Humans are among the most capable long-distance runners in the animal kingdom. However, body shape and composition vary individually, reflecting adaptations to common movement patterns. In running, the extended lower limb swings back and forth.
A lower limb with a “top-heavy bottom-light” mass distribution is mechanically advantageous for swinging, facilitating economical locomotion. This prompts the question: Do habitual runners acquire limbs that are easier to swing through adaptive processes, or do humans possess an inherent ease of lower-limb swing irrespective of fat mass?
In the study published in the Journal of Biomechanics, researchers at University of Tsukuba employed magnetic resonance imaging to construct detailed three-dimensional models of the internal structures—bone, muscle, and fat—of the lower limb of height-matched male runners and non-runners. Then, they precisely calculated the mass and ease of rotation (i.e., moment of inertia) of the lower limbs.
The results showed that although runners had lower lower-limb mass than non-runners, the decrease in moment of inertia was not proportional to the mass reduction.
The finding implies that “the lower limbs of non-runners can more easily swing than expected given their higher body mass.”
In essence, the human body appears to possess a built-in mechanism that preserves running economy by maintaining the ease of lower-limb swing, even with higher body mass. This could be a basic human trait that enables our species to travel economically over long distances.
These findings offer new insights into the structure and function of human legs and may contribute to a deeper understanding of human locomotion.
More information:
Takeshi Edagawa et al, 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, Journal of Biomechanics (2025). DOI: 10.1016/j.jbiomech.2025.113012
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