Aetiology of skeletal muscle ‘cramps’ during exercise: A novel hypothesis

Abstract

The aetiology of exercise-associated muscle cramps (EAMC), defined as 'painful, spasmodic, involuntary contractions of skeletal muscle during or immediately after physical exercise', has not been well investigated and is therefore not well understood. This review focuses on the physiological basis for skeletal muscle relaxation, a historical perspective and analysis of the commonly postulated causes of EAMC, and known facts about EAMC from recent clinical studies. Historically, the causes of EAMC have been proposed as (1) inherited abnormalities of substrate metabolism ('metabolic theory') (2) abnormalities of fluid balance ('dehydration theory'), (3) abnormalities of serum electrolyte concentrations ('electrolyte theory') and (4) extreme environmental conditions of heat or cold ('environmental theory'). Detailed analyses of the available scientific literature including data from recent studies do not support these hypothesis for the causes of EAMC. In a recent study, electromyographic (EMG) data obtained from runners during EAMC revealed that baseline activity is increased (between spasms of cramping) and that a reduction in the baseline EMG activity correlates well with clinical recovery. Furthermore, during acute EAMC the EMG activity is high, and passive stretching is effective in reducing EMG activity. This relieves the cramp probably by invoking the inverse stretch reflex. In two animal studies, abnormal reflex activity of the muscle spindle (increased activity) and the Golgi tendon organ (decreased activity) has been observed in fatigued muscle. We hypothesize that EAMC is caused by sustained abnormal spinal reflex activity which appears to be secondary to muscle fatigue. Local muscle fatigue is therefore responsible for increased muscle spindle afferent and decreased Golgi tendon organ afferent activity. Muscles which cross two joints can more easily be placed in shortened positions during exercise and would therefore decrease the Golgi tendon organ afferent activity. In addition, sustained abnormal reflex activity would explain increased baseline EMG activity between acute bouts of cramping. Finally, passive stretching invokes afferent activity from the Golgi tendon organ, thereby relieving the cramp and decreasing EMG activity.

Long distance triathlon: demands, preparation and performance

“…These aforementioned findings support Schwellnus et al's (1997) 'muscle fatigue' cramping model, suggesting that 'neuromuscular fatigue' leads to abnormal neuromuscular control, namely an increase in muscle spindle afferent activity, a reduction in Golgi tendon organ afferent activity, and a resulting increase in alpha motor neuron activity and the muscle spasm observed (Schwellnus et al, 1997). The practical application of this information is that long distance triathletes need to pace themselves at speeds closer to, or slightly below, those that are practiced (O'Toole, 1989), so as to avoid excessive fatigue and resulting muscle cramps (Laursen et al, 2009).…”

“…Exercise-associated muscle camping is defined as involuntary painful skeletal muscle spasms that occur during or immediately after physical exercise (Schwellnus et al, 1997). In long distance triathletes, these are typically localized in the calf, hamstring or quadriceps muscles.…”

Electrolyte and Plasma Changes After Ingestion of Pickle Juice, Water, and a Common Carbohydrate-Electrolyte Solution

“…Neurologic fatigue and hyperexcitability 24 and imbalances in electrolytes, such as potassium, magnesium, and calcium, have also been postulated 25 to be associated with EAMCs. If given as part of the diet for long durations, acetic acid increases calcium absorption in rat intestines.…”

Sodium Replacement and Plasma Sodium Drop During Exercise in the Heat When Fluid Intake Matches Fluid Loss

“…Although experimental data are lacking, proposed theories for the cause of exerciseassociated cramping include abnormalities in substrate metabolism, fluid balance, and electrolyte concentrations, especially under extreme environmental conditions of heat or cold. 20 Consumption of a carbohydrate-electrolyte solution has been shown to delay the onset of exerciseassociated muscle cramping during calf-fatiguing exercise compared with no fluid ingestion. 21 In a study of athletes participating in an ultra-endurance run, Schwellnus et al 22 found that plasma sodium concentration at the end of the race was lower in those runners who experienced muscle cramping than in those who did not, although at nonhyponatremic values.…”