Proximal-specific reduced mass of lower limbs in male endurance runners does not result in improved mechanical ease of leg swing in proportion to reduced mass

Ease of leg swinging is a cost factor for locomotion. Endurance runners have lighter lower limbs, leading to speculation regarding the ease of leg swinging in their lower limbs. However, since lower-limb mass distribution exhibits inter-individual variability, lower-limb ease of leg swing may not be proportional to reduced lower-limb mass. Here we show that the difference in ease of leg swing between 24 male runners and 18 height-matched controls was not as large as the difference in lower-limb mass. We calculated the lower-limb moment of inertia around the hip joint (I_LowerLimb^Hip) as an index of the ease of leg swing based on fat- and water-only magnetic resonance images as well as information regarding the tissue of each voxel. I_LowerLimb^Hip and lower-limb mass were significantly smaller in runners than controls, but the inter-group difference in I_LowerLimb^Hip (90 % of controls, d = 0.82) was not as large as that in mass (83 % of controls, d = 1.57). The inter-group difference in mass was greater in fat (d = 2.13) than in other tissues (d = 0.71-1.03). The inter-group difference in fat mass was greater in the thigh (d = 2.23) than in other segments (d = 1.57-1.60). We revealed that the proximal-specific reduced mass of the lower limbs in endurance runners did not lead to proportionally improved ease of movement. The current findings in endurance runners, coupled with the heavy but not difficult movement of lower limbs of sprinters in a previous study, suggest that ease of leg swing in humans is relatively invariant to differences in lower-limb mass.