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Muscle Cramping During Exercise: Causes, Solutions, and Questions Remaining

Maughan, R.J. and Shirreffs, S.M., 2019. Muscle cramping during exercise: causes, solutions, and questions remaining. Sports Medicine, 49(2), pp.115-124.

In today’s letter

• Overview of the study breaking down the possible reasons for cramping

• Rapid Results = There are 2 hypothesis of why cramps happen

  1. Disturbances of Hydration and Electrolyte Balance

  2. Altered Neuromuscular Control

• The evidence supporting both sides of the debate is weak and the overall understanding is unclear

• 1 video to check out to further you knowledge about cramps

• Meme of the week: F** around and find out 🤕

Aim of study

To review literature on muscle cramps and provide reasons for possible causes

Did you know?

• Exercise-associated muscle cramps (In this newsletter we will refer to them muscle cramps) are defined as painful, spasmodic contractions of the skeletal muscles that occur during or immediately after physical activity.

• Various studies have tried to determine the prevalence of muscle cramps in different sports, but results are difficult to compare due to differing definitions, measurement periods, and assessment tools.

• Muscle cramps has been reported in 67% of triathletes during or after training or racing, 18-70% of marathoners or endurance cyclists, and 30-53% of American football players.

• Cramping is usually a minor inconvenience: serious muscle cramping occurs in fewer than one per thousand runners, according to Schwabe et al., who studied 65,865 participants in half-marathon and ultra-marathon events.

• For context, Abdulla et al. found that 50% of outpatients aged 65 and older experienced frequent muscle cramps

• The intensity and duration of cramps can vary widely, from minor spasms resolving in seconds to whole-body lockups lasting several minutes.

• In severe cases, muscle pain from cramps may persist for hours or days after the initial contraction, potentially affecting training or competition.

• EIn addition, localised calf cramps in football (soccer) players late in the game differ from the whole-body cramps reported by some American football players and tennis players

• Cramps often occur spontaneously and unpredictably, sometimes with fasciculations or prodromal symptoms, but sometimes without any warning.

• Some cramps happen early during exercise, some after prolonged exercise, and others minutes or hours post-exercise.

• The mechanisms behind different types of cramps are unclear.

• Two main hypotheses for muscle cramps are debated:

  1. A disturbance in water and salt balance

  2. A neurological cause leading to sustained abnormal motor drive to the affected muscles.

 Risk Factors for Exercise‐Associated Muscle Cramp

• Muscle cramps observed in both training and competition across nearly all sports but is more common in endurance activities and team sports.

• Analysis of American football players showed that 95% of cramps occurred during hot weather.

• The incidence of heat cramps among American football players was 37% in the first week of training camp, decreasing to 27%, 18%, and 4% in later weeks.

Schwellnus and colleagues have studied the primary risk factors for cramps in endurance events, noting that muscle cramps in marathon runners is linked to:

1. High intensity

2. Long duration

3. Hilly terrain

• Causing 'premature muscle fatigue' in those with a cramping history.

• The term 'premature' fatigue and its distinction from normal fatigue in endurance events is unclear.

• In a prospective study of 210 Ironman triathletes, Schwellnus et al. found that a history of muscle cramps and competing at higher-than-usual exercise intensity were independent risk factors, while dehydration and serum sodium changes were not predictors.

• Running at a faster pace than usual in training was also identified as a risk factor.

• Additional risk factors included older age, a longer running history, higher BMI, shorter daily stretching time, irregular stretching habits, and a family history of cramping.

Possible Causes of EAMC

• 2 main causes of muscle cramps have been proposed, influencing prevention and treatment strategies depending on which cause is believed.

• It should be noted that the evidence supporting either side of the debate is weak and the overall understanding is unclear.

• A single mechanism is unlikely to explain all types of cramps in every situation

• Regardless of the primary cause, cramps are characterised by active contraction of the affected muscle, indicated by high levels of muscle electrical activity.

Idea 1- Disturbances of Hydration and Electrolyte Balance

• Schwellnus dismissed the role of changes in hydration and electrolyte balance in causing muscle cramps, citing weak scientific evidence and anecdotal observations.

• The strongest evidence linking sweat-related electrolyte imbalances to muscle cramps comes from studies on industrial workers, including miners and steel mill workers in the 1920s and 1930s.

• These early studies, despite methodological limitations, had large populations and kept detailed medical records related to productivity.

Moss's extensive report on coal miners identified

1. High air temperatures

2. Excessive water intake

3. Continued hard work

As factors contributing to cramps.

• Moss observed that cramps often occurred in the 2nd half of a working shift and were more common in less physically fit men

• Cramp was attributed to a combination of chloride loss from sweating, excessive water intake, and temporary paralysis of renal excretion, not dehydration or increased serum electrolyte concentrations.

• Large-scale prospective studies, such as those by Dill et al. at the Hoover Dam and in steel mills, assessed water and salt balance in muscle cramps.

• Dill et al. found that men suffering from cramps showed signs of dehydration and overhydration, with immediate relief from symptoms after isotonic saline injection.

• Recent studies on athletes, including marathon runners and triathletes, have not shown a link between cramps and serum electrolyte changes.

• Serum electrolyte concentrations may be irrelevant as they do not reflect local intracellular and extracellular changes affecting muscle and nerve membrane potential.

• However, blood samples in these studies were often collected long after cramping had resolved, potentially missing relevant changes.

• Ohno and Nosaka showed that a body fluid deficit of 3% of body mass increased muscle cramps during a muscle cramp test in toe flexors, but not in knee extensors.

• This result suggest that sweat loss combined with water intake increases susceptibility to electrically induced muscle cramp, which decreases with high electrolyte content drinks.

• Cramps can occur with large sweat losses during prolonged exercise in heat but also in cool environments without significant sweat loss.

• This indicates that sweat loss and electrolyte balance disturbances alone cannot explain all cramps.

• Despite these observations, evidence from industrial settings shows that cramping is more frequent in hot environments with high sweat losses.

• Cramping's association with hot (though not necessarily humid) environments and high sweat losses is well-documented in large-scale industrial settings.

• Local intracellular and extracellular electrolyte concentrations, rather than plasma concentrations, may be relevant in understanding muscle cramps.

Idea 2- Altered Neuromuscular Control

• The idea that muscle cramps are neurological rather than muscle-related is not new.

• Telegraphists' cramp, affecting hand muscles used in Morse code operation, was investigated by a UK Parliamentary enquiry in 1911.

• The committee concluded that telegraphists' cramp is a central nervous system disease, resulting from a breakdown of cerebral control due to muscle strain.

• Schwellnus et al. back in 1997 hypothesised that cramp is caused by "sustained abnormal spinal reflex activity secondary to muscle fatigue."

• Muscle cramp was attributed to sustained alpha motor neuron activity abnormalities at the spinal level, though the cause of this remains unknown.

• Muscle fatigue is due to its excitatory effect on muscle spindle afferent activity and inhibitory effect on Golgi tendon organ afferent activity.

Explanation:

Muscle spindles are sensory receptors located within muscles that detect changes in muscle length and the rate of that change. Afferent activity refers to the signals sent from these receptors to the central nervous system. This increased activity can lead to heightened muscle tone and contraction in response to muscle stretch, contributing to the sensation of fatigue

Golgi tendon organ are sensory receptors located at the junctions between muscles and tendons. They detect changes in muscle tension. Golgi organs typically help protect muscles from excessive force by inhibiting muscle contraction when tension is too high. With reduced signalling due to fatigue, this protective mechanism is decreased, leading to less effective regulation of muscle tension.

• Evidence supporting this hypothesis includes the observation that passive stretching during a cramp episode can alleviate symptoms via autogenic inhibition by the tendon organ reflex.

However, this does not explain:

1. Why cramp is not an inevitable consequence of exercise-induced fatigue

2. Why it occurs more in high thermal stress environments

3. Why it affects some individuals but not others.

• Athletes prone to cramps show a lower threshold for muscle cramps induced by electrical stimulation of motor nerves.

• Blocking motor nerves with anesthetic does not eliminate these cramps, but higher stimulation frequency is required and cramp duration is reduced, suggesting altered motor unit discharge

• Strong objections to the dehydration/electrolyte loss theory come from studies where preventing dehydration did not affect the onset of electrically-evoked cramps.

• Fatigue alone is unlikely to be the sole cause, though it may contribute; marathon runners often cramp towards the end of races.

A resource

To learn more about cramps

To be honest, all information will lead to the same path, thus herre is 1 video that summarises everything pretty well

1. The GOATS, PhysioTutors 👉️ - LINK

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