LAUSR

During locomotion, force-producing limb muscles are predominantly responsible for an animal's whole-body metabolic energy expenditure. Animals can change the length of their force-producing muscle fascicles by altering body posture (e.g., joint angles), the structural properties of their biological tissues over time (e.g., tendon stiffness), or the body's kinetics (e.g., body weight). Currently, it is uncertain whether relative muscle fascicle operating lengths have a measurable effect on the metabolic energy expended during cyclic locomotion-like contractions. To address this uncertainty, we quantified the metabolic energy expenditure of human participants as they cyclically produced two distinct ankle moments at three ankle angles (90°, 105°, 120°) on a fixed-position dynamometer using their soleus. Overall, increasing participant ankle angle from 90° to 120° (more plantar flexion) reduced minimum soleus fascicle length by 17% (both moment levels, p<0.001) and increased metabolic energy expenditure by an average of 208% across both moment levels (both p<0.001). For both moment levels, the increased metabolic energy expenditure was not related to greater fascicle positive mechanical work (higher moment level, p=0.591), fascicle force rate (both p≥0.235), or model-estimated active muscle volume (both p≥0.122). Alternatively, metabolic energy expenditure correlated with average relative soleus fascicle length (r=-0.72, p=0.002) and activation (r=0.51, p<0.001). Therefore, increasing active muscle fascicle operating lengths may reduce metabolic energy expended during locomotion.