Does increasing the dose of relaxation strain augment the sarcomerogenic response to isometric resistance exercise?

Abstract

Increases in serial sarcomere number (SSN) have been suggested to reduce muscle strain injury risk and improve muscle mechanical performance, however the environmental loading conditions optimizing sarcomerogenesis are not well known. It has been suggested that the magnitude of lengthening which fibres are subject to upon deactivation (i.e., the relaxation strain) may provide a stimulus for sarcomerogenesis. However, no study has attempted to amplify this relaxation strain stimulus to directly test this possible mechanism. Sprague-Dawley rats underwent six weeks (twice weekly) of isometric plantar flexion training, either sustained near-maximal contractions, or intermittent burst type contractions (i.e., contract-relax) in which each repetition consisted of eight brief high-frequency trains to magnify the overall incidence of relaxation strain occurrences across the training period. Groups were otherwise matched for peak torque and torque-time integral. Following training, the torque-angle relationship adapted similarly between groups (training x group interaction P=0.249), with active torque increasing ~30-40% across the angles tested (P<0.001). No changes in maximal shortening velocity were observed with training in either group (P=0.577), however peak power output increased by ~19% in Isometric and ~49% in Burst (training effect P=0.002; interaction P=0.152). For SSN, the gastrocnemius showed a divergent training response (Lateral gastrocnemius: Burst +~11%; Isometric +~2%; interaction P=0.001; Medial gastrocnemius: Burst +~9%; Isometric +~3%; interaction P=0.052), while soleus SSN increased similarly between groups (~2-3%) (training effect P=0.029, interaction P=0.543). Overall, amplifying the relaxation strain stimulus may have a positive effect on stimulating sarcomerogenesis, however further research is needed to clarify the extent of this effect, as well as any underpinning mechanisms.