un canevas de co-conception multidimensionnelle orientée par l'activité[END_REF][START_REF] Reerink | Services technologiques intégrés dans l'habitat des personnes âgées : examen des déterminants individuels, sociaux et organisationnels de leur acceptabilité[END_REF]. Although it does not take into account all the dimensions related to acceptability, Bel's integrative model emphasizes "the existence of a predictive chain [of influencing factors] ranging from a known generic technology that is conceptually close to the unknown technology, to increasingly specific technologies, leading to the prediction of the intention to use [...] the technology that does not yet exist" [START_REF] Bel | Prédire l'utilisation d'une nouvelle technologie : le cas des Systèmes de Transports Intelligents Coopératifs[END_REF]. This model is shown in Fig. 1.

B. The ACCORDS multimodal system

The Signal and Image Processing Laboratory (LTSI), French National Institute of Health and Medical Research -Unit 1099 of University of Rennes 1, the Geriatrics Department of Rennes University Hospital Centre, and the Clinical Investigation Centre -Technological Innovation 1414 of Rennes University Hospital Centre, have joined forces with the industrial companies RF-Track and AZNetwork to develop a multimodal platform for collecting data related to frailty. This project is named ACCORDS, a combinatory approach of connected functionalities for the collection of health data with multimodal aims. Its main objective is to develop one or more algorithms for the early detection of frailty of older people, thanks to the collection of clinical data, associated with paraclinical data obtained through the multimodal platform. The device is entrusted to the volunteer for a maximum of 2 years, to be used in his/her daily living. It is composed of: -a connected weight scale (iHealth HS2 and HS6), to monitor the subject's weight, to be used at least twice a month, -a connected blood pressure monitor (iHealth Track KN-550BT), to monitor blood pressure and heart rate, to be used at least once a month, -a wrist-worn step and calories counter (iHealth AM4), integrated into a wristwatch, to monitor the subject's activity via the number of daily steps, the estimated energy expenditure and the daily travelled distance, to be used at least 15 days, consecutive or not, per month, -a sensor for quantifying physical activity called 'activity tracker' (manufactured by the company RF-Track) comprising an accelerometer, a barometer and a radio transceiver, which can be worn around the neck or the waist, to be used at least 15 days, consecutive or not, per month, -a touch tablet (Samsung Galaxy Tab A 2016, Android 6.0 operating system) allowing access to a health monitoring application for the above-mentioned parameters, which is accessed by the subject at least once per two weeks.

The health monitoring application called Calliopé® was specially developed for this study, based on collaborative work within the team. It allows data to be retrieved from the 3 iHealth sensors, secure data connection and storage, as well as the visualization of the daily data history for the volunteer. This platform is shown in Fig. 2.

III. CLINICAL TRIAL

The project received a favorable opinion for its implementation from the Protection of Persons Committee Ouest VI -1228 HPS2, under the code IDRCB 2019-A02316-51 on 2 January 2020. The French National Commission for Information Technology and Civil Liberties and the French National Agency for the Safety of Medicines and Health Products have been informed of this study. The duration of the inclusion period is 12 months from 1st September 2020. The estimated data processing time is 6 months. The total duration of the study is 54 months, including a 6-month extension of the study due to the SARS-Cov2 epidemic.

A. Participants

The number of subjects to be included was estimated at 30, considering the diversity of older people's profiles. The inclusion criteria for the project were: age ≥ 80 years old, living at home or in a senior residence, assessed as 'robust' or 'pre-frail' according to Fried's frailty phenotype. The criteria for noninclusion were: hospitalization in the month preceding the first visit, cognitive impairment prohibiting proper use of the system, inability to give consent to participate in the study, or being subject to legal protection [START_REF] Op Het Veld | Fried phenotype of frailty: cross-sectional comparison of three frailty stages on various health domains[END_REF]. If the person is classified as 'frail' (3, 4 or 5 Fried's criteria) during the follow-up, the health monitoring is concluded so we can analyze his/her frailty trajectory and the criteria that could indicate the onset of frailty at an early stage.

B. Calendar of home visits

We propose a set of 10 home visits over 2 years. The first inclusion took place on September 2020. In the framework of the SARS-Cov2 pandemic, the barrier measures were respected during home visits. It included the wearing of masks for investigators and respondents, washing hands with a hydroalcoholic solution before and after handling objects, physical distancing avoiding close contacts and room ventilation for the duration of the visit.

C. The collection of data for the a priori acceptability study

The quantitative part of the a priori acceptability study presented here is based on four self-evaluation questionnaires constructed from data in the literature, in particular Bel's integrative model of behavioral prediction of the use of an unknown technology, and reflections of our research team [START_REF] Terrade | L'acceptabilité sociale : la prise en compte des déterminants sociaux dans l'analyse de l'acceptabilité des systèmes technologiques[END_REF][START_REF] Bel | Prédire l'utilisation d'une nouvelle technologie : le cas des Systèmes de Transports Intelligents Coopératifs[END_REF][START_REF] Quiguer | acceptation et appropriation des Systèmes de Transport Intelligents : élaboration d'un canevas de co-conception multidimensionnelle orientée par l'activité[END_REF][START_REF] Reerink | Services technologiques intégrés dans l'habitat des personnes âgées : examen des déterminants individuels, sociaux et organisationnels de leur acceptabilité[END_REF]. These questionnaires include 46 items grouped into eight themes. The items are assessed using a 7-point Likert scale. The first 15-item questionnaire explores three themes: the person's prior experience and attitudes towards information and communication technologies, and prior experience with health p. 4

Colloque JETSAN 2021 monitoring technologies. The second questionnaire of 12 items explores three themes: the 'expected values of the use of the object' also called a priori utility; the 'subjective norms', meaning the influence of family and friends in initiating ICTs' health monitoring; and the a priori intentions to use the system. The third 9-item questionnaire explores one theme, the a priori comfort of use. Eventually, the fourth questionnaire of 10 items explores one theme, the 'perceived behavioral control' also called the a priori usability of the system. The seven possible response options cover the spectrum of opinions, from 'strongly disagree' to 'strongly agree'. For each item, a score is obtained, ranging from 1 for 'strongly disagree' to 7 for 'strongly agree'. For negative sentences, the score is reversed. Then, the participant is asked to give an overall rating of the system out of 10 at the time of inclusion. The scores' means (± standard deviation) were calculated. The closer the mean score is to 1, the more negative the assessment is considered to be. The closer the mean score is to 7, the more strongly the assessment is considered to be positive. About the last item, the participant is asked to give a rate of recommendation of the system to his/her family members and friends at the time of inclusion. The data were recorded using Microsoft Excel (2010) ® software.

IV. RESULTS

A. Characteristics of the population

As of 1 March 2021, twenty-two volunteers have been included in the study. The mean age was 87.0 years (± 3.7). There were sixteen women and six men. Ten participants lived in their own house and twelve in a senior residence. The majority of the participants belonged to a higher socioprofessional category. Most of them had, personally of through their children, a link with the health professional or academic environment.

B. Questionnaires on the a priori acceptability

On average, volunteers were not very familiar with information and communication technologies (mean score for prior experience towards ICTs = 3.7 (± 1.1); mean score for prior experience towards health monitoring technologies = 3.9 (± 1.3)). While they all had a mobile phone, they did not all have a computer or a tablet at home. Older adults living in their own homes were more often equipped with ICTs than individuals living in a senior residence. Overall, attitudes (mean score = 4.8 (± 1.3)) and a priori usability of the device (mean score = 5.0 (± 0.9)) were rated moderately by volunteers, whereas a priori comfort of use (mean score = 5.8 (± 1.2)), a priori utility (mean score = 6.0 (± 1.0)) and a priori intention to use (6.1 (± 0.8)) were better rated. The influence of family members and friends in initiating ICTs' health monitoring was evaluated moderately (mean score = 5.5 (± 1.5). At the end, the overall score out of 10 (average score = 8.0 (± 1.4)) and the rate of recommendation of the system to family members and friends at the time of inclusion (83.0 % (± 17.7)) were favorable to the system, despite a moderate evaluation of the a priori acceptability. This could be explained by the positive feeling of participating in a research protocol, and the excitement of novelty and discovery [START_REF] Fischer | The Importance of User Involvement: A Systematic Review of Involving Older Users in Technology Design[END_REF]. The results of the a priori acceptability of the system are summarized in Table I.

V. DISCUSSION

This study underlines the modulation of the a priori acceptability of an information and communication technology, according to different factors as presented in the 2016 Bel's study. Indeed, according to Bel: "prior experience towards known generic objects indirectly predicted the intention to use the unknown object, via mediation by attitudes towards generic objects, prior experience with specific objects and value/expectations towards the use of the unknown object". Conversely, we reported in the present study that poor or moderate rating of prior experience with known generic and specific objects and attitudes towards ICTs did not appear to alter participants' intention to use it. Besides as reported in Bel's study, our results also highlighted the indirect influence of the subjective norm [mainly the influence of the social environment] on the intention to use of this developing technology [START_REF] Bel | Prédire l'utilisation d'une nouvelle technologie : le cas des Systèmes de Transports Intelligents Coopératifs[END_REF].

The strengths of this study lie firstly in the assessment of the acceptability of an information and communication technology in a population of people aged 80 and over, rarely explored in the literature [START_REF] Grates | New Perspectives on User Participation in Technology Design Processes: An Interdisciplinary Approach[END_REF]. Furthermore, the system is unique in that it is evaluated immediately with the target audience. Indeed, in 2013 Quiguer explained that "by remaining 'impervious to the reasoning that can be deployed in an ecological environment'

A priori acceptability themes

Questionnaires' scores

Mean scores (SD)

Minmax scores

Prior experience towards ICTs 3.7 (± 1.1) 1 -7

Prior experience towards health monitoring technologies 3.9 (± 1.3) 1 -7

Attitudes towards ICTs 4.8 (± 1.3) 1 -7

A priori utility of the system 6.0 (± 1.0) 1 -7

A priori usability of the system 5.0 (± 0.9) 1 -7

A priori comfort of use 5.8 (± 1.2) 1 -7

Influence of family members and friends in initiating ICT's health monitoring 5.5 (± 1.5) 1 -7

A priori intention to use the system 6.1 (± 0.8) 1 -7

Overall rating of the systems at the time of inclusion 8.0 (± 1.4) 1 -10

Rate of recommendation of the system to family members or friends at the time of inclusion 83.0 (± 17.7) 0 -100 p. 5

Colloque JETSAN 2021 (Reerink-Boulanger, 2012, p.104), the knowledge developed through the completion of these questionnaires does not provide the critical content on which to act in order to allow effective appropriation of the systems, which remains the goal of any technological implementation" [START_REF] Quiguer | acceptation et appropriation des Systèmes de Transport Intelligents : élaboration d'un canevas de co-conception multidimensionnelle orientée par l'activité[END_REF]. Overall, user involvement from the start of the design phase of a new technology is essential, so designers can improve the development of the technology at the very beginning of the developing process. Moreover, in this way, older users can develop empowerment and so their well-being [START_REF] Fischer | The Importance of User Involvement: A Systematic Review of Involving Older Users in Technology Design[END_REF][START_REF] Bickel | Les multiples faces du pouvoir d'agir à l'épreuve du vieillissement[END_REF]. Thus, the acceptability of interventions in health promotion would be improved by the integration of individuals throughout the project process [START_REF] Sekhon | Acceptability of healthcare interventions: an overview of reviews and development of a theoretical framework[END_REF].

The main limit of this study is the intrinsic limits of acceptance theoretical models that helped to develop the acceptability questionnaires. According to Bobillier-Chaumon and Dubois in 2009, "new technologies include change factors that cannot always be anticipated a priori". The use of quantitative questionnaires alone does not always reflect real-life situations, and the mean scores obtained from the questionnaires do not usually reflect the reality of the heterogeneity of people, particularly older adults [START_REF] Terrade | L'acceptabilité sociale : la prise en compte des déterminants sociaux dans l'analyse de l'acceptabilité des systèmes technologiques[END_REF][START_REF] Klein | Technologies de la santé et de l'autonomie et vécus du vieillissement[END_REF]. Therefore, we decided to lead a parallel prospective qualitative study assessing acceptability of the system to obtain new perspectives of this topic. Results are in progress.

Finally, if the system under study is scientifically validated in the future, the platform would integrate the 'healthy ageing' strategy [START_REF]Ageing: Healthy ageing and functional ability[END_REF][START_REF]WHO | WHO: Number of people over 60 years set to double by 2050; major societal changes required[END_REF][START_REF]Decade of Healthy Ageing[END_REF]. However, the right to know a level of risk to one's health should not oblige individuals to use this tool. The "agentisation process" triggered by public health policies [START_REF] Delalandre | Activité physique et prévention des chutes, entre médicalisation et capacitation[END_REF] must not, in our view, be a source of guilt for individuals, whether or not they take advantage of this system. Similarly, the right to incompetence remains a right and the empowerment of individuals must not be the only response to improve the individuals' well-being. It must be part of a global health promotion project, via its five areas of reflection, at the risk of increasing social health inequities [START_REF] Op Het Veld | Fried phenotype of frailty: cross-sectional comparison of three frailty stages on various health domains[END_REF][START_REF]Health promotion[END_REF][START_REF]Health Promotion[END_REF][START_REF]WHO | Shanghai Declaration on promoting health in the 2030 Agenda for Sustainable Development[END_REF].

VI. CONCLUSION

The results shown in this study are encouraging and in favor of an intention to use rather favorable to the developing system, despite an a priori acceptability judged rather moderate, probably due to the numerous connected devices to use. Simplicity of use and perfect technical condition seem to be other very important factors for its acceptability. Whereas they are not totally consistent with the literature, those results allow the study to be continued by enriching Bel's model used as a reference in this project and to better understand the actual use of the system beyond its a priori acceptability.

Figure 1 .

 1 Figure 1. Bel's integrative model of the behavioral prediction use of an unknown technology, adapted from 'Bel M. Predicting the use of a new technology: the case of Cooperative Intelligent Transport Systems. 2016.'

Figure 2 .

 2 Figure 2. The ACCORDS platform including a weight scale, a tensiometer, a wrist-worn step and calories counter, an activity tacker and a tablet using the Calliopé® application The present study assesses the a priori acceptability of this system by older adults. It was carried out on the base of collaborative and interdisciplinary work, from its design to its implementation, both in terms of choice of the collected parameters, the development of algorithms and interfaces and in the development of the research protocol for the present study on the ICTs' acceptability. The whole project received funding from the French National Research Agency in 2018 for a period of 4 years.

TABLE I .

 I A PRIORI ACCEPTABILITY OF THE SYSTEM

	ICTs: Information and communication technologies
	SD: Standard deviation

ACKNOWLEDGMENT

This work was supported by the French National Research Agency (ANR) in the context of the ACCORDS Project under Grant ANR-17-CE19-0024-01.


Audrey Leong-Hoi
email: audrey.leong-hoi@altran.com

Julien Imbs

Pierre Pfeiffer

Nicolas Gillard

Jean-Paul Departe

Pauline Coignard

Jacques Kerdraon

Willy Allegre

Jean-Pierre Radoux

Feasibility study of the use of Fiber Bragg Gratings applied to the prevention of pressure ulcers

Keywords: Pressure ulcer prevention, Fiber Bragg Gratings, pressure and shear measurements

I. INTRODUCTION

Although often underestimated, pressure ulcers are a real public health issue. In France, the average prevalence of pressure ulcers among hospitalized patients is about 8.1% in 2014 [START_REF] Barrois | Épidémiologie des escarres en France[END_REF], which further complicates the situation of the already vulnerable people. Besides, pressure ulcers seriously affect the quality of life of patients, causing both physical and mental suffering. Their treatment is painful and long but also expensive. There are currently very few effective ways to prevent pressure ulcers. In practice in hospitals, healthcare professionals try to prevent them by moving patients every two hours to alternate areas of prolonged support.

The use of a multi-sensor device could prevent the appearance of pressure ulcers. These sensors should therefore measure pressure, shear, temperature and humidity in the patient's risk areas. Indeed, pressure ulcers are wounds that form during prolonged immobilization, in a lying or sitting position, because of pressure that can be associated with shear. The originality of this project is to propose a bedsore prevention device based on optical fibers adapted to the patient's context, and therefore integrable and embeddable in an everyday object, such as a cushion for patients in wheelchairs or in a mattress topper for bedridden patients. But, before incorporating optical fibers in adapted materials, the first objective is of this project is to study the feasibility of the use of a sensor based on Fiber Bragg Gratings (FBG) to prevent pressure ulcer occurrence. FBG generate interest in sensor technology, since they can measure physical values such as temperature or pressure. A first step consists in developing a FBG-based prototype able to measure vertical and horizontal deformations, corresponding respectively to pressure and shear. Once the feasibility is proven, we will have our calibrations and control measurements and then we will focus on the embedding of these FBG in a material and the impact of this integration in the measurements.

II. TECHNOLOGY BACKGROUND

A. Use of Fiber Bragg Gratings 1) Principle of the FBG

The principle of a Fiber Bragg Grating is based on the modulation of the refractive index of the optical fiber core over a defined length [START_REF] Her | Simultaneous Measurement of Temperature and Mechanical Strain Using a Fiber Bragg Grating Sensor[END_REF]. When the spectrum of a broadband source passes through the core of the optical fiber, only a narrow part of the incident spectrum is reflected at a resonant wavelength: the Bragg wavelength, noted as λ B (Fig. 1).

p. 2 Colloque JETSAN 2021

The Bragg wavelength is defined by:

λB = 2ne Λ (1)
with n e the effective core index of the optical fiber, and Λ the pitch of the grating. The operating principle of FBG sensors is based on the variation of the Bragg wavelength when the optical fiber is subjected to strain or temperature changes. This variation is noted Δλ B .

Figure 1. Operating principle of a FBG optical fiber [START_REF] Demirel | Contribution à l'optimisation des mesures de température et de déformations par capteur à fibre optique à réseau de Bragg : application au procédé de fabrication des composites par infusion de résine[END_REF].

2) Use of FBG as a temperature sensor

It can be shown that the shift of the Bragg wavelength Δλ B due to a change in temperature can be written as follows [START_REF] Demirel | Contribution à l'optimisation des mesures de température et de déformations par capteur à fibre optique à réseau de Bragg : application au procédé de fabrication des composites par infusion de résine[END_REF]:

(

with the thermo-optical coefficient due to a variation in the refractive index. It is equal to 7.3.10 -6 /°C at the temperature T = 20°C for an optical fiber of pure silica [START_REF] Vacher | Capteurs à fibres optiques pour le contrôle de l'élaboration et la caractérisation mécanique des matériaux composites[END_REF]. The coefficient α corresponds to the relative elongation of the Bragg grating pitch with respect to a temperature variation [START_REF] Demirel | Contribution à l'optimisation des mesures de température et de déformations par capteur à fibre optique à réseau de Bragg : application au procédé de fabrication des composites par infusion de résine[END_REF]. α is therefore the coefficient of thermal expansion and is equal to 0.55.10 -6 /°C [START_REF] Demirel | Contribution à l'optimisation des mesures de température et de déformations par capteur à fibre optique à réseau de Bragg : application au procédé de fabrication des composites par infusion de résine[END_REF]. Besides, the thermal sensitivity of a FBG can be obtained by:

(

Typical values of the FBG temperature response are 6.8, 10 and 13 pm/°C for Bragg wavelengths of 830, 1300 and 1550 nm respectively [START_REF] Vacher | Capteurs à fibres optiques pour le contrôle de l'élaboration et la caractérisation mécanique des matériaux composites[END_REF], [START_REF] Rao | In-fibre Bragg grating sensors[END_REF].

3) Use of FBG as a pressure sensor

Pressure measurement is not obtained directly. It is first necessary to calculate the strain undergone by the optical fiber that we note ε, which corresponds to a deformation. In our study, we consider that the FBG is isotropic and homogeneous. By applying a uniaxial force (for example a vertical force), the fiber is subjected to a strain and is deformed along its axis. The cross section of the fiber is also reduced by the Poisson effect. The Poisson's coefficients are noted ν. It can also be shown that the variation of the refractive index as a function of deformation can be expressed by the Pockels coefficients p ij [START_REF] Hill | Fiber Bragg grating technology fundamentals and overview[END_REF]- [START_REF] Narasimhamurty | Photoelastic and Electro-Optic Properties of Crystals[END_REF].

Finally, the Bragg wavelength shift due to uniaxial deformation can be written as follows [START_REF] Demirel | Contribution à l'optimisation des mesures de température et de déformations par capteur à fibre optique à réseau de Bragg : application au procédé de fabrication des composites par infusion de résine[END_REF]:

(4)
Sensitivity to deformation can be written as follows:

(5)

with:

(6)
Typical values for a germanium-doped optical fiber are [START_REF] Hill | Fiber Bragg grating technology fundamentals and overview[END_REF], [START_REF] Othonos | Fiber Bragg gratings[END_REF]:

 p 11 = 0.113  p 12 = 0.252  ν = 0.16  n e = 0.148
Knowing the deformation ε of the fiber, the corresponding elongation can be calculated. Indeed, the relationship between deformation and the length of the fiber is:

(7)
For a Bragg wavelength of 1550 nm, the sensitivity to distortion is of the order of 1.3 pm/µε [START_REF] Hill | Fiber Bragg grating technology fundamentals and overview[END_REF], [START_REF] Othonos | Fiber Bragg gratings[END_REF]. The total force applied to the fiber is therefore:

(8)
The coefficient k is the stiffness constant of the springs used and m*g is the weight applied to the optical fiber. The pressure P is easily determined, as it is the ratio of the force F applied to the bearing surface S of the system:

B. FBG-based prototype

For our experiment, a prototype able to measure vertical strain and horizontal shear forces was designed and engineered at the ICube Laboratory. The FBG used have a length L of 5 cm. A tunable laser is used with a sweeping range between 1545 and 1560 nm and a center wavelength λ B0 of 1554.22 nm (cf. Table 1).

The wavelengths sent by the laser pass through a circulator that transmits the light to the optical fiber (1 to 2: blue arrow). The Bragg wavelength is reflected by the grating and passes back p. 3

Colloque JETSAN 2021 through the circulator (3: red arrow) to be sent to a detector. The operating principle of the circulator is shown in figure 2.

First, a rigorous approach consists in sending a light source in the optical fiber with a tunable laser to measure the Bragg wavelength associated with a known strain by putting known weights on the prototype for example. The IL Client software is used to acquire the Bragg wavelength. Two optical fibers are attached, each of them between two rigid attachment points. The bash-plate that applies pressure on one of the optical fibers is set on a rectangular support. This support is in contact with four springs, which allow the system to return to its initial state when no more force is applied. The complete system is shown in the figures 3 and 4. FBG1 and FBG2 are the optical fibers used to measure vertical and horizontal strains, respectively. Horizontal strains are considered as shears. The bash-plate presses on the FGB1 when a vertical force is exerted (cf. Fig. 5). The two rods deform the FBG2 when there are horizontal movements that corresponds to shears (cf. Fig. 6). First, a measurement of the Bragg wavelength λ B0 is performed without any applied force. Then, different weights are put on the measuring system to determine the corresponding Bragg wavelength offset Δλ B0 .

III. RESULTS

The measurements results of the Bragg wavelengths without any applied force are given in Table 2.

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Table 3 gives the measured center wavelengths (λB0) for different weights put on the bearing surface according to the temperature room (cf. Fig. 7). In order to study the response of the FBG to strain, we measured the wavelength shifts when weights are exerted and then removed on the fiber. This experiment allows us to check if the fiber returns to its initial position after being deformed. This allows us therefore to validate the setup of the prototype.

Figure 8 shows the variation of the wavelength as a function of the weights applied to FBG1. In red, the measurements were obtained by adding weights and in blue, by removing weights from the prototype. It can be noticed that both curves are almost superimposed, which can be assumed that the measurements are repeatable. In the same way, figure 9 shows the variation of the wavelength according to millimeter horizontal shift applied to the FBG2 with different weights put on the prototype. The study of horizontal shifts allows us to analyze the response of the fiber to shear. The different curves show a linear response of the system to shear despite the different weights applied as illustrated in figure 7. However, it can be noticed a deviation of the measurements after shifting the prototype of 1.5 mm (cf. red circle). Those errors in the measurements have been obtained certainly because of the mechanical stop in the system.

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The sensitivity of the fiber to deformation is 1.3 pm/µε at 1550 nm. ε can then be obtained with the equation ( 5). The stiffness constant of the springs used is 11 960 N/m. The bearing surface on which the weights are applied is 0.015 m².

With these data, it is therefore easy to calculate, from the wavelength shift measurements, the deformation exerted on the FBG, and then the strain and the shear applied to the fiber.

IV. DISCUSSIONS

The results obtained are in agreement with the theory. Indeed, the linear dependence of the Bragg wavelength defined by equation ( 5) is verified. However, a slight drift of the Bragg wavelength from its original value (1554.22 nm) is observed. The FBG is fixed and tightened at two attachment points. Finally, Bragg wavelength measurements show good repeatability and reproducibility.

In the same way as we have shown in this article, the temperature can be calculated by measuring the wavelength shift after calibration of the FBG and then the deformation of the fiber (cf. equation ( 2)). However, the simultaneous measurements of strain and temperature will be more complicated. Indeed, the theory shows that a change in temperature or deformation has the same effect on the Bragg wavelength shift Δλ. Several methods have been proposed in the literature to discriminate between temperature and strain [9]- [START_REF] Du | Fiber Bragg grating cavity sensor for simultaneous measurement of strain and temperature[END_REF]. Experimental systems are more complex and would not necessarily be suitable for the project. Contrary to the strain and shear measurements which have to be performed in a finer way, the temperature measurement can be performed in a more coarse way. For this reason, a solution is to set up thermistors to measure temperature on different areas of the prototype to reduce the number and therefore the cost of FBG that are dedicated to the measurements of strain and shear.

V. CONCLUSIONS AND PERSPECTIVES

The use of FBG for pressure ulcer prevention is one possible solution. These optical fibers are very sensitive to deformations and can therefore be used for strain and shear measurements applications.

An important next step in this project will be to incorporate the FBG-based prototype in a material suitable for patients such as gel, foam or fabric. This will be a complicated process since it is necessary to ensure that the optical fiber does not break and that it does not damage the material (crack, breakage, etc...).

Figure 3 .

 3 Figure 3. Support system (front view).

Figure 4 .

 4 Figure 4. Support system (top view).

Figure 5 .Figure 6 .

 56 Figure 5. Scheme of the FBG1 deformation with a vertical force (front view).

Figure 8 .

 8 Figure 8. Wavelengths as a function of the weights applied to the FBG1.

Figure 9 .

 9 Figure 9. Wavelengths as a function of horizontal shift applied to the FBG2 with different weights.

TABLE I

 I

		.	SPECIFICATIONS OF THE FBG USED
	Serial number	λB0 [nm]	Sensibility (deformation) [m/ε]	Maximal deformation [ε]
	6920170551	1554.220	1.30E-06	5.10 -3

Figure 2. Operating principle of the circulator.

TABLE II

 II

	.	BRAGG WAVELENGTH MEASUREMENTS
			λ B0 [nm]
			1556.6916
		Weight: 0 g (without any applied force)	1556.6620 1556.6315
			1556.7785
		Mean	1556.6909
		Standard variation	0.0633
	TABLE III.
			0	1556,7085
		23,7	2329 4658	1557,2309 1557,8358
			6987	1558,6462
			0	1556,7088
		24,3	2329 4658	1557,2484 1557,8538
			6987	1558,6387
			0	1556,6705
		24.6	2329 4658	1557,1929 1557,7628
			6987	1558,5835

MEASUREMENTS OF BRAGG WAVELENGTH FOR DIFFERENT WEIGHTS APPLIED ON THE SUPPORTING SUPPORT DEPENDING ON ROOM TEMPERATURE

Room temperature [°C] Weight (g) ΔλB0 [nm]

Figure

7

. Photo of weights applied to the prototype.

ACKNOWLEDGMENT

This project was initiated in 2019 and brings together several players such as Kerpape, the ICube Laboratory and Altran. This project is carried out within the "Handicap Innovation Territory" project (HIT), supported by the French Government as part of "Territoires d'innovation" (French program for innovative territories), administered by the "Banque des Territoires".

Thanks are extended to students of Télécom Physique Strasbourg for their helps.


Helene Pillet
email: helene.pillet@ensam.eu

Bruno Watier

Development of a wearable framework for the assessment of a mechanical-based indicator of falling risk in the field

Keywords: locomotion, gait instability, wearable sensors, falling risk

This paper presents both normative thresholds of the distance of the Body Center of Mass (BCOM) to the Minimal Moment Axis (MMA) during able-bodied level walking that can be used as a proxy of whole-body angular momentum variations to characterize the instability of gait and a wearable framework to assess this indicator in the field. The results are based on experimental analysis of the gait of able-bodied subject from a motion capture system and on a preliminary comparison of wearable-based against optoelectronicbased assessment of gait instability in one asymptomatic subject and one person with transfemoral amputation.

I. INTRODUCTION

Quantifying the instability of gait is of crucial importance for the monitoring of falling risk. In the literature, variation of the whole-body angular momentum has been identified as a good mechanical candidate to this aim, but its interpretation can be a bit challenging in a clinical context [START_REF] Popovic | Angular Momentum Regulation during Human Walking: Biomechanics and Control[END_REF]. In the same time, the ability to compute such indicator in the field is also essential for its implementation in relevant wearable monitoring tools of the actual risk of falling. Thus, there is a need for visual indicator of the mechanics of gait that can be interpreted and understood by clinicians and patients. Contrary to whole body angular momentum, the distance of the Body Center of Mass (BCOM) to the Minimal Moment Axis (MMA) referred hereafter as appears as an alternative easy to represent and accessible in the field [START_REF] Bailly | A human-inspired mechanical criterion for multi-contact locomotion in humanoids To cite this version : A human-inspired mechanical criterion for multicontact locomotion in humanoids[END_REF][START_REF] Al-Abiad | A Mechanical Descriptor of Instability in Human Locomotion : Experimental Findings in Control Subjects and People with Transfemoral Amputation[END_REF].

The 3D kinetics of the BCoM are usually acquired with either force plates or the combination of inertial models and optical motion capture system (OMCS), generally not available in clinical routine (cost, set-up time, …). Therefore, the use of magneto-inertial measurement units (MIMUs) has been proposed as a wearable alternative for the estimation of BCoM-derived parameters. Shahabpoor et al. proposed a method to select a reduced number of MIMUs for the estimation of vertical BCoM acceleration [START_REF] Shahabpoor | Estimation of vertical walking ground reaction force in real-life environments using single IMU sensor[END_REF]. This method has been recently extended and validated considering the 3D nature of BCoM movement to obtain accurate 3D BCoM acceleration of one person with transfemoral amputation [START_REF] Simonetti | Development of a wearable framework for body center of mass acceleration assessment in people with transfemoral amputation[END_REF].

The aim of the present abstract is (i) to give the normative thresholds of associated to able-bodied level walking (ii) to describe a framework that could be used to assess from fully wearable sensors and to present bottlenecks that still have to be overcome for the final implementation of this framework.

II. DISTANCE OF THE MINIMAL MOMENT AXIS TO THE BODY CENTER OF MASS AS A PROXY OF WHOLE BODY ANGULAR MOMENTUM VARIATIONS DURING LOCOMOTION

A. Theoretical background 1) Minimal Moment Axis

Contact mechanical actions can be represented at any point , by a force and a moment . and define a moment field that can be expressed at any point B as:

(1)
where is the position of with respect to .

There exists one axis called the Minimal Moment Axis (MMA) such that, at each point of this axis, the moment is collinear to [START_REF] Shimba | AN ESTIMATION OF CENTER OF GRAVITY fr[END_REF]. Assume that point . is computed as in [START_REF] Bailly | A Mechanical Descriptor of Human Locomotion and its Application to Multi-Contact Walking in Humanoids[END_REF]:

(2)

2) External mechanical actions during gait

Considering the contact mechanical actions between the foot and the ground during gait and if we now assume that B is the BCoM

p. 2

Colloque JETSAN 2021 When , Q is the orthogonal projection of the COM onto , and the distance is the minimal distance between and the BCoM. The vector will be called hereafter:

(4)

From equation ( 4), we compute the Euclidian norm of the distance from BCoM position and force platform data:

(5)

Interestingly, it can be noted that this distance can also be linked to the angular momemtum at the BCoM by considering the relation : where is the derivative of the angular momentum. Thus: [START_REF] Shimba | AN ESTIMATION OF CENTER OF GRAVITY fr[END_REF] With is the angle between and .

B. Material and Methods

1) Experimental procedure

The study, under approval of the ethics committee, included 11 able-bodied volunteers. Each volunteer was equipped with 47 retro-reflective skin markers tracked at 200 Hz using a 13 T10 Vicon system (Oxford Metrics, UK). Ground reaction forces and moments (GRF) were simultaneously measured at 2000 Hz by three force plates (AMTI, USA) and filtered with a 4th order Butterworth low-pass filter with a 15 Hz cutoff frequency. Each subject performed several trials at selfselected walking velocity.

2) Data treatment

From GRF, the MMA along which the moment of the external contact forces is minimal was obtained. 3D BCOM position was determined from a regression method. The vector between the BCOM and its projection on the MMA was computed at each instant of time (4), the norm of this vector and its components in the global reference frame denoted as on the Medio-Lateral, Anterior-Posterior and Vertical axes respectively.

C. Normative thresholds of the distance of the Minimal Moment Axis to the body Center Of Mass during able bodied gait

The results confirm that the components of the vector between the BCOM and its projection on the MMA remains low for able-bodied subjects walking on level ground (Table 1). The pattern of (Figure 1) also shows that the norm increases at the end of the single support phase.

A. Framework overview

The developed framework is based on the methodology described by Simonetti et al. [START_REF] Simonetti | Development of a wearable framework for body center of mass acceleration assessment in people with transfemoral amputation[END_REF] to obtain the 3D acceleration of the BCoM. Briefly three steps are necessary: i) computing the 3D acceleration of each segment's center of mass (sCoM) from MIMUs data, ii) merging sCoM accelerations in a consistent common global frame , and iii) estimating the 3D BCoM acceleration from a weighted average of selected sCoM accelerations. Adapting the approach from Shahabpoor et al. [START_REF] Shahabpoor | Estimation of vertical walking ground reaction force in real-life environments using single IMU sensor[END_REF], the trunk, pelvis, thighs and feet were identified as the major contributors in 3D BCoM acceleration in both ablebodied and people with amputation. Therefore, 6 MIMUs, manually aligned with the longitudinal axes of the segments, are adopted. For further details, the reader can refer to Simonetti et al. [START_REF] Simonetti | Development of a wearable framework for body center of mass acceleration assessment in people with transfemoral amputation[END_REF]. From the computation of 3D BCoM acceleration, 3D BCoM velocity and position can be retrieved from an integration process assuming integration constants. The feasibility of such approach has already been demonstrated from force plates data [START_REF] Whittle | Three-dimensional motion of the center of gravity of the body during walking[END_REF]. To determine the distance of the Minimal Moment Axis to the body Center Of Mass, an estimation of the center of pressure would be useful and could be provided by wearable insoles.

B. Framework implementation 1) Experimental protocol

One male individual with transfemoral amputation (mass: 83 kg, stature: 1.69 m) and one female able-bodied subject (mass:

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Colloque JETSAN 2021 56 kg, stature: 1.69m) gave their written informed consent to participate in the study. They were instrumented with a fullbody marker set [START_REF] Al-Abiad | A Mechanical Descriptor of Instability in Human Locomotion : Experimental Findings in Control Subjects and People with Transfemoral Amputation[END_REF] and 6 MIMUs (Xsens, 100 Hz) on the feet, thighs, pelvis, and trunk, each mounted on a 3D-printed plastic support with housings for 4 reflective markers. An OMCS (VICON, 200 Hz) recorded the markers' positions while 4 photographs (front, back, both sides) were taken. Then, starting from a static standing posture, the participant walked at selfselected speed along an 8 m pathway, with 3 force plates (AMTI, 1000 Hz) in the middle. Synchronization between instruments was achieved by a trigger signal. Only trials with three successive foot contacts on the force plates (i.e. a complete stride), were considered for further analysis.

2) Preliminary data processing

Data were filtered using a zero-phase 4 th -order Butterworth filter (cut-off frequencies identified using a trial-and-error approach: 5 Hz markers and MIMUs, 10 Hz force plates).

Reference BCoM acceleration was computed from force plates' signals.

C. Preliminary results and discussion

High consistency between reference and MIMU 3D BCoM acceleration patterns can be observed (Figure 2).

IV. CONCLUSION AND PERSPECTIVES

The present article gives two important First, reference values of the distance of the Body Center of Mass (BCOM) to the Minimal Moment Axis (MMA) during ablebodied gait are computed and constitute the normative thresholds that could be used to characterize pathological or instable gait. Second, the potential of a fully wearable framework has been demonstrated based on a preliminary experiment with two individuals that revealed very encouraging results for the assessment of 3D BCoM acceleration. Next steps include the obtention of BCoM trajectory and the synchronization with pressure insoles to estimate the position of the Minimal Moment Axis. The accuracy of these parameters remains to be investigated.

Figure 1 :

 1 Figure 1: Evolution of over a gait cycle (average and corridor +/-1 standard deviation for the 11 participants)

Figure 2 :

 2 Figure 2: BCoM acceleration obtained with force plates (dotted lines) and MIMUs (straight lines).

Table 1 :

 1 Average and standard deviation for the 11 participants of the ranges of

	and	in mm

ACKNOWLEDGMENT

This work was supported by the national research agency (ANR), through the projects: ANR-HOBIS [18CE27-0010] and ANR-COBOT [18CE10 -0003] and by the Fondation de l'Avenir (grant number AP-RM-20-001)