What causes muscle cramps in exercise?
Muscle cramping during exercise is a common problem among athletes that involves sudden, involuntary and painful muscle contraction during or after exercise. The occurrence of cramps is quite unpredictable, and the causes are not well understood, though there are two hypotheses.
Types and prevalence of muscle cramps
Many athletes have experienced muscle cramping during or after exercise, at some point in their sporting career. It is difficult to assess how many athletes suffer from muscle cramps, as some athletes may experience cramps only occasionally, whilst cramping may be a recurring problem for others. There are also different types of cramps, from small cramps in small muscles that resolve quickly to large whole-body cramps that cause pain for hours or even days. Studies may also report and define cramping differently (1). These factors make estimating the prevalence of cramping difficult, but one large survey of 2600 triathletes suggested that 67% of the participants reported some level of cramping during or after exercise, whilst 4% had experienced severe cramping (2).
Cramping appears to be more common in athletes participating in endurance sports, perhaps due to the long duration and repetitive nature, though it can occur in any sporting activity. A number of risk factors have been associated with cramping, including older age, cardiovascular disease, hot and humid conditions. However, most of these risk factors are only correlated with cramping, and are not the causes of cramping. There are two main possible causes, each with merits, though neither can fully explain cramping alone.
Disturbed electrolyte balance and hydration status
This is the classical explanation for muscle cramps, that dehydration and loss of electrolytes through sweating can cause muscle cramps. Because electrolytes are required for proper muscle function, including contracting and relaxing, electrolyte depletion (both in the blood and the muscle) could plausibly result in disrupted and uncontrolled muscle contraction (i.e. cramping). The classical explanation for muscle cramps is that dehydration and loss of electrolytes through sweating can cause muscle cramps
The initial evidence for this explanation is from studies from the 1920s and ‘30s involving heavy industry workers (miners, steel mill workers and ship stokers), who worked physically demanding jobs, often in hot conditions with thick protective clothing on. These studies mostly showed that cramping was associated with loss of electrolytes through sweat, though dehydration was not always implicated as the workers drank large amounts of plain water. Hyponatremia (low sodium concentrations in the blood) specifically was implicated, which can cause symptoms if left long enough such as nausea, confusion or even coma. One interesting study gave workers in one steel mill plain water to drink throughout the day, whilst in another nearby mill, workers were given a sodium drink. In the mill where sodium drinks were provided, the incidence of cramps fell drastically. Larger losses in sodium during exercise tend to occur in athletes who experience cramps, who also tend to drink more plain water compared to electrolyte beverages.
More recent evidence has looked at cramping specifically in athletes. Although many of the studies have been small, they have shown that larger losses in sodium during exercise tend to occur in athletes who experience cramps, who also tend to drink more plain water compared to electrolyte beverages, which may contribute. Other studies have tried to trigger cramps with exhaustive exercise protocols, with one showing that drinking carbohydrate-electrolyte drinks to replace sweat losses delayed the length of time it took for subjects to experience cramps. Of course, the working conditions of early 20th Century industrial workers are not directly comparable to sporting conditions, and similarly arduous conditions are uncommon. However, these studies provided the earliest suggestions that electrolyte imbalance could cause muscle cramps.
Altered neuromuscular control
Although cramping often occurs in prolonged exercise in the heat, cramping can also occur without dehydration or electrolyte imbalance and in cool environments. Therefore, there must be other causes for cramps that occur in these conditions. Cramping can be triggered by activities beyond exercise, including repetitive, small muscle group activities like typing, writing or pressing buttons. It was suggested that cramps could be caused by abnormal activity of the nerve that control muscle activity, originating in the central nervous system. However, the cause of the ‘abnormality’ was, and is, unclear, though it is suggested to be caused by increasing fatigue. The increased fatigue is thought to cause increased muscle activation, whilst the inhibition of excessive activation that normally controls contraction is reduced. This leads to uncontrolled contraction, leading to a muscle cramp.
Nervous system control of cramping is difficult to study
To study the impact of nervous system control of cramping, studies were developed to trigger cramping. Cramps during exercise are notoriously unpredictable and therefore difficult to study, so electrical activation of muscles was used in a number of studies to cause it. In one study subjects were provided with an electrolyte beverage or plain water, but this did not reduce the incidence of cramps triggered by electrical activation. Athletes who are prone to cramps have also been shown to require less electrical stimulation to the nerves to trigger cramps. When these nerves are ‘blocked’ using an anaesthetic more stimulation is needed to trigger cramps. Together this evidence supports the idea that there is a nerve-related mechanism that can trigger cramps. Whether these electrically stimulated cramps are similar to those cramps caused by exercise is not known, but they are one of the easiest ways to study muscle cramps. This evidence supports the idea that there is a nerve-related mechanism that can trigger cramps.
What causes cramps?
Cramping is certainly more common in exercise in the heat, where sweating rates are high and electrolyte depletion results. However, cramping can also occur at times when electrolyte depletion/dehydration has not occurred. In these conditions, cramps are more likely to be caused by the altered control of muscle contraction by nerves, as a result of fatigue. However, the exact mechanisms by which this happens are not well understood because of the difficulties associated with studying cramping. Each mechanism applies differently in different situations, and although we do not fully understand the mechanisms, this is not necessary to know whether a treatment is effective or not. Watch this space for a blog on prevention and treatment of muscle cramps.