The energy cost of cyclic muscle contractions at different initial muscle-tendon unit lengths derived from near-infrared spectroscopy

During locomotion, the plantarflexor muscle fascicles appear to operate on the ascending limb of its maximal force-length relationship, producing less force per unit activation and elevating the energy cost (EC) compared with contractions performed at optimal length (Lo). The EC of the medial gastrocnemius muscle was quantified at different initial muscle-tendon unit lengths by having participants perform 30 submaximal fixed-end contractions at an ankle joint angle associated with 0.85Lo, Lo, and 1.15Lo in a random order, cyclically targeting 50% of the maximal force at 0.85Lo. EC was quantified from near-infrared spectroscopy during blood flow occlusion and EMG quantified MG muscle activity. Mean EC was 36 ± 24% higher at 0.85Lo (P = 0.005) compared with Lo. Mean EC at 1.15Lo (2 ± 27% lower) was similar to that at Lo (P = 0.81), despite lower forces at 0.85Lo (P = 0.02), and similar absolute fascicle shortening (P = 0.10), and shortening velocity (P = 0.52). Muscle activity was approximately twofold higher at 0.85Lo (P = 0.001). The EC per unit activation was similar across lengths (P = 0.45), whereas the EC per unit force was significantly higher at 0.85Lo compared with Lo and 1.15Lo (P = 0.008). Together, these results demonstrate a significant increase in the cost of cyclically producing force at short initial muscle-tendon unit lengths, due to a lower force potential at that initial muscle fascicle length, which we surmise is a result of a lower cross-bridge force and not a higher energetic cost of activation.