Based on these studies, it will be possible to specifically investigate reasons for reduced muscular performance without apparent cause – as in certain cases diagnosed as chronic fatigue syndrome.
The extent to which we are able to activate our muscles can depend on motivation, physical condition/muscle fatigue, and the perception of muscle fatigue.
For a long time, the research on muscle fatigue was largely confined to changes in the muscle itself. Now, a Swiss research project has shifted the focus to brain research - with significant implications not just for optimizing physical performance but for the investigation of reasons for reduced muscular performance related, for example, to certain medications and various diseases.
Headed by neuro-psychologist Kai Lutz (University of Zurich) in collaboration with Prof Urs Boutellier (Institute of Human Movement Sciences & Sport at ETH Zurich), the researchers discovered neuronal processes for the first time that are responsible for reducing muscle activity during muscle-fatiguing exercise.
The third and final part of this series of experiments has now been published in the European Journal of Neuroscience.
Muscle's nerve impulses inhibit motoric area in the brain
In the initial study(1), the researchers showed that nerve impulses from the muscle - much like pain information - inhibit the primary motoric area during a tiring, energy-demanding exercise.
They were able to prove this using measurements in which study participants repeated thigh contractions until they could no longer attain the force required.
If the same exercise was conducted under narcotization of the spinal chord (spinal anesthesia), thus interrupting the response from the muscle to the primary motoric area, the corresponding fatigue-related inhibition processes became significantly weaker than when the muscle information was intact.
Other brain areas analyze nerve signals
In a second step, using functional magnetic resonance imaging, the researchers were able to localize the brain regions that exhibit an increase in activity shortly before the interruption of a tiring, energy-demanding activity and are thus involved in signalizing the interruption.(2)
These brain areas are the thalamus and the insular cortex – both areas which analyze information that indicates a threat to the organism, such as pain or hunger.
Insular cortex regulates message to motoric area
The third study has now shown that the inhibitory influences on motoric activity are actually mediated via the insular cortex.(3)
In tests using a bicycle ergometer, the researchers determined that the communication between the insular cortex and the primary motoric area became more intensive as the fatigue progressed.
"This can be regarded as evidence that the neuronal system found not only informs the brain, but also actually has a regulating effect on motoric activity," explains University of Zurich researcher Lea Hilty, who conducted the experiment as part of her doctoral thesis.
And the results open up a new research field, says Dr. Lutz
"The findings are an important step in discovering the role the brain plays in muscle fatigue," he says. "Based on these studies, it won't just be possible to develop strategies to optimize muscular performance, but also specifically investigate reasons for reduced muscular performance in various diseases."
• Prolonged reduced physical performance is a symptom that is frequently observed in daily clinical practice.
• It can also appear as a side effect of certain medication.
• However, so-called chronic fatigue syndrome is often diagnosed without any apparent cause.
1. “Limitation of Physical Performance in a Muscle Fatiguing Handgrip Exercise Is Mediated by Thalamo-Insular Activity.” Human Brain Mapping, Dec 10, 2010. Lea Hilty, Lutz Jäncke, Roger Luechinger, Urs Boutellier, Kai Lutz.
2. “Spinal opioid receptor-sensitive muscle afferents contribute to the fatigue-induced increase in intracortical inhibition in healthy humans.” Experimental Physiology, Feb 11, 2011. Lea Hilty, Kai Lutz, Konrad Maurer, Tobias Rodenkirch, Christina M. Spengler, Urs Boutellier, Lutz Jäncke, Markus Amann.
3. “Fatigue-induced increase in intracortical communication between mid ⁄anterior insular and motor cortex during cycling exercise.” European Journal of Neuroscience, Nov 21, 2011. Lea Hilty, Nicolas Langer, Roberto Pascual-Marqui, Urs Boutellier, Kai Lutz.
Source: Based on University of Zurich Press Release, Dec 5, 2011