Muscle Fatigue - Energy And Fatigue: Chronic Fatigue

Content provided by the Faculty of the Harvard Medical School

Muscle fatigue

To understand how muscles become fatigued, it's essential to understand the chain of events that makes them work.

The first part of the muscle contraction process occurs not in the muscles but in the brain. Let's say you want to hit a golf ball. The motor cortex, the area of the brain that controls movement, sends a nerve impulse to muscles in your arms and hands to adjust your grasp on your golf club and prepare to swing. To reach the appropriate muscles, the nerve impulse travels down the spinal cord and through a pathway of nerves to the neuromuscular junction, a tiny space between the end of a nerve and the surface of a muscle. A chemical signal travels across the neuromuscular junction to land on the muscle. This, in turn, triggers a series of chemical reactions in the muscle cells that ultimately lead to the breakdown of ATP. It is the breakdown of ATP, the last link in this long chain of events, which provides the energy that enables the muscles to contract. You adjust your grip, lift your arms, and swing your golf club.

When muscles become fatigued, they don't contract as forcefully or as quickly as muscles that are not tired. Fatigued muscles have certain characteristics, including a buildup of acid, particularly lactic acid. But it's not known whether the acid buildup causes the muscle to fatigue, or whether it occurs along with the fatigue. Therefore, it's unclear whether attempts to change the acid concentration of exercising muscle — as has been proposed by some fitness professionals — would allow a muscle to work longer and harder without getting fatigued. Although some experts suggest that gradually increasing the intensity of your training and eating a combination protein-carbohydrate snack within 15 minutes of working out will help reduce lactic acid, there's no proof that this will increase your endurance or energy level.

Last updated:	January 23, 2007