Neural and mechanical properties of vastus lateralis and vastus medialis at different rectus femoris muscle lengths

This study investigated how altering the length of one muscle influences motor unit discharge behavior of its synergists. Eighteen participants performed isometric knee extensions at 10% and 30% of maximal voluntary contraction (MVC) with the hip joint at 90{degrees} (shortened rectus femoris, RF) and 180{degrees} (lengthened RF). High-density surface electromyograms were recorded from vastus medialis, RF and vastus lateralis, and decomposed into motor unit spike trains. Mean discharge rate and coefficient of variation of interspike interval were analyzed for tracked units in the vasti and non-tracked in the RF. While no changes were observed in RF motor units, lengthening the RF led to increased discharge rates of vasti motor units at 10% MVC, but not 30% MVC. To further explore these force-dependent changes, two further experiments were conducted. The first showed that the discharge rate at recruitment during ramp-up contractions increased with RF lengthening, but only for low-threshold vasti units. In the second, electrically evoked twitch contractions in the vasti revealed significantly reduced twitches at 180{degrees} during low-frequency, but not high-frequency stimulation. These findings collectively suggest that the force-dependent changes in the vasti motor unit discharge rates are likely driven by RF-length dependent changes in the vasti muscles contractile properties.