Fascicle gearing dynamics: Unveiling 3D rotation effects in muscle elongation

Detailed insights into muscle and tendon movement mechanisms during stretching are essential to improve our overall mobility and flexibility. It is not only important for optimum athletic performance, but also crucial for preventing musculoskeletal injuries. When an individual stretches, 50% to 70% of the elongation is absorbed into the muscle belly, i.e., the fleshy part of the muscle containing most fibers.

However, in skeletal muscles with fascicles, the muscle fibers are shorter than the muscle belly and attach to the tendon at an angle. This angle between the fascicles and the tendon changes in response to the length of the muscle belly through a mechanism known as ‘gearing’. However, despite advancements in tissue-imaging technologies, our understanding of these geometrical dynamics has been confined to a 2D perspective.

Against this backdrop, a team of researchers led by Dr. Yasuo Kawakami from Waseda University, including Dr. Katsuki Takahashi from Doshisha University, Dr. Hiroto Shiotani from Waseda University, Dr. Pavlos E. Evangelidis from the University of Exeter, and Dr. Natsuki Sado from the University of Tsukuba embarked on an innovative research project, the findings of which were published in Volume 55, Issue 11 of Medicine & Science in Sports & Exercise® in November 2023. Their goal was to better understand the three-dimensional dynamics of fascicles during stretching, particularly in pennate muscles.

The team used diffusion tensor imaging to reconstruct the fascicles of 16 healthy adults in three dimensions. They specifically measured changes in fiber length and angles in both the sagittal and coronal planes during passive ankle dorsiflexion. Dr. Kawakami explains, “The effect is somewhat like what happens upon opening a book. As you open a book, the page angle changes – they fan out – but the pages themselves do not get longer.” Similar to this, as the muscle stretches, the fibers change their angle, allowing the muscle to extend, without any significant change in the length of each fiber.

This gearing mechanism not only contributes to the overall elongation of the muscle but reduces the elongation of individual fascicles at any given time, preventing them from overstretching and getting injured. “These findings challenge and extend our current understanding of how muscles work, and they underscore the value of diving into the finer details of familiar territory, as this can lead to unexpected and intriguing results,” adds Dr. Kawakami.

Comprehensive knowledge of how muscles naturally adapt during stretching can help us develop better techniques and practices in sports training and physical therapy. Additionally, by understanding the limits and capabilities of muscle stretching, we can gain insights into how to train more effectively and safely. This can help prevent injuries and aid in the rehabilitation of muscle-related injuries.

Going forward, the team plans to extend their research to other muscles, each with their own unique structural characteristics to further enhance our understanding of human muscle movement.

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Reference

DOI: https://doi.org/10.1249/MSS.0000000000003229

Authors: Takahashi, Katsuki^1,2,3; Shiotani, Hiroto^4,5; Evangelidis, Pavlos E.⁶; Sado, Natsuki⁷; Kawakami, Yasuo^4,5

Affiliations      

¹Faculty of Health and Sports Science, Doshisha University, Kyoto, JAPAN

²Graduate School of Sport Sciences, Waseda University, Saitama, JAPAN

³Japan Society for the Promotion of Science, Tokyo, JAPAN

⁴Faculty of Sport Sciences, Waseda University, Saitama, JAPAN

⁵Human Performance Laboratory, Comprehensive Research Organization of Waseda University, Tokyo, JAPAN

⁶Faculty of Health and Life Sciences, University of Exeter, Exeter, UK

⁷Institute of Health and Sport Sciences, University of Tsukuba, Ibaraki, JAPAN

About Waseda University

Located in the heart of Tokyo, Waseda University is a leading private research university that has long been dedicated to academic excellence, innovative research, and civic engagement at both the local and global levels since 1882. The University has produced many changemakers in its history, including nine prime ministers and many leaders in business, science and technology, literature, sports, and film. Waseda has strong collaborations with overseas research institutions and is committed to advancing cutting-edge research and developing leaders who can contribute to the resolution of complex, global social issues. The University has set a target of achieving a zero-carbon campus by 2032, in line with the Sustainable Development Goals (SDGs) adopted by the United Nations in 2015.

To learn more about Waseda University, visit https://www.waseda.jp/top/en

About Professor Yasuo Kawakami from Waseda University

Dr. Yasuo Kawakami is a professor at the Faculty of Sport Sciences at Waseda University, Japan. He holds a Ph.D. from the University of Tokyo, where he also served as an Associate Professor before joining Waseda University. His primary research area of interest is biodynamics (biomechanics, exercise physiology and functional anatomy), and he specializes in muscle mechanics. Specifically, he is investigating the in vivo skeletal muscle behavior during various human movements, and the effects of exercise training, growth, aging, and fatigue on the human musculoskeletal system. In a career spanning more than 30 years, he has over 200 papers to his credit. At Waseda University, he is also serving various administrative roles including the Vice President of Research Communications and Heath & Wellbeing.