Impairments in muscle shape changes affect metabolic demands during in-vivo contractions

The uncoupling behaviour between muscle belly and fascicle shortening velocity (i.e. belly gearing), affects mechanical output by allowing the muscle to circumvent the limits imposed by the fascicles' force-velocity relationship. However, little is known about the 'metabolic effect' of a decrease/increase in belly gearing. In this study, we manipulated the plantar flexor muscles' capacity to change in shape (and hence belly gearing) by using compressive multidirectional loads. Metabolic, kinetic, electromyography activity and ultrasound data (in soleus and gastrocnemius medialis) were recorded during cyclic fixed-end contractions of the plantar flexor muscles in three different conditions: no load, +5 kg and +10 kg of compression. No differences were observed in mechanical power and electrophysiological variables as a function of compression intensity, whereas metabolic power increased as a function of it. At each compression intensity, differences in efficiency were observed when calculated based on fascicle or muscle behaviour and significant positive correlations (R2 range: 0.7-0.8 and p > 0.001) were observed between delta efficiency (ΔEff: Effmus-Efffas) and belly gearing (Vmus/Vfas) or ΔV (Vmus-Vfas). Thus, changes in the muscles' capacity to change in shape (e.g. in muscle stiffness or owing to compressive garments) affect the metabolic demands and the efficiency of muscle contraction.