Muscle fatigue is a decreased capacity to perform a physical action. Fatigue causes a decline in performance and a decrease in your ability to exert force. Maximum strength is the greatest force you can exert in a single contraction. Once your muscles begin to fatigue, maximum strength declines. The muscle may continue to contract but not at the highest level possible.
Normal muscle contraction involves a variety of physiological processes. Chemicals called neurotransmitters travel between the brain and the muscles. Minerals such as calcium and sodium are used to signal across cell membranes, and electrical activity results in contractions of the muscle. Your muscles must be able to respond to these chemical and electrical stimuli. Any disruption of these processes can result in muscle fatigue.
Metabolic byproducts such as lactic acid were once thought to be the cause of muscle fatigue. When you exercise, your body breaks down sugar to produce energy. If you exercise vigorously, you may be unable to get enough oxygen, and lactic acid accumulates in the muscles. Older theories of muscle fatigue held that lactic acid built up in the muscles, causing them to hurt and burn, thereby increasing fatigue. In reality, muscles contract more efficiently when lactic acid builds up, according to Dr. Gabe Mirkin, a former marathon runner and author who is board certified in sports medicine.
Factors for Muscle Fatigue
According to a report published in the “Journal of Physiology” in 2008, muscle fatigue probably results from a variety of causes acting in concert. Among the possible factors noted in this article were changes in blood flow to the muscle, oxygenation of the muscle, a change in the electrical activity in the muscle, electrolyte changes, age of the study participants, gender of the study participants and the buildup of metabolic byproducts.
When your muscles fatigue, your maximum strength decreases. One study reported in the January 2011 issue of “Sports Medicine, Arthroscopy, Rehabilitation, Therapy and Technology” found that muscle fatigue in shoulder muscles resulted in a 17-percent decrease in maximum strength. Researchers also found that the total amount of work that study participants were able to perform decreased by 42 percent during the last third of the testing process.
In another study reported in the November 2010 “European Journal of Neuroscience,” researchers evaluated hand-grip strength and dexterity in healthy adults. Study participants were asked to grip and lift an object repeatedly. After a sustained maximum grip that produced muscle fatigue, maximum grip strength declined by 60 percent and overall grip strength fluctuated during the grip-and-lift process.
- Eastern Kentucky University: Human Physiology -- Muscle
- Journal of Physiology: Muscle fatigue: What, Why and How it Influences Muscle Function
- DrMirkin.com: What is Lactic Acid?
- Sports Medicine, Arthroscopy, Rehabilitation, Therapy and Technology: Shoulder Muscle Endurance: The Development of a Standardized and Reliable Protocol
- The European Journal of Neuroscience: Change in Manipulation with Muscle Fatigue
- Physiology: Muscle Fatigue: Lactic Acid or Inorganic Phosphate the Major Cause?
- Physiological Reviews: Skeletal Muscle Fatigue: Cellular Mechanisms