Effects of long-term spaceflight on mechanical properties of muscles in humans

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Abstract
The effects of long-term spaceflight (90–180 days) on the contractile and elastic characteristics of the human plantarflexor muscles were studied in 14 cosmonauts before and 2–3 days after landing. Despite countermeasures practiced aboard, spaceflight was found to induce a decrease in maximal isometric torque (17%), whereas an index of maximal shortening velocity was found to increase (31%). In addition, maximal muscle activation evaluated during isokinetic tests decreased by 39%. Changes in musculotendinous stiffness and whole joint stiffness were characterized by means of quick-release movements and sinusoidal perturbations. Musculotendinous stiffness was found to be increased by 25%. Whole joint stiffness decreased under passive conditions (21%), whereas whole joint stiffness under active conditions remained unchanged after spaceflight (−1%). This invariance suggests an adaptive mechanism to counterbalance the decrease in stiffness of passive structures by an increased active stiffness. Changes in neural drive could participate in this equilibrium.

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