Findings could inform inform strategies for improving strength and endurance, whether for athletes, people with muscular disorders, or patients recovering from injury
October 20, 2025

When you're running up stairs or out on a jog, your muscles eventually start to feel heavy and weak. That's fatigue setting in, a sign that the muscles’ energy reserves are becoming depleted. But a team of researchers led by Rensselaer Polytechnic Institute (RPI) biology professor Doug Swank, Ph.D., have discovered something surprising: certain muscle fibers have a built-in backup system that fights back against fatigue, potentially helping us keep going when we'd otherwise have to stop.
The secret lies in a phenomenon called "stretch activation": when a muscle is stretched just before it contracts, it can produce a short burst of extra force. Stretch activation has been studied extensively in the context of insect flight muscle and heart muscle contraction in mammals, but its effects have long been assumed to be physiologically irrelevant for the big skeletal muscles we use for day-to-day activities like walking around.
The new study, published in the Journal of General Physiology, shows that assumption was wrong, at least when it comes to certain fast-twitch muscle fibers used to produce 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 said. "But we realized no one had tested what happens during fatigue, when the chemical environment inside muscle fibers changes significantly."
The researchers tested individual muscle fibers from mice under three conditions: normal, early fatigue (with chemical changes that mimic the state of tired muscles), and severe fatigue. They found that while the fibers' normal force production dropped dramatically as expected, in certain fibers the stretch-activated force stayed the same or even increased.
In the most fatigued state, stretch activation contributed up to 30% of the total force these fast-twitch fibers were generating.
“What was dismissed as too small to matter may actually be an important fatigue-fighting mechanism that's been hiding in plain sight,” Swank said.
The effect was specific to fast-twitch fibers, which are used to generate rapid, powerful movements like sprinting and jumping. Slow-twitch fibers, which are used during endurance tasks like long-distance running or cycling, are more fatigue-resistant to begin with, and showed almost no stretch activation response.
Understanding how muscles naturally combat fatigue could eventually inform strategies for improving strength and endurance, whether for athletes, people with muscular disorders, or patients recovering from injury.
Swank and his colleagues are following up on their findings by conducting more detailed explorations of how stretch activation contributes to force generation in both low-intensity and high-intensity exercise. The research is funded by a five-year, $2.7 million National Institutes of Health grant to Professor Swank.