Author(s): KUNIMASA, Y., OKUSHIMA, D., AMANO, T., KOGA, S., Institution: NIIGATA UNIVERSITY, Country: JAPAN, Abstract-ID: 1857

INTRODUCTION:
The muscle’s in vivo force-length relationship dictates the magnitude of muscle activation and metabolic cost required to achieve a given force (Fletcher & MacIntosh, 2017) . Muscle contraction at longer or shorter lengths than optimal length results in less isometric force (Gordon et al. 1966). On the other hand, the necessary level of activation and muscle metabolic cost can be minimized if the muscle is operating near optimal length. However, force production efficiency relative to metabolic cost and muscle activation in vivo in humans remains poorly understood. The purpose of this study was to determine the influence of ankle joint angle (muscle length) on muscle O₂ extraction and neuromuscular efficiency (NME) during sustained submaximal plantar flexion.
METHODS:
Nine males and nine females participated in this study. Participants were seated upright in an isometric dynamometer with the knee fully extended. Baseline measurements were obtained at rest, with the ankle positioned at 20° plantar flexion. Participants performed maximal voluntary isometric contractions (MVC) with the ankle in neutral position. Subsequently, nine 2-min isometric plantar flexion contractions were performed at three joint angles: 10° dorsiflexion (DF10), neutral position (NP), and 10° plantar flexion (PF10), and at three contraction intensities (10, 20, and 30% MVC) in a randomized order with 3-min rest between trials. We measured ankle plantar flexion torque and muscle activity in medial and lateral gastrocnemius (MG and LG) and soleus (SOL) using electromyography (EMG) to calculate NME (ankle plantar flexion torque/EMG). Muscle O₂ extraction of MG and SOL was estimated from deoxygenation of hemoglobin and myoglobin (deoxy [Hb+Mb]) by time-resolved near-infrared spectroscopy.
RESULTS:
Ankle plantar flexion torque was greater in NP than in PF10 at 20% and 30% MVC (p<0.05). NME tended to be highest at DF10, followed by NP and PF10 for LG (p<0.01) and SOL (p<0.05) across all contraction intensities. However, MG showed greater NME at DF10 than at PF10 only at 10% MVC (p<0.05). The increase in deoxy [Hb+Mb] was significantly smaller at DF10 than at NP and PF10, only at 30% MVC for MG (p<0.05, respectively).
CONCLUSION:
Ankle joint angle influenced muscle O₂ extraction and neuromuscular efficiency during isometric plantar flexion torque production. Shorter muscle length (PF10) was associated with greater muscle O₂ extraction and muscle activation compared with longer muscle length (DF10). However, muscle O₂ extraction and neuromuscular responses differed depending on the muscle and contraction intensity. Overall, these findings suggest that longer muscle lengths of the triceps surae would allow more efficient torque production relative to muscle O₂ extraction and muscle activity.