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

The effects of long-term spaceflight (90-180 days) on the contractile and e lastic characteristics of the human plantarflexor muscles were studied in 1 4 cosmonauts before and 2-3 days after landing. Despite countermeasures pra cticed aboard, spaceflight was found to induce a decrease in maximal isomet ric 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 a nd whole joint stiffness were characterized by,means of quick-release movem ents and sinusoidal perturbations. Musculotendinous stiffness was found to be increased by 25%. Whole joint stiffness decreased under passive conditio ns (21%), whereas whole joint stiffness under active conditions remained un changed after spaceflight (-1%). This invariance suggests an adaptive mecha nism to counterbalance the decrease in stiffness of passive structures by a n increased active stiffness. Changes in neural drive could participate in this equilibrium.