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6 min readrunning / endurance

Pace predicted cramps in 210 triathletes, blood sodium did not

Salt tablets are standard kit in ultra running. I race 100 km, and the advice circulating in the sport has not moved in years: drink more, take more salt, your electrolytes are low. A research group took blood from 210 Ironman triathletes before and after a race, then asked who cramped. The 43 who did had the same serum electrolytes and the same body mass change as the 166 who did not 1.

What the research shows.

Schwellnus, Drew and Collins ran that cohort as a prospective study. Across every pre-race and post-race serum electrolyte measure, and across body mass lost during the race, the cramping and non-cramping groups matched. Two variables came out as independent risk factors. The first was cramp history, specifically the number of episodes an athlete reported in their last 10 races. The second was overall finishing time: the faster triathletes cramped more 1.

The same group had tested distance runners seven years earlier and found the same pattern. No difference in plasma volume, in body mass lost, or in plasma electrolyte concentrations between the runners who cramped and those who did not 2.

Braulick and colleagues came at the question from the other side. They dehydrated subjects on a cycle ergometer in the heat until they lost 5% of body mass or quit, then measured how much electrical stimulation it took to trigger a cramp. Cramp threshold frequency was 15 Hz euhydrated and 13 Hz hypohydrated (p = 0.12). Cramp intensity and cramp EMG amplitude did not move either 3. Ten subjects is a small sample, and the cramps were induced in a small foot muscle rather than a racing quad, so read it as one controlled data point: losing 5% of body mass in sweat left cramp susceptibility where it started.

The mechanism Schwellnus and others propose is sustained abnormal spinal reflex activity secondary to muscle fatigue. As a muscle fatigues, excitatory drive from the muscle spindles rises while inhibitory drive from the Golgi tendon organs falls, and the motor neuron pool keeps firing after you wanted it to stop 4. Racing at an intensity your training has not prepared the muscle for brings that fatigue forward, which lines up with the finishing-time result in the triathlete cohort.

Miller and colleagues tested relief and landed in the same place. They induced cramps in the flexor hallucis brevis of hypohydrated men, then gave 1 mL per kg of body mass of either deionized water or pickle juice right after the cramp started. Cramps lasted 84.6 s after pickle juice against 133.7 s after water, a difference of 49.1 s 5. Plasma composition had barely moved 5 minutes after drinking, so nothing was absorbed in time to replace anything. The authors credit a reflex triggered by receptors in the mouth and throat.

In 2022, Miller, McDermott, Yeargin, Fiol and Schwellnus graded 16 recommendations for treating and preventing exercise-associated muscle cramps. Gentle static stretching of the cramping muscle until it abates carries the top grade. Vinegar and capsaicin-type foods come next. Pre-event intravenous fluid given purely to prevent cramping is advised against, and the review tells clinicians to find each athlete's own risk factors instead of issuing one electrolyte plan to everybody 6.

Electrolytes still explain some cases.

Maughan and Shirreffs reviewed the field in 2019 and declined to crown a single cause. Some cramps do track disturbances of water and salt balance. Others look like fatigue-driven spinal reflex activity. No remedy works consistently across cases, and they argue that the search for one strategy covering prevention and treatment is unlikely to succeed 4.

Those field studies have one limit worth stating: a faster finishing time predicts cramping in an observational cohort without proving that the pace caused it. Within those bounds, cramp history and race intensity beat hydration status as predictors, and the best-graded treatment costs nothing.

The protocol.

  1. Score your history before the next race. Count how many of your last 10 races involved cramping, and note which muscle and at what point in the race. Episodes in the last 10 races was one of the two independent predictors in the triathlete cohort 1. Three or more out of 10 puts you in the group that should plan for it.

  2. Set a first-half pace you have held in training for that long. The faster finishers cramped more 1. If your longest run at goal pace lasted 75 minutes, racing 4 hours at that pace bets on a muscle you have never fatigued that way.

  3. Finish your long runs at race intensity. Put the last 20 to 30 minutes of the long run at goal pace so the fatigue arrives in training rather than at kilometre 32. No trial has tested this against cramp rates; it follows the fatigue mechanism 4 and the intensity signal in the cohort data.

  4. Stretch the muscle, do not walk it off. When a muscle locks, stop and hold a static stretch on it until the cramp releases. That is the highest-graded treatment in the 2022 review 6. Calf: heel flat on the ground, push the hips toward a wall or a tree. Hamstring: straight leg, hinge from the hip.

  5. Carry 60 to 80 mL of pickle juice if you cramp often. 1 mL per kg at cramp onset cut duration by 49.1 s in the lab 5. A mustard sachet or a vinegar shot works on the same proposed reflex 6. Try the taste and your stomach tolerance on a long run before you race with it.

  6. Keep sodium for sweat replacement and stop expecting it to stop cramps. Sodium earns its place in a long hot race for other reasons. On cramping, three studies found no electrolyte or hydration difference between crampers and non-crampers 123, and the 2022 review advises against pre-race intravenous fluid for cramp prevention 6.

Cramps that arrive at rest, always in the same muscle, or alongside confusion and dark urine are a medical question rather than a pacing one. Take those to a doctor.

Sources.

  1. Schwellnus, M. P., Drew, N., & Collins, M. (2011). 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. British Journal of Sports Medicine, 45(8), 650-656. doi:10.1136/bjsm.2010.078535
  2. Schwellnus, M. P., Nicol, J., Laubscher, R., & Noakes, T. D. (2004). Serum electrolyte concentrations and hydration status are not associated with exercise associated muscle cramping (EAMC) in distance runners. British Journal of Sports Medicine, 38(4), 488-492. doi:10.1136/bjsm.2003.007021
  3. Braulick, K. W., Miller, K. C., Albrecht, J. M., Tucker, J. M., & Deal, J. E. (2013). Significant and serious dehydration does not affect skeletal muscle cramp threshold frequency. British Journal of Sports Medicine, 47(11), 710-714. doi:10.1136/bjsports-2012-091501
  4. Maughan, R. J., & Shirreffs, S. M. (2019). Muscle cramping during exercise: causes, solutions, and questions remaining. Sports Medicine, 49(Suppl 2), 115-124. doi:10.1007/s40279-019-01162-1
  5. Miller, K. C., Mack, G. W., Knight, K. L., Hopkins, J. T., Draper, D. O., Fields, P. J., & Hunter, I. (2010). Reflex inhibition of electrically induced muscle cramps in hypohydrated humans. Medicine & Science in Sports & Exercise, 42(5), 953-961. doi:10.1249/MSS.0b013e3181c0647e
  6. Miller, K. C., McDermott, B. P., Yeargin, S. W., Fiol, A., & Schwellnus, M. P. (2022). An evidence-based review of the pathophysiology, treatment, and prevention of exercise-associated muscle cramps. Journal of Athletic Training, 57(1), 5-15. doi:10.4085/1062-6050-0696.20

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