Muscle cramps during exercise are a frustrating challenge for many athletes, with 80% of triathletes and 50% of football players experiencing cramps during competition or training. At one time, it was thought that dehydration and/or low levels of electrolytes such as sodium, potassium, and magnesium were the cause. In fact, many athletes tout the benefits of pickle juice, which is high in electrolytes, to relieve muscle cramps. Research, however, demonstrates that when most individuals are cramping, blood levels of electrolytes, as well as hydration levels are normal (Sulzer et al., 2005). So what really causes exercise-associated muscle cramps?
Recent research suggests that the nervous system is the likely cause. Alpha motor neurons are nerves located in the brain stem and spinal cord. These nerves communicate with muscle, telling it when to contract. Muscles fatigue increases the activity of alpha motor neurons, stimulating the muscle to maintain a sustained contraction (Schwellnus et al., 1997, Miller et al, 2010b), producing muscle cramps.
When a muscle fatigues, receptors in the muscle (muscle spindle and Golgi Tendon Organ) alter activity in the alpha motor neurons. For example, muscle fatigue increases the activity of the muscle spindle which is responsible for the stretch reflex. When triggered, the stretch reflex causes the muscle to contract by increasing alpha motor neuron activity; this means more messages to the muscle for contraction. In addition, other spinal reflexes are triggered which block muscle relaxation (Golgi Tendon Organ, GTO), further contributing to muscle cramps.
Now, you might be saying, wait a minute, pickle juice works for me. Yes, there is research showing pickle juice shortens muscle cramp duration, but not because of its high electrolyte concentration. In fact, it takes about 30 minutes for even small volumes (2/3 cup) of pickle juice to leave the stomach (Miller et al., 2010a). Hence, blood electrolyte levels would not increase quickly enough to explain cramp relief. However, the acetic acid in pickle juice is ‘noxious tasting’ and proposed to chemically stimulate a reflex in the back of the throat. This reflex has been shown to decrease activity in the alpha motor neurons which causes muscle relaxation. You don’t even have to swallow the pickle juice to trigger the reflex, which can relieve cramps in less than 3-4 minutes (Miller, 2010b). It is possible that other noxious tasting substances may also provide relief from exercise-associated muscle cramps.
Note that hydration and electrolyte balance during exercise are still important for optimal performance. Our body is 60% water by weight, and muscle is approximately 75% water. Dehydration decreases the ability of the cardiovascular system to provide adequate blood flow to both the working muscle and skin for cooling. The right balance of electrolytes is important to keep the heart and muscle functioning properly during exercise.
Miller, K.C., Mack, G.W., & Knight, K.L. (2010a). Gastric emptying after pickle-juice ingestion in rested, euhydrated humans. Athletic Training. 45(6), 601-8. doi: 10.4085/1062-6050-45.6.601
Miller, K.C., Mack, G.W., Knight, K.L., Hopkins, J.T., Draper, D.O., Fields, P.J., & Hunter, I. (2010b). Reflex inhibition of electrically induced muscle cramps in hypohydrated humans. Medicine and Science in Sports and Exercise. 42(5). doi: 10.1249/MSS.0b013e3181c0647e
Schwellnus, M.P., Derman, E.W., & Noakes, T. D. (1997). Aetiology of skeletal muscle ‘cramps’ during exercise: A novel hypothesis. Journal of Sports Sciences. 15, 277-285. Retrieved from http://dx.doi.org/10.1080/026404197367281
Sulzer, N.U., Schwellnus, M.P., & Noakes, T.D. (2005). Serum electrolytes in Ironman triathletes with exercise-associated muscle cramping. Medicine and Science in Sports and Exercise.37(7), 1081-5. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/16015122