Can Your Brain Fight Fatigue?
.....many (but not all) physiologists now believe that exhaustion isn’t just in the muscles but also involves the brain. “What we now think is that the muscle isn’t acting on its own,” he says. “There’s an interplay of central processing and muscular exertion.” From the outset of exercise, “the brain asks for and gets constant feedback from the muscles and other systems especially about body temperature” and checks on “how are things going,” says Carl Foster, a professor in the department of exercise and sports science at the University of Wisconsin in La Crosse. Through mechanisms that aren’t fully understood, the brain tracks and calibrates the amount of fuel that is in the muscles, as well as the body’s core temperature. As the amount of fuel drops and the temperature rises, the brain decides that some danger zone is being approached. It starts reducing “the firing frequency of motor neurons to the exercising muscle, leading to a loss of force production,” says Ed Chambers, a researcher at the School of Sport and Exercise Sciences at the University of Birmingham in England and an author of the carbohydrate-drinks study. In other words, the mind, recognizing that the body may be going too hard, starts sending fewer of the messages that tell the muscles to contract. The muscles contract less frequently and more feebly. In a sensation familiar to anyone who exercises, your legs die beneath you.
The limit to exercise tolerance in humans: mind over muscle?
In exercise physiology, it has been traditionally assumed that high-intensity aerobic exercise stops at the point commonly called exhaustion because fatigued subjects are no longer able to generate the power output required by the task despite their maximal voluntary effort. We tested the validity of this assumption by measuring maximal voluntary cycling power before (mean ± SD, 1,075 ± 214 W) and immediately after (731 ± 206 W) (P < 0.001) exhaustive cycling exercise at 242 ± 24 W (80% of peak aerobic power measured during a preliminary incremental exercise test) in ten fit male human subjects. Perceived exertion during exhaustive cycling exercise was strongly correlated (r = −0.82, P = 0.003) with time to exhaustion (10.5 ± 2.1 min). These results challenge the long-standing assumption that muscle fatigue causes exhaustion during high-intensity aerobic exercise, and suggest that exercise tolerance in highly motivated subjects is ultimately limited by perception of effort.