Axial spondyloarthropathy (SpA) is the most common inflammatory rheumatism (1% of the general population) with important medico-economic consequences.

Fatigue is a major feature of SA. It can be defined as a feeling of reduced muscle capacity, lack of energy and exhaustion. The fatigue reaches an abnormally high level (fatigue severity score (FSS) ≥4, called severe fatigue in this protocol) in more than two thirds of patients with SA.

Skeletal muscle repercussions are present during SA. It is characterized by a decrease in exercise capacity independently of pain and ankylosis but is associated with a decrease in strength and muscle mass, the importance of which varies from one study to another.

The link between fatigue (subjective sensation) and the skeletal muscular impact (objective) of SA has never been studied.

This work will:

1. Study for the first time the implication of the objective skeletal muscular impact in fatigue during SA.
2. Better characterize the skeletal muscle impact of SA;
3. Characterize the evolution of the fatigue and the muscular impact under targeted therapy

Inclusion criteria:

• Axial SA according to the ASAS criteria;
• Targeted therapy naïve patients
• Indication to start a targeted therapy;
• Initiation of targeted therapy ≤ 15 days before inclusion
• ≥ 18 years old, no upper age limit;
• Subject affiliated to a social health insurance reimbursement;
• Subject able to understand the aims and risks of the research and having signed a dated and informed consent
• Subject informed of the results of the preliminary medical examination
• Woman in childbearing age: negative beta-HCG test and effective contraception;
• Sufficient understanding of French to follow the protocol.

Exclusion criteria:

• Targeted therapy in progress for > 15 days prior to inclusion
• Contraindication to the use of targeted therapy
• Systemic corticosteroids in the 15 days preceding the V0 visit
• Associated extramuscular inflammatory disease (s) (excluding ocular and cutaneous involvement) eg chronic inflammatory bowel disease
• Associated fibromyalgia (Questionnaire score FiRST ≥5) achieved during the V0 visit
• History of coronary artery disease: exercise angina, acute coronary syndrome and / or coronary angioplasty,
• History of lower extremity arterial disease: vascular claudication and / or lower extremity angioplasty
• COPD
• Neuromuscular pathology
• Insufficiency of organ (renal, hepatic pulmonary heart)
• Sleep apnea
• Impossibility of giving the subject informed information (subject in emergency situation, difficulties in understanding the subject, etc.)
• Subject under the protection of justice
• Subject under guardianship or curatorship
• Breastfeeding
• Pregnancy
• Subject in exclusion period defined by another clinical study or participating in a study likely to impact the results of the research