Mechanical output following muscle stretch in forearm supination against inertial loads

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Abstract
Muscle stretch enhances force produced in both single fibers and voluntarily activated human muscle. This study determined how initial conditions of muscle stretch (and associated eccentric work), muscle length, and load inertia contributed to human concentric muscular output during maximal voluntary forearm supination. Outputs of angular velocity and concentric work over specific displacements and times of motion were calculated. Multiple regression analysis was performed using these outputs and initial conditions as dependent and independent variables, respectively. Initial conditions were shown to be significant and systematic determinants of muscle output in concentric contraction. Evidence of a temporary shift in the force-velocity curve was found and discussed regarding its beneficial contribution to load movement. Greater benefit was considered to be due to the fact that muscle stretch allows time for achievement of maximal muscular recruitment prior to concentric contraction. This produces large forces at the onset of the concentric phase, in comparison with contractions starting from rest. These findings were discussed with regard to both single- and multi-segment movement patterns.