Lower limb in sprinters with larger relative mass but not larger normalized moment of inertia

Purpose Sprinters exhibit inhomogeneous muscularity corresponding to musculoskeletal demand for sprinting execution. An inhomogeneous morphology would affect the mass distribution, which in turn may affect the mechanical difficulty in moving from an inertia perspective; however, the morphological characteristics of sprinters from the inertia perspective have not been examined. Here we show no corresponding differences in the normalized mass and normalized moment of inertia between the sprinters and untrained non-sprinters. Methods We analyzed fat- and water-separated magnetic resonance images from the lower limbs of 11 male sprinters (100 m best time of 10.44–10.83 s) and 12 untrained non-sprinters. We calculated the inertial properties by identifying the tissue of each voxel and combining the literature values for each tissue density. Results The lower-limb relative mass was significantly larger in sprinters (18.7 ± 0.7% body mass) than in non-sprinters (17.6 ± 0.6% body mass), while the normalized moment of inertia of the lower limb around the hip in the anatomical position was not significantly different (0.044 ± 0.002 vs. 0.042 ± 0.002 [a. u.]). The thigh relative mass in sprinters (12.9 ± 0.4% body mass) was significantly larger than that in non-sprinters (11.9 ± 0.4% body mass), whereas the shank and foot relative masses were not significantly different. Conclusion We revealed that the mechanical difficulty in swinging the lower limb is not relatively larger in sprinters in terms of inertia, even though the lower-limb mass is larger, reflecting their muscularity. We provide practical implications that sprinters can train without paying close attention to the increase in lower-limb mass and moment of inertia.