Meeting Abstract
Intrinsic properties enable muscles to change force output in response to changes in load, length and velocity instantaneously, without feedback from the nervous system. Muscle force increases with active stretching and decreases with active shortening. The force velocity relationship describes how muscle force decreases with increasing shortening velocity, up to maximum shortening velocity, Vmax. The force-velocity relationship is typically attributed to the attachment and detachment kinetics of the cross bridges, but is often only studied in supra-maximally activated muscles in after-loaded isotonic contractions. After-loaded isotonic contractions remove the contribution of the series elastic elements, theoretically permitting the study of active cross bridges alone. This relationship can also be obtained from experiments in which muscles experience isovelocity or cyclical length changes that include series and parallel muscle elasticity. The goal of the present study was to investigate how the force velocity relationship of muscles varies with the changes in strain trajectories. Soleus muscles from mice were isolated and attached to a force lever to measure muscle force and length. Muscles were stretched and shortened under isovelocity, isotonic or cyclical strain trajectories over a range of lengths from ±2% to ±10% of optimum muscle length (L0). We fit all force-velocity curves in the shortening domain to a modified Hill equation. Data suggest that maximum shortening velocity and power are highest in maximally stimulated muscles undergoing cyclical contractions. Afterloaded isotonic contractions showed an intermediate maximum power. Predicted Vmax was similar in under isotonic and isovelocity contractions. This study demonstrates that the force-velocity relationship changes depending on strain trajectories.
