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 to contractions performed at optimal length (L o ). 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 0.85L o , L o , and 1.15L o in a random order, cyclically targeting 50% of the maximal force at 0.85L o . EC was quantified from near-infrared spectroscopy during blood flow occlusion and EMG quantified MG muscle activity. Mean EC was 36±24% higher at 0.85L o ( p=0.005) compared to L o . Mean EC at 1.15L o (2±27% lower) was similar to that at L o (, p=0.81), despite lower forces at 0.85L o ( p=0.02), and similar absolute fascicle shortening ( p=0.10), and shortening velocity ( p=0.52). Muscle activity was ~2-fold higher at 0.85L o ( 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 0.85L o compared to L o and 1.15L o ( 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 crossbridge force and not a higher energetic cost of activation.