Niigata University

How do we humans move our bodies? Electrical signals sent from the brain are transmitted to the muscles, where force is generated. The force exerted by the muscles is then transmitted to the bones via tendons, causing the joints to move. Ultimately, the force exerted at the joints acts as a rotational force on the ground, objects, water, and other surfaces, enabling us to perform physical movements such as walking, running, jumping, throwing, and swimming.

Physical performance is ultimately determined by the force exerted at the joints and the energy required to generate that force. These are made possible through the intricate coordination of the structure and function of the skeleton, muscles, and tendons. For example, the gastrocnemius muscle (the calf muscle), which plays a crucial role in force generation during running, has a feather-like structure and because the muscle fibers attach at an angle to the tendon, there is an increased number of muscle fibers within the muscle, thereby efficiently increasing the force the muscle can generate. Furthermore, the gastrocnemius is attached to the Achilles tendon, which is very long and does not consume energy. The interaction between the muscle and the tendon allows us for efficient muscle force production during walking and running. Understanding those mechanisms of the human body is incredibly fascinating and truly brings home just how well-designed our bodies are.

Ultrasound image of the gastrocnemius muscle (calf muscle), one of the feather-like muscles The yellow lines indicate “muscle fascicle,” which are groups of muscle fibers. By attaching to the tendon (red line) at an angle, there is an increase in the number of fibers within the muscle, thereby efficiently increasing the force the muscle can exert.

Let me introduce one of my previous research projects. Have any of you watched the Summer Olympics or world track and field championships? If you focus on middle- and long-distance running events (such as the marathons and the 1,500-meter run), you’ll likely notice that most of the medalists are from East Africa (such as Kenya and Ethiopia). Why do athletes from a specific region dominate certain events? Why do East African athletes demonstrate such phenomenal ability in middle- and long-distance running? Our research group tried to uncover the secret by focusing on the muscle-tendon structure and functions of their lower legs (from the knee down) and directly measuring physiological data using techniques such as ultrasound and surface electromyography. Our findings revealed that East African runners possess short, small calf muscles,long Achilles tendons, and andAchilles tendon moment arm, and during running, they utilize these muscular, tendinous, and skeletal characteristics to generate effective ankle joint torque while minimizing muscle activity. This study is the first in the world to demonstrate that East African middle-and-long-distance runners enhance their performance by leveraging the structural and functional characteristics of their muscles, tendons, and skeleton to enable efficient force generation at the joints (Related paper: Kunimasa et al. Scand J Med Sci Sports. 2014; Sano et al. Eur J Appl Physiol. 2015; Kunimasa et al. Acta Physiol. 2022; Kunimasa et al. J Physiol Anthropol. 2023) .

Since I began studying human movement from a neuromuscular perspective using vivo imaging techniques, I’ve come to realize just how fascinating and sophisticated the human body is. I believe that unraveling the mechanisms behind human movement will not only allow us to push the limits of our physical capabilities but also improve the efficiency of our daily physical activities, help us recover our physical abilities and stamina from injuries and illnesses, and ultimately enrich our daily lives. I hope to continue unraveling the mechanisms behind the human body’s intricate movements and generate insights that will enrich our lives.

Ph.D. (Sports Science). Specializes in biomechanics. After completing the Ph.D. program at the Graduate School of Sports Science at the Osaka University of Health and Sport Sciences, then serving as a postdoctoral researcher at Aix-Marseille University, she has been an Assistant Professor in the Academic Assembly Institute of Humanities and Social Sciences -(Faculty of Education) and the Faculty of Engineering at Niigata University since July 2022. She was selected as faculty for the Niigata University Young Faculty Swing-By Program (2nd cohort).

*Article content and profile information are current as of July 2026.

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