There is a point in exercise when your body will not allow you to do any additional lifts at the same weight or intensity as before. Muscular fatigue may result from energy system depletion, lactic acid accumulation in the muscles, nervous system fatigue and failure of the muscle fiber contractile mechanism, according to Jack H. Wilmore and David L. Costill, authors of "Physiology of Sport and Exercise."
Energy System Depletion
There are three different sources of energy in your body that may cause muscular fatigue: Adenosine Triphosphate - phosphocreatine or ATP-PCr, anaerobic glycolysis and oxidation. ATP-PCr is the simplest of the energy systems and is anaerobic for it used during extremely short and intense bouts of exercise such as weight training, sprinting and jumping. During repeated maximal contractions, fatigue coincides with PCr depletion. The anaerobic glycolysis system lasts no more than 2 minutes in duration. Oxidation or aerobic energy systems rely on oxygen for energy. Anaerobic glycolysis and oxidation systems are dependent upon the glycogen available in the muscles. Glycogen is made from glucose, and once your glucose is used up, you may experience muscular fatigue or other hypoglycemic symptoms.
Lactic Acid
Lactic acid accumulation in the muscles often is associated with muscular fatigue. However, it is not the lactic acid that causes the fatigue but rather the hydrogen ion generated by lactic acid that causes fatigue. The accumulation of the hydrogen ion decreases muscle pH, which impairs the cellular processes that produce energy and muscle contraction, according to Wilmore and Costill.
Neuromuscular Fatigue
Fatigue also can occur at the neuromuscular junction. This prevents the nerve impulse from transmitting to the muscle fiber. Some evidence suggests calcium retention within the sarcoplasmic reticulum -- storage site for calcium within the muscle fiber -- may lead to a decrease in calcium available for muscle contraction, according to Wilmore and Costill. There are still uncertainties on why neuromuscular fatigue occurs, and further research is needed to determine its cause.
Central Nervous System
Generally, you will perceive to be at a higher level of discomfort psychologically before you are physiologically. Most people, unless they are highly motivated, will terminate the exercise before their muscles are physiologically exhausted, according to Wilmore and Costill. Athletes train to increase this tolerance for muscular fatigue in order to outperform competitors.
References
- Real Solutions Mag: Muscle Fatigue
- "Physiology of Sport and Exercise"; Jack H. Wilmore, David L. Costill; 2004
