Background Despite the high prevalence of Exercise Associated Muscle Cramping (EAMC) in endurance athletes, the aetiology and risk factors for this condition are not fully understood.

Objective The objective of this prospective cohort study was to identify risk factors associated with EAMC in endurance triathletes.

Setting Field study during an international Ironman triathlon.

Participants 210 triathletes competing in an Ironman triathlon.

Assessment Prior to the race, subjects completed a detailed validated questionnaire and blood samples were taken for serum electrolytes. Immediately before the race, pre-race body weight was obtained. Body weight and blood samples for serum electrolyte concentrations were obtained immediately post-race. Clinical data on EAMC experienced during or immediately after the race were also collected.

Main outcome measurements Training data, serum electrolyte and body weight changes were compared between 44 triathletes who reported EAMC (CR group) and 166 triathletes who did not report EAMC (NC group). A regression analysis was conducted to determine independent risk factors for EAMC in triathletes.

Results There were no significant differences between groups in any pre-post race serum electrolyte concentrations and body weight changes. The development of EAMC was associated with faster predicted race times and faster actual race times, despite similarly matched preparation and performance histories in subjects from both groups. A regression analysis identified faster overall race time (and cycling time) and a past history of cramping (in the last 10 races) as the only two independent risk factors for EAMC.

Conclusion The results from this study add to the evidence that dehydration and altered serum electrolyte balance are not causes for EAMC. Rather, endurance runners competing at a fast pace, which suggests that they exercise at a high intensity, are at risk for EAMC.