Since the racing season is in full swing I hear from many athletes that they suffered from exercise-associated muscle cramping (EAMC) during the race, be it on the bike or in the swim, but first of all on the run or after the finish line. It became an unquestioned state-of the-art to see cramps as a result of the dehydration and the salt deficit. But if we measure the electrolyte levels of the cramping athlete, we rarely find any deterioration of electrolytes. If the cause of the cramps were out-of-range electrolytes, the cramping would show a different pattern of muscles groups being affected. Usually these kind of cramps start in the small muscles, spread and finally turn into generalized phenomenon. It’s important to know that these muscle cramps are not related to the workload the muscles are exposed to.
Exercise-associated muscle cramping – a definition of the term
Exercise-associated muscle cramping is one of the most common medical problems encountered by the competitors in an Ironman event. Exercise-associated muscle cramping can be defined as involuntary painful skeletal muscle spasms that occur during or immediately after physical exercise. These kinds of cramps usually present as painful localized muscle cramping in different exercising muscle groups – usually the calf, hamstring or quadriceps muscles. Exercise-associated muscle cramping should be distinguished from generalized involuntary muscle contractions in non-exercising muscles which can be associated with a number of acute or chronic and congenital or acquired medical conditions.
The traditional hypotheses state that heat, dehydration and electrolyte depletion are the cause of EAMC. But believe me the evidence supporting these hypotheses is sparse. In contrast, very recent data from prospective cohort studies showed no relationship between serum electrolyte changes and dehydration in the development of EAMC. Despite these findings triathletes, coaches and exercise scientists still need to be convinced that the development of EAMC is not necessarily related to dehydration and electrolyte depletion.
Neuromuscular fatigue leads to muscle cramping
Fatigue caused by the strenuous workload the exercising muscles are undergoing during a competition weakens the mechanisms that control the muscle movements and coordination.
In 1997 Schwellnus and his co-workers offered already a hypothesis that seems much more obvious to me than the salt and dehydration narratives. They introduced a new hypothesis that became known as the ‘muscle fatigue’ hypothesis. This hypothesis suggests that the development of EAMC goes along with the development of ‘neuromuscular fatigue’. As a consequence neuromuscular control deteriorates.
It has been documented that fatigue increases the afferent activity of the muscle spindle and decreases afferent activity of the Golgi tendon organ which then may result in an increased alpha motor neuron activity. What actually happens is that the uncontrolled firing of neurones on the muscle and the subsequently dramatically reduced relaxation phase of the muscle fibers lead to the emergence of cramps. As muscle fatigue progresses, control mechanisms vanish and the risk for developing exercise-associated muscle cramping increases.
An increased electromyographic baseline activity has been measured in the cramping muscle of triathletes who were suffering from cramps immediately after the race, while non-exercising muscles presented a normal activity pattern.
In general the endurance athletes who compete at a pace that is faster and harder than their usual training pace (higher relative exercise intensity) and those whose recovery has not been complete may develop muscle fatigue earlier during a race and may therefore be at a higher risk for exercise-associated muscle cramping.
Risk factors for developing exercise-associated muscle cramping
To find out more about the risk factors that increase susceptibility to cramping the group around Schwellnus performed a study*. 1136 triathletes (970 male (85.4%) and 166 female (14.6%)) were enrolled in the study. All of them did an Ironman race (3.8 km swim, 180 km cycle and a 42.2 km run) and were considered as potential candidates for developing cramps.
The study identified two risk factors. One critical factor turned out to be the history of muscle cramping reported in the last 10 races. The second risk is the racing pace. The faster your overall racing speed the more your are threatened by cramping.
The study results showed that the cramping group was more ambitious to perform better. Data collected on the actual race day confirmed that both cycling and overall performance times of the cramping group of triathletes were significantly faster compared with the non-cramping group. Furthermore, there was a trend for the run times to be faster in the cramping group.
Although the study cannot directly link the faster race pace to the development of muscle fatigue, data from observational studies in animals and laboratory-based studies in humans support the hypothesis that repetitive muscle contraction results in fatigue that can lead to cramping, probably by altering neuromuscular control.
No electrolyte changes found in either study group. The pre-race and the post-race serum electrolyte (sodium and chloride) concentration changes did not differ between the cramping and the non-cramping triathletes nor did the body weight changes. The hypothesis that either serum electrolyte changes or dehydration is related to exercise-associated muscle cramping is not supported by the findings of this study. It is the largest prospective study investigating possible risk factors for exercise-associated muscle cramping in endurance athletes. The results of this study are therefore a strong argument.
The results of the Schwellnus study confirm that there is no correlation between exercise-associated muscle cramping and changes in electrolyte concentrations or changes in hydration status. Rather, this study shows that EAMC is associated with a history of muscle cramping and an overall faster race time during the Ironman triathlon. Prolonged exercise at a relatively higher intensity, compared with training, is therefore associated with a higher risk to develop exercise-associated muscle cramping in triathletes.
Schwellnus MP, Drew N, Collins M: Increased running speed and previous cramps rather than dehydration or serum sodium changes predict exercise-associated muscle cramping: a prospective cohort study in 210 Ironman triathletes. Br J Sports Med, 1-7, Dec. 9, 2010