The popular theory that exercise-induced muscle cramping (EAMC) is caused by fluid imbalances, particularly dehydration and abnormalities in blood electrolyte levels, has been overturned by a South African study of ultra-distance runners.

Electrolyte and fluid disturbances have been associated with muscle cramps in certain clinical conditions, explain the researchers, and it is therefore often assumed that exercise-induced muscle cramping (EAMC) has the same cause despite a lack of evidence to that effect.

They set out to determine whether acute exercise-induced muscle cramping (EAMC) in distance runners is related to changes in serum electrolyte concentrations and hydration status. A cohort of 72 male runners participating in the Two Oceans Ultra-marathon, a 56k road race held annually in Cape Town, were asked about their history of exercise-induced muscle cramping (EAMC) and then followed up for the development of the condition during the race.

All subjects were weighed before and immediately after the race to assess changes in hydration status. Blood samples were taken before, immediately after and 60 minutes after the race and analysed for glucose, protein, sodium, potassium, calcium and magnesium concentrations, as well as various markers of hydration status.

Of the 72 runners in the study, 45 had a history of exercise-induced muscle cramping (EAMC), while 27 had no previous experience of muscle cramping. In the event, 21 of the 45 runners with a history of cramping suffered acute EAMC either during the race or within 60 minutes of completing it, while 22 of the 27 runners with no history of cramping formed a ‘control’ group for comparison purposes.

Key findings were as follows:

• All episodes of cramping occurred in the latter half of the race or immediately afterwards, with most affected runners reporting three or more episodes. Most commonly affected muscles were hamstrings (48%) and quadriceps (38%). Most cramps were moderate-to-severe in intensity and best relieved by slowing the pace or passive stretching;
• There were no significant differences between the groups for pre- or post-race body weight, per cent change in body weight, blood volume, plasma volume, or red cell volume, indicating no difference in hydration status;
• Immediate post-race serum sodium concentration was significantly lower in the cramp group, while serum magnesium concentration was significantly higher. However, these differences were considered to be too small to be of clinical significance.

‘Furthermore,’ report the researchers, ‘the decrease in serum sodium concentration following the race in the cramp group is probably related to an increased fluid intake during the race in this group. Although drinking patterns were not measured directly, increased drinking in the cramp group is likely because of the well publicised belief that cramping is caused by dehydration.’

This supposition was supported by the finding that runners with EAMC were less dehydrated than non-cramping runners immediately after the race, with per cent decreases in body weight (preto post-race) of 2.9% and 3.6% respectively.

‘The results of our study,’ conclude the researchers, ‘do not support the common hypotheses that exercise-induced muscle cramping (EAMC) is associated with either changes in serum electrolyte concentrations or changes in hydration status following ultra-distance running. An alternative hypothesis to explain the [cause] of exercise-induced muscle cramping (EAMC) must therefore be sought.’

Br J Sports Med 2004; 38:488-492