When you’re sprinting up stairs or jogging through the park, that familiar sensation of heaviness and weakness in your muscles is fatigue setting in. This occurs as the muscles’ energy reserves deplete. However, a groundbreaking discovery by researchers led by Doug Swank, Ph.D., a biology professor at Rensselaer Polytechnic Institute (RPI), reveals that certain muscle fibers possess a built-in backup system to combat fatigue, potentially allowing us to push past our limits.
The secret to this endurance boost lies in a phenomenon known as “stretch activation.” This occurs when a muscle is stretched just before it contracts, resulting in a brief burst of extra force. While stretch activation has been extensively studied in the context of insect flight muscles and heart muscle contraction in mammals, its significance for the large skeletal muscles used in everyday activities has been largely overlooked—until now.
Challenging Long-Held Assumptions
The new study, published in the Journal of General Physiology, challenges the long-standing belief that stretch activation is physiologically irrelevant for skeletal muscles. This is particularly true for fast-twitch muscle fibers, which are responsible for quick, powerful movements.
“For decades, stretch activation in skeletal muscle was considered physiologically insignificant because it contributes a relatively small amount of force under normal conditions,” Swank explained. “But we realized no one had tested what happens during fatigue, when the chemical environment inside muscle fibers changes significantly.”
The researchers conducted experiments on individual muscle fibers from mice under various conditions: normal, early fatigue (mimicking the chemical changes in tired muscles), and severe fatigue. They discovered that while the normal force production of these fibers decreased significantly, the stretch-activated force in certain fibers remained constant or even increased.
In the most fatigued state, stretch activation contributed up to 30% of the total force these fast-twitch fibers were generating.
Fast-Twitch vs. Slow-Twitch Fibers
The study’s findings highlight the specific role of fast-twitch fibers, which are crucial for rapid, powerful movements such as sprinting and jumping. In contrast, slow-twitch fibers, which are more fatigue-resistant and used for endurance activities like long-distance running or cycling, showed minimal response to stretch activation.
“What was dismissed as too small to matter may actually be an important fatigue-fighting mechanism that’s been hiding in plain sight,” Swank noted.
Implications for Athletes and Medical Applications
Understanding how muscles naturally combat fatigue could have significant implications for enhancing strength and endurance. This knowledge could benefit athletes seeking to improve performance, individuals with muscular disorders, and patients recovering from injury.
The research was supported by a five-year, $2.7 million grant from the National Institutes of Health. Swank and his team are now delving deeper into how stretch activation contributes to force generation during both low-intensity and high-intensity exercises.
This development follows a growing interest in the physiological mechanisms that underpin muscle performance and endurance. As researchers continue to explore these pathways, the potential applications for sports science and rehabilitation are vast.
Meanwhile, the study opens new avenues for understanding muscle function and fatigue, inviting further research into how these mechanisms can be harnessed to improve human health and performance.
