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INTRODUCTION

Skeletal muscle cramps are a common affliction in sports. Highly fit athletes must

sometimes succumb to debilitating cramping episodes, and some often compete with

concern, knowing that these painful, involuntary muscle contractions can appear seemingly

without warning or apparent cause.

TWO PERSPECTIVES: FATIGUE VERSUS ELECTROLYTE DEFICIT

Muscle Overload and Fatigue

During sports competition and training repeated or extended loading on muscles can lead to

muscle or tendon strain and local fatigue. The neural mechanisms designed to inhibit muscle

contraction, in response to muscle tension detected by the Golgi tendon organ, are disrupted

or depressed. At the same time, enhanced excitatory activity from the muscle triggers an

intense and sustained involuntary muscle contraction that is unopposed by Golgi tendon

organ control. Notably, shortened muscles with sustained contraction may be particularly

vulnerable to such cramping.

Predisposing risk factors associated with overload and fatigue-related muscle cramping

might include

- older age

- poor stretching habits

- insufficient conditioning

- cramping history

- excessive exercise intensity and duration

- related metabolic disturbances.

Electrolyte Deficit

With exertional heat cramps, an athlete typically has been sweating extensively with

appreciable sweat electrolyte losses as well, particularly sodium and chloride. Whether

during a single long race, match, game, or training session or consequent to multiple sameor

repeated-day exercise bouts, a sizeable whole-body exchangeable sodium deficit

develops when sweat sodium and chloride losses measurably exceed salt intake (6,7,57).

The deficit threshold required to prompt muscle cramping is not well described; however, an

estimated sweat-induced loss of 20%-30% of the exchangeable Na+ pool has been noted

with severe muscle cramping (6,29).

How readily this occurs depends upon

- sweating rate (10)

- sweat sodium concentration (typically 20-80 mmol·L-1) (7,12,28),

- dietary intake (27).

- continuous physical activity over an extended period of time (e.g., 3-4 h or more).

A high sweat sodium concentration generally stays high, even as whole-body water and

sodium deficits progressively increase. This is possible because sweating rate remains fairly

consistent during such long-term activity and serum sodium concentration is typically

maintained or elevated, along with potential changes in sweat gland function or sympathetic

nervous system activity that would tend to increase sweat sodium concentration (33).

Other electrolytes also are lost in sweat to a much lesser degree, and several of these

(namely calcium, magnesium, and potassium) have been implicated falsely as the cause of

muscle cramping during or after exercise when purported deficiencies are suspected

(3,15,23,24,31,56,62,63). However, exertional heat cramp-prone athletes characteristically

develop a sodium deficit because their sweat sodium and chloride losses are not offset

promptly and sufficiently by dietary intake (6,7,57).

RECOVERY AND PREVENTION

Muscle Overload and Fatigue

Overload and fatigue-related muscle cramps remain localized to the overworked muscle(s),

and these cramps often can be resolved readily by passive stretching, massage, active

contraction of the antagonist muscle group, or icing of the affected muscles. Lowering

overall exercise intensity and altering the load on the distressed muscle(s) can be effective

as well. Preventive measures include reducing training and competition intensity and

duration, as well as improving conditioning and range of motion through appropriate and

regular individualized progressive fitness and stretching programs. Adjustments to

equipment configuration and selection (e.g., bicycle seat and handle position, shoes),

biomechanics, and relaxation techniques may also help to avert or delay fatigue-induced

muscle cramping (4,23,30,42,50,54,56).

Electrolyte Deficit

At the first sign of muscle twitches or mild exertional heat cramps, a prompt oral bolus of a

high-salt solution (e.g., 0.5 L of a carbohydrate-electrolyte drink, with 3.0 g of salt added

and thoroughly mixed, consumed all at once or over 5-10 min) has been a proven effective

field strategy in relieving cramping or preventing muscle fasciculations from developing

into a more severe and debilitating condition (6).

Massaging and applying ice to the affected area can assist in relaxing the muscles and

relieving some of the discomfort while waiting for the ingested fluid and salt to be absorbed

adequately into circulation, although the effects of an oral salt solution often can be seen in

just a few minutes (6), as the ingested beverage is rapidly absorbed (14) and plasma sodium

levels quickly begin to change (21).

After such a high-salt solution bolus, athletes can often promptly continue and immediately

resume training or competition effectively without muscle cramping or twitching symptoms

for an hour or more (6), while additional lower-sodium fluid is consumed appropriately at

subsequent regular intervals. Continuation of activity at the same intensity likely would not

be possible if muscle overload or fatigue was the sole or primary contributing factor to the

muscle cramping.

After the training or competition session, any remaining body water and electrolyte deficits

need to be replaced with a particular emphasis on salt intake, in order to help retain (52) and

distribute the ingested fluid, so that all fluid compartments are restored sufficiently (32).

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