Researchers at the University of Tokyo reveal the way our legs adapt to fast movements. When people hop at high speeds, key muscle fibers in the calf shorten rather than lengthen as forces increase, which they call "negative stiffness." This counterintuitive process helps the leg become stiffer, allowing for faster motion.
The findings could improve training, rehabilitation, and even the design of prosthetic limbs or robotic exoskeletons. When you hop, run or jump, your legs behave like springs, absorbing and returning energy with each step. But what happens to your muscles and tendons to make this possible? Associate Professor Daisuke Takeshita and doctoral student Kazuki Kuriyama from the Department of Life Sciences sought to investigate how muscles and tendons work together during bouncing movements, specifically hopping, as it serves as a proxy for common activities like running.
"Human movements like hopping and running are often characterized by a spring-mass model, where the leg acts as a spring supporting the body mass as it bounces off the ground," said Takeshita. In our study, we specifically examined hopping under constrained conditions, instructing participants to maintain extended knees and minimize ground contact time. These constraints were crucial as they allowed us to isolate the role of ankle joint mechanics and focus on what's known as the plantarflexor muscle-tendon dynamics.
" Daisuke Takeshita, Associate Professor, Department of Life Sciences, University .