Muscle Cramping in the Marathon: Dehydration and Electrolyte Depletion vs. Muscle Damage.
Level 3 - non-randomized controlled study
Prospective observational cohort study comparing outcomes between runners with and without muscle cramps
PubMed 32796418 · doi:10.1519/JSC.0000000000003713
What was done
Ninety-eight runners entered a prospective observational study during a road marathon, undergoing pre-race cardiopulmonary exercise testing and baseline measurements. Pre-race, immediate post-race, and 24-hour post-race blood and urine samples and body mass measurements were taken to compare hydration indicators (body mass change, urine specific gravity), serum electrolytes (sodium, potassium), and muscle damage biomarkers (creatine kinase [CK], lactate dehydrogenase [LDH]) between runners who developed exercise-associated muscle cramps (EAMC) and those who did not. Pacing and training history were also evaluated.
What was found
Of 88 marathon finishers, 20 (24%) developed EAMC during or immediately post-race. Crampers and non-crampers did not differ in body mass change, post-race urine specific gravity, or serum sodium and potassium concentrations. However, runners with EAMC exhibited significantly higher post-race CK (464.17 ± 220.47 vs. 383.04 ± 253.41 UI/L, p = 0.034) and LDH (362.27 ± 72.10 vs. 307.87 ± 52.42 UI/L, p = 0.002). These elevations persisted at 24 hours post-race for both CK (2,438.59 ± 2,625.24 vs. 1,166.66 ± 910.71 UI/L, p = 0.014) and LDH (277.05 ± 89.74 vs. 227.07 ± 37.15 UI/L, p = 0.021). Fewer crampers reported regular strength training compared to non-crampers (25% vs. 47.6%, p = 0.074), and relative running speed diverged between groups from the 25th kilometer onward (p < 0.05).
Why it matters
This study provides evidence that marathon-associated muscle cramping aligns with muscle damage rather than dehydration or electrolyte depletion, supporting neuromuscular fatigue models over fluid-loss theories.
Limits
Observational design cannot establish whether muscle damage causes cramping or results from prolonged muscle contractions. The sample of crampers was small (n = 20), and the abstract does not report dietary/electrolyte intake during the race, ambient race conditions, or direct neuromuscular fatigue measurements.
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