Sprinters’ legs: very muscular, still maneuverable

Previous studies have shown that sprinters have inhomogeneous muscle development, with well-developed hip flexors, extensors and muscles in the proximal femur. Muscle strength in sprinters corresponds well with the demands of the musculoskeletal system when sprinting, which can be a positive aspect in terms of strength-related factors. At the same time, however, the morphological differences would imply differences in mass distribution and thus maneuverability. More muscular legs would lead to greater mass and, in turn, to general speculation about the trade-off between greater torque-exertion capacity and greater difficulty in moving the lower limb.

This raises an interesting question: how do sprinters balance muscle strength and brisk movements? Contextually, not much is known about the maneuverability of sprinter legs from an inertial perspective. This knowledge gap caught the attention of a research team led by Professor Yasuo Kawakami of Waseda University’s Faculty of Sports Sciences, who, together with his colleagues Hoshizora Ichinose of Nihon University and Dr. Natsuki Sado of the University of Tsukuba examined the leg morphology of athletes. from an inertial perspective.

“As former athletes, we have been interested in the relevance of the human body in sport. Athletes train hard, but in many cases they do not know how their body specifically responds to training. We wanted to address this and help develop effective training strategies ,” says Kawakami, explaining the motivation behind their research.

In their study, the researchers analyzed fat- and water-separated MRI scans from 11 male sprinters, all trained athletes with careers lasting more than 7 years, and 12 male non-sprinters, individuals who had not undergone resistance training or practiced sports within the past two years at the time of the survey. By comparing the MRI features, they found that the sprinters had higher relative lower limb mass than the non-sprinters. This increase in muscle strength in turn promoted energy production while running. These findings have been published in Medicine & Science in Sports & Exercise. To understand the interplay between joint torque and segment maneuverability, the researchers further calculated the moment of inertia of the lower limbs around the hip. This comparison showed that moment of inertia did not differ between sprinters and non-sprinters, suggesting that increased muscle mass does not compromise lower limb maneuverability in sprinters.

When comparing different parts of the lower limbs between sprinters and non-sprinters, the researchers found that while the relative mass of the thighs differed between sprinters and non-sprinters, the relative mass of the shank and foot did not vary. Thus, the researchers conclude that these top-heavy, underlight features of sprinters’ limbs help them perform athletic movements with ease, judging from an inertia perspective.

This new study sheds light on the unique characteristics of sprinters’ lower limbs. The findings will be useful for the development of new training strategies. Based on their findings, what do the researchers suggest athletes and trainers do to train more effectively? Kawakami notes: “Sprinters can focus on training the muscles of their lower extremities without having to worry about the increased mass associated with sprint-induced lower extremity muscle strength and the resulting difficulty in moving due to a larger moment of inertia. This strategy will help balance running power and limb movements.”