
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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