Inhomogeneous muscle hypertrophy in response to daily motor executions is a morphological adaptation in humans. Inhomogeneous morphological adaptation would change the mass distribution, which in turn may affect the mechanical difficulty in moving from an inertia perspective; however, the morphological adaptation from the inertia perspective has not been examined. Here, we show no corresponding differences in the normalized mass and normalized moment of inertia between the sprinters and the untrained non-sprinters. We analyzed the 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. 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 similar between them (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 similar. We revealed that the adaptation to sprinting in humans does not relatively increase the mechanical difficulty to swing the lower limb in terms of inertia, even though the lower-limb mass increases, reflecting muscular hypertrophy. This is suggested to be one of the significant advantages of human morphology for adaptability to explosive motor tasks.

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 nonsprinters. 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 nonsprinters. 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 nonsprinters (17.6% ± 0.6% body mass), whereas 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 nonsprinters (11.9% ± 0.4% body mass), whereas the shank and foot relative masses were not significantly different. Conclusions: 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.

followed two weeks of active or placebo treatment at home, followed by post-treatment measurements. Then participants had two weeks with no intervention or placebo to wash-off any potential effects, followed by the repetition of the procedure with the other active or placebo device, again followed by two weeks of washout. The Wilcoxon signed-rank test was used to compare the change in pre to post-treatment measurement within subjects and between the active treatment and placebo. Values are mean (SD) in N. RESULTS: Under active treatment, strength significantly improved compared to the baseline (Pre,162.64 (37.51); Post,185.56 (33.95), p=0.01) and compared to the placebo (Active,22.64 (24.01); Placebo,-4.12 (32.24), p=0.02). No significant difference in strength was reported with placebo (p=0.68). Endurance time did not change significantly with either active treatment (p=0.52) nor placebo (p=0.85). CONCLUSIONS: Two-week administration of PBMT may improve muscle strength in persons with mild-moderate MS.

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