Human movements like hopping and running can be explained using a spring-mass model. In this model, the leg works like a spring that supports the body’s weight and helps it bounce off the ground.
How do our muscles and tendons make this possible?
Daisuke Takeshita and Kazuki Kuriyama delved into how muscles and tendons work together during hopping. To isolate the mechanics of the ankle joint, they set strict conditions: participants had to keep their knees extended and reduce ground contact time. Their research focused on the dynamics of the plantar flexor muscle tendon.
The findings revealed that muscle fibers react differently based on hopping speed. At slower paces, the fibers barely change length. But during faster hops, the fibers actively shorten even as force rises, a phenomenon they termed “negative stiffness.” This surprising behavior boosts overall leg stiffness, enabling quicker, more efficient movements.
Doctoral student Kazuki Kuriyama from the Department of Life Sciences said, “Our findings provide a new framework for understanding muscle function during various activities. Rather than viewing muscles as simply generators of force, we’ve shown that they actively modulate the mechanical properties of the leg through their dynamic interaction with tendons. This perspective opens new avenues for research in sports science, rehabilitation medicine, and biomechanical engineering.”
Scientists successfully printed a robotic hand with bones, ligaments, and tendons
To conduct this study, researchers utilized a cutting-edge system integrating ultrasound imaging, motion capture, and force plate data. The synchronization of these tools ensured precise alignment of data points, helping them analyze the sequence of events unfolding within the muscles during hopping movements.
Leg-sensing apparatus. A participant jumps on the spot while wearing the team’s combination of sensors to provide test data for their analysis. ©2025 Kazuki Kuriyama and Daisuke Takeshita CC-BY-ND
Kazuki Kuriyama meticulously digitized muscle fiber data from thousands of ultrasound frames—a labor-intensive process requiring exceptional attention to detail. This effort captured subtle changes in muscle fiber length during each hopping cycle. Every step, from data collection to interpretation, presented unique challenges that demanded innovative technical solutions and creative problem-solving.
Researchers now aim to expand their study beyond the lab and onto the running track. Observing real-world movements, they hope to understand better how lower leg muscles drive athletic performance. These findings could enhance training techniques for athletes and aid in rehabilitation efforts, bridging the gap between controlled lab studies and the complexity of real-world human motion.
Journal Reference
Kazuki Kuriyama and Daisuke Takeshita, “Leg stiffness adjustment during hopping by the dynamic interaction between the muscle and tendon of the medial gastrocnemius,” Journal of Applied Physiology: March 27, 2025, DOI: 10.1152/japplphysiol.00375.2024