Spondylarthropathies (SpA) are among the most common chronic inflammatory rheumatisms in adults, responsible for a muscular loss or sarcopenia which can be very disabling, not much documented in the literature, and not yet studied in France. In addition, sarcopenia is associated with decreased physical activity and increased risk of falls and fractures. A better characterization of the determinants of this muscular loss will allow to better detect and take care of it.