Vastus lateralis maximum force-generating potential occurs at optimal fascicle length regardless of activation level

Despite the fact that everyday movements are hardly ever performed with muscles contracting maximally, our understanding of the force-length relationship is mostly based on in vitro studies using maximal activation. In this study, the in vivo submaximal and maximal force-length relationships of vastus-lateralis were investigated. Force-length relationships were obtained based on maximal and submaximal levels of force and, also, on EMG activation.

Nine subjects performed isometric knee extensor contractions at ten knee angles (80°-170°). Knee extensor torque, and vastus-lateralis EMG and fascicle lengths were acquired simultaneously. Fascicle lengths and knee angles at peak force occurrence were compared across maximal and submaximal conditions.

The submaximal force-fascicle length relationships depend crucially on the approach used: in the force-based approach, peak forces are constrained to occur at the same MTU length and, because of series elasticity, occur at longer fascicle lengths for decreasing force levels. In contrast, in the activation-based approach, peak force occurrence is not constrained to a given muscle length for submaximal contractions and occurs at similar fascicle lengths but shorter MTU lengths (more extended knee angles) as force decreases.

Our results support the hypothesis that vastus-lateralis fascicle length for maximal force production is about constant for maximal and submaximal levels of activation, presumably taking advantage of optimal myofilament overlap at that fascicle length. This result implies that optimal vastus-lateralis lengths occur at different knee angles for different levels of activation, which is in stark contrast to findings in the literature in which submaximal force-fascicle length relationships were based on force rather than activation.