To what extent do the muscles and tendons influence metabolic cost and exercise tolerance in the hypermobile Ehlers-Danlos Syndrome and Hypermobility Spectrum Disorders?

BACKGROUND: Individuals with hypermobile Ehlers-Danlos Syndrome (hEDS) and Hypermobility Spectrum Disorders (HSD) often experience chronic pain, muscle fatigue, and exercise intolerance, potentially due to altered muscle-tendon mechanics. This study investigated the influence of Achilles tendon (AT) compliance and plantar flexor muscle function on the metabolic cost of walking in individuals with and without HSD/hEDS. METHODS: Eleven individuals with HSD/hEDS and 11 age- and sex-matched controls completed walking trials at, below, and above their preferred walking speed. Achilles tendon stiffness, medial gastrocnemius muscle shortening, AT energy storage, pain, and lower limb electromyographic activity were evaluated during stance phase. The energy cost of walking was also computed. FINDINGS: The hypermobile conditions were associated with significantly lower AT stiffness, higher energy cost of walking and increased pain. Muscle fascicle shortening, shortening velocity, muscle energy cost and mechanical efficiency were similar between groups. Greater muscle activation and antagonist coactivation were observed in HSD/hEDS, particularly during early stance, likely reflecting compensatory mechanisms for joint instability. INTERPRETATION: Elevated cost of walking in HSD/hEDS appears driven not by increased plantar flexor work, but by a redistribution of joint work to more proximal joints. These findings suggest that altered muscle-tendon properties and neuromuscular control strategies contribute to exercise intolerance and fatigue in the Hypermobility Spectrum Disorders and hypermobile Ehlers-Danlos Syndrome.