Relationships between cross sectional area of lower limb muscles and forward velocity in underwater undulatory swimming

This study aimed to investigate the relationship between cross-sectional area (CSA) of lower limb muscles and UUS velocity on swimmers.Eighteen male swimmers at the regional to national level participated in this study.Kinematic data during maximum-effort UUS were collected using an underwater motion capture system to determine the mean UUS velocity.T1-weighted cross-sectional magnetic resonance (MR) images of the lower limb muscles were acquired using a 3T MR imaging system to determine the CSA of 17 muscles on the swimmers' right side.Relationships between UUS velocity and the CSA of lower limb muscles were examined using correlation coefficients.A positive significant correlation was found between CSA of rectus femoris and UUS velocity (absolute value: r = 0.475 p = 0.046, relative value: r = 0.548 p = 0.019).However, UUS velocity was not significantly correlated to CSA of other lower limb muscles (r = -0.445~ 0.469).These results suggest that the CSA of rectus femoris has a moderate relationship between UUS velocity among the lower limb muscles, regardless of the body size.Revealing the relationships between muscle size and UUS velocity can provide an important insight regarding which lower limb muscles should be strengthened to enhance UUS performance for swimmers, coaches and researchers.