The self-administered Bath Ankylosing Functional Index (BASFI) measures functional consequences of ankylosing spondylitis (AS). The patient indicates its level of ability in ten domains of daily life: put the socks, bend to pick a pen from the floor, reach up to a high shelf, get up from a chair and from the floor, stand unsupported for 10 minutes, climb steps, look over the shoulder, do physically demanding activities and do a full days activities. Each question is answered on a 10 cm horizontal visual analog scale, the mean of which gives the BASFI score (0-10). Higher is the score, higher are the functional consequences of AS.

Gait pattern will be captured by Physilog sensors (Gait Up ®) and Opal sensors (APDM®) during a 10-m walk test in single and dual task, the iTUG and the i6MWT.

Patients will fill in the self-assessment questionnaire: International Physical Activity Questionnaires (IPAQ) to measure health-related physical activity. Data will be summarized according to each physical activity domain (walking, moderate and vigorous activities) and estimated time spend sitting per week. Then data will be used to estimate Metabolic Equivalent of Task (MET) energy expenditure assigned to each category.

Patients will fill in the self-assessment questionnaire: Short Form-36 (SF-36) to measure quality of life. Eight domains will be assessed : physical activity, daily life limitations, physical pain, health, social life, psychological health, psychological limitations, health evolution.

Patients will fill in the self-assessment questionnaire: Epworth score for evaluation of daytime sleepiness. Patients wil rate the chances that they would doze off or fall asleep while in eight different situations, from no chance to doze or sleep (0) to strong chance to sleep (3).

Patients will fill in the self-assessment questionnaire: Pichot score for evaluation of daytime tiredness. This scale assess the importance of tiredness to carry out daily activities.Patients rate how the eight proposals correspond to their feeling, from not at all (0) to extremely (4).

Lower limb range of motion will be assessed using a goniometer in each joint.

Foot function will be assessed with the validated and commonly used Foot Function Index. Participants will rate 0 to 10 on a numeric scale, in 3 domains concerning feet, i.e. pain, disability and activity restriction. The score is out of 230 and translated out of 100. Higher scores are related to worse outcome.

The instrumented six minute walk test (i6MWT) will be performed in a 30-m corridor.

Patients will performed the iTUG : they have to stand up from a chair, walk 7 meters, turn and go back to the chair, as fast as possible

The validated Work Productivity and Activity Impairment scale (WPAI) will be assessed to evaluate absenteeism, presenteeism, overall work productivity loss and activity impairment. WPAI outcomes are expressed as impairment percentages, with higher numbers indicating greater impairment and less productivity, i.e., worse outcomes.

Smoking status will be assessed with the calculation of number of pack-year smoked. It is calculated by multiplying the number of packs of cigarettes smoked per day by the number of years the person has smoked.

Smoking status will be assessed with the validated and commonly used Fagerström index, giving a score from 0 to 10, with high scores related to high dependence

Social support will be assessed with the validated and commonly used Medical Outcome Study (MOS) Social Support Survey in French. The survey consists of four separate social support subscales and an overall functional social support index. A higher score for an individual scale or for the overall support index indicates more support.