Can't serve an ace? Could be muscle fatigue

Fatigue could reduce skills and cause injuries and muscle weakness during sport because the brain does not consider the extra effort required for movement, Monash University researchers have found.

Professor Uwe Proske, from Monash's Department of Physiology, found when muscles were weakened from overuse or fatigue, limb control was affected, particularly if the person couldn't see their limbs.

The study, which has been published in the Journal of Physiology, showed muscles needed to work harder to compensate for fatigue, which led to uncertainty about where the limb was.

"In the absence of sight, people judge the position of their limbs based on the amount of effort required to lift them against the force of gravity," Professor Proske said. "Take gravity away, as happens to astronauts, and they have real trouble carrying out skilled movements.

"In our experiments we found that when the muscles in one arm were fatigued, the effort required to maintain a set arm position was much greater. When asked to keep both arms in a similar position, where one arm was fatigued and the other was not, the arms did not align.

"That was unexpected. Previously it was believed that fatigue had nothing to do with the body's sense of position," he said.

Professor Proske said the findings could have implications for sports that required skilled actions such as serving a tennis ball, throwing a javelin or shooting a bow and arrow.

He said it could also result in loss of control over stride length during running - leading to stretched hamstrings and other injuries.

"When a tennis player is serving, they don't watch where their shoulders are, they rely on the brain's knowledge of how much effort is required to maintain the best position, to get the ball where it needs to go.

"However, if the limbs are fatigued the brain must activate them harder and this leads to errors about where the different body parts are located."

Professor Proske said the research could also offer insight into the symptoms of some motor system diseases such as Parkinson's disease. The abnormal movements were likely to relate, in part, to a disturbed sense of effort, he said.

Source : Research Australia