Effectiveness Index of Mechanical Energy Utilization in Male 400-m Sprinters and the Relation Between Muscle Cross-Sectional Area of the Trunk and the Lower Limb.

The purpose of this study was to identify the morphological characteristics of trunk and lower limb muscles associated with 400-m sprint performance and "effectiveness index of mechanical energy utilization (EI)." Twelve male 400-m sprinters (age: 21.4 ± 1.7 years, 400-m best time: 48.38 ± 1.80 s) participated in this study. Using a 3-T magnetic resonance imaging system, we measured muscle cross-sectional area (CSA) of the key trunk and lower limb muscles, including the rectus abdominis, lateral abdominal wall, erector spinae, psoas major, adductors, quadriceps femoris, hamstrings, dorsiflexors, and plantar flexors. The CSA of the trunk muscles was summed bilaterally, while the CSA of the lower limb muscles was assessed on the right leg. In addition to absolute CSA, relative CSA, normalized by the two-thirds power of lean body mass, was used in the analysis to account for individual body size differences. Participants completed a 400-m sprint on an official outdoor track, with running motion in early phase (around the 160-m point) analyzed using high-speed video cameras. Our findings indicate that the specific muscle groups' relative CSA, particularly the lateral abdominal wall and adductors, were significantly correlated with 400-m sprint time (r = -0.604 to -0.748, p = 0.005-0.038) and EI (r = 0.598-0.599, p = 0.040). Additionally, the thigh-to-lower leg CSA ratio was significantly correlated with 400-m sprint time (r = -0.643, p = 0.024) and EI (r = 0.577, p = 0.049). These results suggest that the relative size of proximal muscles plays a crucial role in economical sprinting in 400-m sprinters. Thus, optimizing the development of proximal muscles relative to distal muscles may enhance 400-m sprint economy, providing a valuable reference for designing training programs focused on economical running techniques.