Early structural adaptations to unloading in the human calf muscles

Aim: The present study investigated the influence of muscle architectural changes on muscle torque during 3-week unilateral lower limb suspension (ULLS). Methods: Plantarflexion maximal voluntary contraction (MVC), soleus (SOL), gastrocnemius medialis (GM) and lateralis (GL) muscle volume (VOL), GL fascicle length (Lf) and pennation angle (θ), physiological cross-sectional area (PCSA), and electromyographic (EMG) activity were assessed in eight healthy men (aged 19 ± 0 years) after days 14 and 23 of ULLS. Results: After 14 day of ULLS, MVC and SOL EMG decreased (P < 0.05) by 10% and 29%, respectively, but did not further decline between days 14 and 23. SOL, GM and GL muscle VOL decreased by 5%, 6% and 5%, respectively (P < 0.05), on day 14, and by 7% (SOL), 10% (GM) and 6% (GL) on day 23. In GL, θ and Lf were reduced by 3% (P < 0.05) and 2% (NS), respectively, on day 14, and by 5% (P < 0.05) and 4% (P < 0.05), respectively, on day 23. Consequently, GL PCSA declined by 3% (P < 0.05) on day 14, but did not further decrease on day 23. Similarly, the 7% (P < 0.05) loss in GL force/PCSA observed on day 14 persisted until the end of the unloading period. Conclusion: These findings suggest that rapid muscle architecture remodelling occurs with lower limb unloading in humans, with changes occurring within 14 days of weight bearing removal. These adaptations, mitigating the decline in muscle PCSA, might protect from a larger loss of muscle force. © 2008 The Authors