A new study from York University is challenging a widely held belief about what causes fatigue during intense exercise.
When graduating Faculty of Health student Alireza Vaziri was in high school, a foot correction surgery left him facing six months of recovery. As he convalesced, he became fascinated by how his body worked, particularly how quickly strength could fade and return.
“During the rehabilitation period, I was amazed by how easily muscles could succumb to fatigue, lose their strength, and yet find their way back,” he says.
It sparked an interest in how musculature works, which grew into a focus on neuromuscular fatigue and led him to York’s Muscle Health Research Centre, overseen by Faculty of Health Associate Professor Arthur Cheng.
He built on that interest, studying the brain’s role in muscle performance and central fatigue, an exercise‑induced impairment in the ability of the brain and nervous system to fully engage musculature.
It’s the kind of limitation people might notice near the end of an all‑out sprint, when their legs feel like they won’t respond, he describes.
That question of what causes that breakdown, and how the brain and body respond during intense exercise, is published in new research on central fatigue, a study led by Vaziri, supervised by Cheng, and co‑authored with Faculty of Health researcher Michael Paris.
A key driver of the work, Vaziri says, is that this type of exhaustion is still not fully understood. One widely held idea is that intense exercise creates chemical changes in the muscles – the burning feeling often linked to “lactic acid” – which signal the brain to ease off.
However, Vaziri says there are growing indications that these signals do not always lead to central fatigue or cause the brain to scale back its control of muscles.
His work tested this by taking a closer look at how the brain responds during repeated bouts of high‑intensity exercise. Vaziri enlisted healthy, recreationally active young adults and asked them to complete six rounds of 30‑second, all‑out exercise emphasizing dorsiflexors, groups of muscles at the front of the lower leg which control the feet. Each round was followed by a 60‑second effort where participants tightened the muscle as hard as possible without moving it.
That moment right after intense exertion is critical as this type of exhaustion can recover within seconds after exercise stops, making it difficult to detect.
“We were committed to designing a protocol that captured central fatigue after each interval set without allowing it to recover,” he says.
To do that, researchers applied brief electrical pulses to the nerve controlling the muscle – a quick test of whether it had more force to give. If the pulse created a small extra burst – visible as a brief spike in the measurements – it suggested the brain wasn’t fully activating the area. If not, it meant the brain was already driving it at full strength. By measuring this right away, and continuing during sustained effort, researchers gave central fatigue every chance to show up. That meant if they didn’t see it, the result would be meaningful.
Despite the demanding protocol, neural control remained consistent throughout. Even after repeated bouts, there was little indication it was easing off.
What did change, however, was the muscle itself. Strength dropped significantly over time, but the exhaustion originated within the tissue, not the nervous system.
Taken together, the findings suggest the musculature that lift feet are surprisingly resistant to central fatigue, even under repeated strain. Put another way, the signals from tired muscles may not influence the brain as strongly as once believed.
“The results from this study add nuance to our knowledge,” says Vaziri. “Filling this gap matters because knowing whether fatigue originates from the central nervous system or in skeletal muscle could reshape how interventions are designed.”
The findings have implications well beyond the lab.
“The dorsiflexors are critical for gait, balance and everyday movement, and understanding fatigue in this muscle group could inform rehabilitation strategies,” says Vaziri.
For athletes, the results could influence how training and performance limits are approached and point to the importance of building resilience versus using mental effort – or “pushing through” – for movements involving the lower leg.
In rehabilitation settings, this insight could help guide recovery for people rebuilding strength after injury – like Vaziri himself once did. In that way, this work returns him to where he began: trying to understand why muscles tire, and how they find their way back.