Background EAMC is a common clinical syndrome in endurance runners, and can range in severity. Risk factors for EAMC have been identified and include age, gender, racing distance, a history of chronic diseases, medication, training and competition load, and running injuries. However, to date, risk factors for more severe EAMC (recurrent cramping) have not been investigated.

Objective To document risk factors associated with EAMC in distance runners, including more severe EAMC.

Design Data analysis from a 2-year prospective cohort study.

Patients (or Participants) 41 698 distance runners (21.1 km and 56 km) who completed an online pre-race medical screening questionnaire.

Main Outcome Measurements Independent risk factors associated with EAMC (model 1: binary outcome), and risk factors associated with severe EAMC (model 2: defined as recurrent cramping history).

Results Significant risk factors (p<0.05) for EAMC (model 1: binary outcome) were: males, age>40 yrs, increased BMI, history of any GIT or kidney/bladder disease; chronic or regular medication use, history of a running injury in the last 12 months, running the 56 km race, recreational running for <5 yrs, training/racing <3 times/week, and slower runners (>6 min/km). In model 2, severe EAMC was associated with all the risk factors for EAMC in model 1, but also included a history of any cardiovascular disease (CVD) symptoms. In model 2, a lower BMI and running in 21.1 km race were also specific risk factors for severe EAMC. Training volume and pace weren't risk factors in model 2.

Conclusions Risk factors for the clinical syndrome of EAMC in distance runners are identified, including novel risk factors associated with more severe EAMC (a history of CVD symptoms, lower BMI and shorter race distance). Physicians need to consider secondary causes (including underlying chronic diseases) in the diagnosis and prevention of EAMC, and also consider different risk factor for recurrent (severe) EAMC in endurance athletes.