Experimental assessment of the effects of epimuscular myofascial force transmission in the sensory level

It has been shown that non-tendinous structures play a major role in force transmission: epimuscular myofascial force transmission. Such force transmission was shown to cause substantial strain distributions along muscle fibers indicating serial heterogeneity of sarcomere lengths. Recent studies showed evidence on sizable inter-antagonistic epimuscular myofascial force transmission. It is hypothesized in this study that epimuscular myofascial force transmission can play a role in afferent signals generated in muscle sensory organs. The goal of our present study was to test this hypothesis by measuring the afferent firing rates of antagonistic muscles of the lower leg. Gastrocnemius muscle of the frog (Rana ridibunda) was given 1-5 mm of ramp-and-hold stretch via a pulley mechanism connected to its distal tendon. Keeping the ankle and knee angles fixed (at 100° and 120°, respectively), sensory unit recordings were taken from both tibial and peroneal branches of sciatic nerve simultaneously: afferent signals generated from both the lengthened gastrocnemius muscle and the restrained antagonistic muscles were recorded. Remarkably, imposing passive stretch resulted in a significant increase in the firing rates of the units of not only the lengthened muscle, but also of the restrained antagonists (p<;0,05 n=12). This novel finding suggests that due to epimuscular myofascial force transmission, stretching of the target muscle causes local length changes sensed by the sensory organs within the fibers of the antagonistic muscles, despite being restrained. Our results therefore provide a preliminary support to our hypothesis and are likely to have major implications on our understanding of the functioning of muscular mechanoreceptors.