Origin and Development of Muscle Cramps
Origin and Development of Muscle Cramps
Cramps are sudden, involuntary, painful muscle contractions. Their pathophysiology remains poorly understood. One hypothesis is that cramps result from changes in motor neuron excitability (central origin). Another hypothesis is that they result from spontaneous discharges of the motor nerves (peripheral origin). The central origin hypothesis has been supported by recent experimental findings, whose implications for understanding cramp contractions are discussed.
A muscle cramp is a sudden, involuntary, painful contraction of a muscle or part of it, self-extinguishing within seconds to minutes and is often accompanied by a palpable knotting of the muscle. The cramp contractions are associated with repetitive firing of motor unit action potentials. This myoelectric activity has been referred to as "cramp discharge".
Cramps may occur in patients with lower motor neuron disorders, neuropathies, metabolic disorders, and acute extracellular volume depletion. However, they also often occur in healthy subjects with no history of nervous or metabolic disorders, such as during sleep, pregnancy, and strenuous physical exercise. The latter cramps have been defined as "benign cramps" or "idiopathic cramps" or "cramps with no apparent cause".
Muscle cramping during or immediately after physical exercise was first reported more than 100 yr ago in miners working in hot and humid conditions. Dehydration (and/or electrolyte depletion) often is given as an explanation for muscle cramps occurring in workers and athletes, al though this claim is not supported by scientific evidence. The main risk factors for exercise-associated muscle cramps include family history of cramping, previous occurrence of cramps during or after exercise, increased exercise intensity and duration, and inadequate conditioning for the activity.
Norris et al. reported a high prevalence of benign cramps in a wide group of healthy young subjects enrolled in an exercise class; 115 (95%) of 121 had experienced spontaneous muscle cramps at least once. Jansen et al. reported an overall yearly incidence of cramps in 37% of healthy adults. Similarly, the lifetime prevalence of cramps in athletes (marathon runners) and sedentary healthy subjects (aged 65 yr or older) has been reported to be as high as 30% to 50%.
A cramp can be distinguished from spasms (i.e., any involuntary and abnormal muscle contraction, regardless of whether it is painful) or generic painful contractions based on clinical and electrophysiological criteria. For example, muscle contractures resemble cramps because they are involuntary and painful. However, they are electrically silent. Dystonias, such as cervical dystonia (spasmodic torticollis) or focal hand dystonia (so called musician's or writer's cramp), are different from cramps because they are involuntary sustained cocontractions of several muscles producing slow, twisting, and repetitive movements or abnormal postures that are not relieved by muscle stretching. Conversely, cramps present unique clinical features: a) they are acutely painful (this may result in persistent soreness and increased levels of circulating muscle proteins); b) they present an involuntary explosive onset and gradual spontaneous resolution or sudden termination with muscle stretching; c) only one muscle or a part of it is involved; d) they are associated with both modest and forceful contractions, especially in shortened muscles; and e) they preferably occur in calf and foot muscles, followed by the hamstrings and the quadriceps.
The purpose of this review is to present recent experimental findings providing new insights into cramp pathophysiology and to discuss their implications for understanding cramp contractions in pathology and consequent to exercise.
Abstract and Introduction
Abstract
Cramps are sudden, involuntary, painful muscle contractions. Their pathophysiology remains poorly understood. One hypothesis is that cramps result from changes in motor neuron excitability (central origin). Another hypothesis is that they result from spontaneous discharges of the motor nerves (peripheral origin). The central origin hypothesis has been supported by recent experimental findings, whose implications for understanding cramp contractions are discussed.
Introduction
A muscle cramp is a sudden, involuntary, painful contraction of a muscle or part of it, self-extinguishing within seconds to minutes and is often accompanied by a palpable knotting of the muscle. The cramp contractions are associated with repetitive firing of motor unit action potentials. This myoelectric activity has been referred to as "cramp discharge".
Cramps may occur in patients with lower motor neuron disorders, neuropathies, metabolic disorders, and acute extracellular volume depletion. However, they also often occur in healthy subjects with no history of nervous or metabolic disorders, such as during sleep, pregnancy, and strenuous physical exercise. The latter cramps have been defined as "benign cramps" or "idiopathic cramps" or "cramps with no apparent cause".
Muscle cramping during or immediately after physical exercise was first reported more than 100 yr ago in miners working in hot and humid conditions. Dehydration (and/or electrolyte depletion) often is given as an explanation for muscle cramps occurring in workers and athletes, al though this claim is not supported by scientific evidence. The main risk factors for exercise-associated muscle cramps include family history of cramping, previous occurrence of cramps during or after exercise, increased exercise intensity and duration, and inadequate conditioning for the activity.
Norris et al. reported a high prevalence of benign cramps in a wide group of healthy young subjects enrolled in an exercise class; 115 (95%) of 121 had experienced spontaneous muscle cramps at least once. Jansen et al. reported an overall yearly incidence of cramps in 37% of healthy adults. Similarly, the lifetime prevalence of cramps in athletes (marathon runners) and sedentary healthy subjects (aged 65 yr or older) has been reported to be as high as 30% to 50%.
A cramp can be distinguished from spasms (i.e., any involuntary and abnormal muscle contraction, regardless of whether it is painful) or generic painful contractions based on clinical and electrophysiological criteria. For example, muscle contractures resemble cramps because they are involuntary and painful. However, they are electrically silent. Dystonias, such as cervical dystonia (spasmodic torticollis) or focal hand dystonia (so called musician's or writer's cramp), are different from cramps because they are involuntary sustained cocontractions of several muscles producing slow, twisting, and repetitive movements or abnormal postures that are not relieved by muscle stretching. Conversely, cramps present unique clinical features: a) they are acutely painful (this may result in persistent soreness and increased levels of circulating muscle proteins); b) they present an involuntary explosive onset and gradual spontaneous resolution or sudden termination with muscle stretching; c) only one muscle or a part of it is involved; d) they are associated with both modest and forceful contractions, especially in shortened muscles; and e) they preferably occur in calf and foot muscles, followed by the hamstrings and the quadriceps.
The purpose of this review is to present recent experimental findings providing new insights into cramp pathophysiology and to discuss their implications for understanding cramp contractions in pathology and consequent to exercise.
