Background: For people diagnosed with a spondyloarthritis (SpA) e.g. ankylosing spondylitis or undifferentiated spondyloarthritis, physical activity and exercise are important components in the self-management. Exercise, in addition to physical and mental symptoms related to the disease can easily feel overwhelming to exercise, and low adherence may result. By studying the effects of high-intensity interval training (HIIT) in comparison with training as usual on physiological, inflammatory, and self-reported disease parameters in patients with SpA, we intend to further investigate the short-term and longitudinal training effects, and refine the knowledge to tailor, coach, and stimulate to self-performed HIIT.

Objective: The purpose of this study is to investigate the short- and long-term effects of high- intensity interval training (HIIT) on physiological, inflammatory, and self-reported health parameters in patients with SpA. The aim is also to study the adherence to physical activity and exercise recommendations.

Design: A randomized controlled trial (RCT) design. Participants: One hundred adults with a confirmed axial SpA from rheumatology clinics in southern Sweden will be recruited and randomized into two groups, the intervention group and the control group. Intervention: Three high-intensity training sessions per week for three months, of which two interval sessions, with coaching both from a clinical physiotherapist and digital devices (watch and app) followed by nine months with sporadic coaching. The control group will go on with exercise as usual. Primary outcome: Self-reported disease activity, inflammatory biomarkers (acute phase proteins). Secondary outcomes: Physical fitness (aerobic capacity (VO2max), blood pressure, grep strength), body composition and self-reported physical function, health status, well-being, pain, fatigue, adherence to physical activity and exercise recommendations, and confidence in one's own ability to manage pain, symptoms and high-intensity exercise, and additional serum biomarkers.