Secondary outcomes:
fatigue, pain
DISEASE
Evaluation of the effect of the program on disease activity assessed by the change of JADAS-27 score compare to baseline, at 26 and 50 weeks ± 1 week.Evaluation of the physical activity level: The difference in the average level of physical activity will be assessed by continuous measures (over 7 days) of the time spent in MVPA (Moderate to Vigorous Physical Activity) Actimetry measurements will be performed before the start of the intervention to determine a value for V0, at V1-1 week, V2-1 week and V3-1 week, and pedometric measurements will be performed throughout the study but collected one week per month to estimate a monthly value.Quantification of the difference in muscle strength (expressed in Newtons) by evaluation of the grip strength, using a dynamometer at weeks 14, 26 and 50 ± 1 week.Quality of life, functional capacities, pain and fatigue at weeks 14, 26 and 50 ± 1 week: a) Quality of life will be assessed by the difference over baseline on the score obtained on the PedsQL (Pediatric Quality of Life Inventory) tool (questionnaire and VAS). The PedsQL MCID is 4.4 points (Assessment of the feasibility of the program: The feasibility endpoint will focus on participation, completion, adherence, compliance, and watch use time. Participation is defined as the proportion of subjects who agree to participate in the study among eligible patients. Completion will be assessed by the proportion of subjects who complete the full program, and by the proportion of subjects who complete all the exercise sessions. Adherence is defined as the proportion of sessions attended by each subject. Compliance is the proportion of prescribed exercise actually performed during the session. These four criteria will be assessed during the period of the APA sessions, from week 2 to week 13. Finally, watch use is defined as the amount of time the pedometer watch was used over the duration of the study.