By Brian B. Parr, Ph.D., ACSM Certified Clinical Exercise Physiologist
Muscle cramps are a common condition experienced during endurance exercise and many sports. Known as exercise-associated muscle cramps (EAMC), these sustained and painful muscle contractions tend to occur during prolonged exertion including exercise and physical labor that results in fatigue. Despite their frequency, the cause and treatment for muscle cramps is not well understood by many recreational and competitive athletes.
Muscle cramps that occur during exercise are widely thought to be caused by dehydration and electrolyte imbalances in the muscle. This makes sense, as muscle cramps seem to be more likely during prolonged exercise in a hot, humid environment in which sweat loss could cause a loss of both water and salt from sweat. Sometimes, EAMC are erroneously referred to as “heat cramps,” even though they can occur in cooler conditions. Furthermore, dehydration and electrolyte imbalances would affect the entire body, whereas muscle cramps occur only in the working muscles, typically the gastrocnemius in the lower leg. It is also worth noting that the most effective treatment for muscle cramps – stretching the affected muscle – does not replace fluid or electrolytes. These observations suggest that EAMC have a cause other than fluid and electrolyte loss during exercise.
Research also does not support the notion that dehydration or electrolyte abnormalities are the cause of muscle cramps that occur during exercise. For example, in one study of competitors in an Ironman triathlon, there were no significant differences between those who experienced muscle cramps and those who did not in blood electrolyte concentrations or body weight changes (an indicator of dehydration). The development of EAMC was related to faster race times and a history of cramping. Other studies in a laboratory setting show that muscle cramps still occur even when electrolyte balance and fluid replacement is maintained. These findings and other research led to the development of an alternate theory of the cause of muscle cramps during exercise.
Understanding the cause of EAMC requires a brief primer in muscle physiology. Muscle contraction occurs when nerves, from the spinal cord, called motor neurons, stimulate the muscles to shorten and produce force. The force production by the muscle is controlled by a host of receptors that either stimulate or inhibit the muscle. One of these, the muscle spindle, responds to stretch and causes muscle excitation. Another, the Golgi tendon organ (GTO), inhibits muscle activation and causes relaxation, in part to prevent excessive force production. Working together, the muscle spindle and GTO regulate force production during exercise.
You can easily experience the muscle spindle and GTO in action. The patellar tendon reflex, in which a tap on the patellar tendon causes your quadriceps muscles to contract and “kick” your lower leg, is due to simulation of the muscle spindle. You have probably noticed that you can stretch farther if you hold a stretch for a long time. This is caused by the GTO inhibiting the stretched muscle, allowing it to relax and lengthen even more.
Just as repeated muscle contraction over time can lead to muscle fatigue, the pattern of the motor neurons stimulating the muscle can also be altered during prolonged, intense exercise. This causes an excessive stimulation of the muscle spindle and decreased activation of the GTO, resulting in uncontrolled muscle contraction – a cramp. This also explains why stretching is an effective treatment for muscle cramps. By engaging the GTO, the muscle relaxes and the cramp is eventually relieved.
The prevention of muscle cramps is obviously of great interest, especially for athletes who are prone to develop them. Given that muscle fatigue is a condition underlying most cramps, adequate training and conditioning to increase endurance is important. Acclimating to hot, humid conditions and staying as cool as possible during exercise may also help delay fatigue. There are a number of popular preventive strategies, including increasing fluid and electrolyte intake, consuming specific foods and beverages like pickle juice and bananas prior to exercise, and taking substances like quinine. However, experimental evidence for these strategies to reduce EAMC is lacking and their effectiveness is based mostly on anecdotal reports.
There is another potential approach to prevent and treat muscle cramps. Remarkably, ingesting spicy foods or food extracts like cinnamon, ginger, or peppers may reduce the intensity and/or duration of muscle cramps. The mechanism for this involves the activation of nervous system receptors in the mouth that send signals along the spinal cord, which can inhibit muscle contraction. Limited research suggests that this strategy may reduce muscle cramps during exercise, but it is too soon to know how effective it is. In the meantime, the best way to prevent muscle cramps is to prepare yourself through training and nutrition for both the environmental conditions and exercise itself. And if you get a muscle cramp during exercise, immediate stretching is the best way to relieve it.
Brian B. Parr, Ph.D., is an ACSM Certified Clinical Exercise Physiologist and Associate Professor in the Department of Exercise and Sports Science at the University of South Carolina Aiken.