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 (<i>L</i>_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 0.85<i>L</i>_o, <i>L</i>_o, and 1.15<i>L</i>_o in a random order, cyclically targeting 50% of the maximal force at 0.85<i>L</i>_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 0.85<i>L</i>_o (<i>P</i> = 0.005) compared with <i>L</i>_o. Mean EC at 1.15<i>L</i>_o (2 ± 27% lower) was similar to that at <i>L</i>_o (<i>P</i> = 0.81), despite lower forces at 0.85<i>L</i>_o (<i>P</i> = 0.02), and similar absolute fascicle shortening (<i>P</i> = 0.10), and shortening velocity (<i>P</i> = 0.52). Muscle activity was approximately twofold higher at 0.85<i>L</i>_o (<i>P</i> = 0.001). The EC per unit activation was similar across lengths (<i>P</i> = 0.45), whereas the EC per unit force was significantly higher at 0.85<i>L</i>_o compared with <i>L</i>_o and 1.15<i>L</i>_o (<i>P</i> = 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.<b>NEW & NOTEWORTHY</b> Cyclically producing force performed at short muscle-tendon unit lengths incurs a higher metabolic cost of contraction compared with those contractions performed at optimal or long muscle-tendon unit lengths, respectively. We confirm this through direct observation of muscle fascicle length and muscle energy cost measurements. We further offer a unique and novel explanation for why muscles consume more energy at short compared with optimal or long initial muscle-tendon unit lengths.