While it's an overstatement to say that muscle fatigue may all be in a person's head, a series of studies suggests that the body's cue to slow down during tiring physical activities does start there.

It is known that the extent to which people can use their muscles voluntarily depends on motivation as well as the physical condition and level of fatigue of the muscles. But less is known about the specific mechanisms involved involved with the sensation of fatigue. 

While most research to date on muscle fatigue has studied muscle tissue itself, a series of studies by the University of Zurich has shifted the focus to include the brain, as well. The researchers say the studies show a link between muscle fatigue and changes in the interaction between neuronal structures, specifically, processes responsible for reducing muscle activity during muscle-fatiguing exercise.

In the first study, researchers found that nerve impulses from muscles inhibit the primary motoric area in the brain during a fatiguing, energy-demanding exercise. This was found to be similar to how pain stimuli are conveyed. Study participants repeated thigh contractions until they could no longer achieve the force required to carry out the task. When the same exercise was conducted under spinal anesthesia, consequently interrupting the response from the muscle to the primary motoric area, it was found that the corresponding fatigue-related inhibition processes became markedly weaker than when the brain had received all the muscle information intact.

In a second step, functional magnetic resonance imaging was used to pinpoint those regions of the brain that showed an increase in activity shortly before being interrupted by a tiring, energy-demanding activity. These were found to be the thalamus and the insular cortex, respectively, areas that analyze information that could pose a threat, such as hunger or pain.

The third and final study in the series, published in the European Journal of Neuroscience, found that the neuronal system has a regulating effect on muscle performance. Specifically, the study found that that inhibitory influences on motoric activity are mediated by a part of the brain known as the insular cortex.

Tests using a bicycle ergometer showed that the communication between the insular cortex and the primary motoric area increased as fatigue wore on.

Taken together, the results of the studies suggest that it may be possible to develop ways to optimize muscular performance, the researchers said, opening up new avenues for research and therapy.

References:

 1. Lea Hilty, Lutz Jäncke, Roger Luechinger, Urs Boutellier, Kai Lutz. Limitation of physical performance in a muscle fatiguing handgrip exercise is mediated by thalamo-insular activity. Human Brain Mapping, 2011; 32 (12): 2151 DOI: 10.1002/hbm.21177

2. Lea Hilty, Kai Lutz, Konrad Maurer, Tobias Rodenkirch, Christina M. Spengler, Urs Boutellier, Lutz Jäncke, Markus Amann. Spinal opioid receptor-sensitive muscle afferents contribute to the fatigue-induced increase in intracortical inhibition in healthy humans. Experimental Physiology, 2011; DOI: 10.1113/expphysiol.2010.056226

3. Lea Hilty, Nicolas Langer, Roberto Pascual-Marqui, Urs Boutellier, Kai Lutz. Fatigue-induced increase in intracortical communication between mid/anterior insular and motor cortex during cycling exercise. European Journal of Neuroscience, 2011; DOI: 10.1111/j.1460-9568.2011.07909.x

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