Please tell us about your research topic.
Kusagawa : My research focuses on the anatomical and functional characteristics of the human foot, particularly the bones and muscles located below the ankle joint. I aim to understand how the foot morphology relates to human movement and physical activity.
The human foot possesses unique structural characteristics that distinguish it from those of other primates that engage in quadrupedal and arboreal locomotion. These features are believed to have evolved as adaptations for bipedal locomotion. One representative example is the pronounced medial longitudinal arch, which is considered a uniquely human characteristic.
As people age, however, foot-related health problems such as pain, stiffness, and deformities become increasingly common. Conditions including hallux valgus (bunions), tailor’s bunions, claw toe, and mallet toe are frequently observed, particularly among older adults and women. In addition, previous experiments, including our own, have shown that toe flexor strength declines with ageing.
These problems are closely associated with reductions in physical function, including impaired walking ability and poor postural control, and decreased physical activity levels. Foot problems can also negatively affect quality of life (QOL), limiting daily activities and reducing opportunities for social participation, thereby influencing psychological and social well-being. Our research seeks not only to clarify the anatomical mechanisms underlying foot health, but also to translate scientific findings into practical applications through community-based activities. Ultimately, our goal is to contribute to healthy longevity and well-being across the lifespan.
Could you please describe your research in detail?
Kusagawa : The muscles involved in foot movement are broadly classified into two groups, namely, extrinsic and intrinsic foot muscles. The intrinsic foot muscles, which are located entirely within the foot, consist of ten individual muscles per foot, each has distinct anatomical functions. In my research, I use magnetic resonance imaging (MRI) to quantitatively evaluate the volume of these muscles. This approach allows us to examine differences in the volume of individual intrinsic muscles among various populations.
Muscle morphology is known to be closely related to functional properties, such as maximum force production and contraction speed. Based on this concept, I investigated whether intrinsic foot muscles could be functionally categorized from a morphological perspective. Specifically, I measured muscle length and volume in 26 healthy young adult males using MRI and estimated physiological cross-sectional area (reflecting maximum force production) and muscle fiber length (reflecting maximum contraction speed) for each individual muscle. Using these variables, I conducted a clustering analysis and found that intrinsic foot muscles could be classified into four groups according to their contraction properties. These findings suggest a novel classification system that incorporates functional characteristics in addition to conventional anatomical classification.
Please tell us about your latest research findings.
Kusagawa : Recently, I have focused on understanding the functions of intrinsic foot muscles from a morphological perspective. Toe flexor strength plays an important role in maintaining posture during unstable situations such as single-leg standing. In older adults, declines in toe flexor strength are associated with slower walking speed, impaired postural balance, and an increased risk of falls. Both extrinsic and intrinsic foot muscles contribute to toe flexor strength. However, the relative contribution of each individual muscle has not been fully understood. To address this issue, we investigated the relationship between volume in both extrinsic and intrinsic muscles and toe flexor strength.
In a study involving 17 healthy young adult males, we measured the volume of extrinsic and intrinsic muscles using MRI and assessed the toe flexor strength using a dynamometer. Statistical analysis revealed that several intrinsic foot muscles showed significant positive correlations with toe flexor strength. Among these muscles, the oblique head of the adductor hallucis was identified as the primary determinant of the toe flexor strength.
Could you tell us what you are currently working on and what your outlook for the future is?
Kusagawa : Going forward, I hope to further expand my research and promote its application in real-world settings. Currently, we are conducting an intervention study investigating the effects of toe flexor strength training. In this project, healthy young adults participate in a 12-week training program, after which we will evaluate changes in muscle volume, strength, foot structure, and balance ability. Through this work, I aim to elucidate the physiological and functional benefits of toe flexor strength training and contribute to the development of evidence-based training methods for maintaining foot health.
In parallel with laboratory-based research, I am also actively engaged in community outreach activities to improve foot health literacy. Since AY2023, I have been organizing hands-on educational and research events in Kusatsu and surrounding areas in Shiga Prefecture, using actual research equipment to help participants better understand foot health and physical function. I have also conducted a community-based survey at a research facility on Ritsumeikan University Biwako–Kusatsu Campus.
Through research, education, and public engagement, I hope to continue contributing to healthier aging and the realization of well-being for people of all generations.