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Ouch! What Causes Muscle Cramps?

By Len Kravitz, PhD on Sep 11, 2019

Muscle cramps can stop athletes in their tracks. Although they usually self-extinguish within seconds or minutes, the abrupt, harsh, involuntary muscle contractions can cause mild-to-severe agony and immobility, often accompanied by knotting of the affected muscle (Minetto et al. 2013). And cramps are common; 50%–60% of healthy people suffer muscle cramps during exercise, sleep or pregnancy or after vigorous physical exertion (Giuriato et al. 2018). There is no gender difference with skeletal muscle cramps, but they appear to occur more often in endurance athletes and in the elderly (Naylor & Young 1994).

During endurance exercise, muscle cramps correlate with the length and intensity of workouts. Fitness pros and clients frequently talk about muscle cramps, but until recently, little has been known about their actual physiology.

Types of Muscle Cramps

Giuriato et al. categorized muscle cramps into three groups:

  1. Nocturnal cramps occur during sleep without any clear trigger.
  2. Pathological cramps are a consequence of having diabetes, nerve dysfunctions or metabolic disorders.
  3. Exercise-associated muscle cramps occur during or after exertion. The first scientific confirmation of these types of cramps dates to 1908, when they were described in miners working in hot and humid conditions.

Muscle Cramp Risk Factors

With marathon runners, research has found certain risks associated with the occurrence of a muscle cramp (Schwellnus, Derman & Noakes 1997). These risks include a longer history of running, advanced age, higher body mass index, shorter daily stretching time, irregular stretching habits and a family history of cramping. Schwellnus, Derman & Noakes report that the two most important observations from the research are that exercise-associated cramps for marathoners are muscle fatigue (linked to longer runs) and poor stretching habits.

See also: The Many Dimensions of Pain

Early Theories About Muscle Cramp Causes

Theories on the Cause of muscle cramps
Early theories on the source of muscle cramps focused on electrolytes, dehydration and the environment.

Schwellnus, Derman & Noakes analyzed three early theories on the causes of exercise-associated muscle cramps.

SERUM ELECTROLYTE THEORY

Blood plasma contains electrolytes, such as sodium, potassium, chloride, bicarbonate, calcium and phosphate. Although electrolyte depletion is often blamed for causing cramps, there currently is no solid explanation of how low serum electrolyte concentrations can result in the condition. Schwellnus, Derman & Noakes pointed to two studies that measured serum electrolyte concentrations in endurance runners at prerace, immediate postrace and at 60-minute postexercise recovery. Neither study found a connection between postrace recovery, muscle cramps and changes in serum electrolyte concentrations.

DEHYDRATION THEORY

In the past, studies have suggested treating muscle cramps in workers and firefighters with fluids and electrolytes. But those studies did not measure hydration. More recent studies that have estimated blood volume and plasma volume do not support the theory that dehydration has a direct link to exercise-associated cramps.

ENVIRONMENTAL THEORY

This theory sprang from the condition referred to as “heat cramps.” While exercising in a hot, humid environment may be correlated with the development of muscle cramps, no evidence shows cramps are linked to an increase in core body temperature.

Current Theory on Muscle Cramps

The newest concept of muscle cramps is a neuromuscular theory (Giuriato et al. 2018). This theory has evolved to point to two origins: a central (spinal column) and a peripheral (neuromuscular junction).

The central or spinal origin theory suggests that the involuntary contraction of a muscle occurs when nerve messages to the spinal column are altered, perhaps due to muscle fatigue (see “The Neuromuscular Theory of Skeletal Muscle Cramps,” below). This results in an imbalance of excitatory (from muscle spindles) and inhibitory (from Golgi tendon organs) spinal messages to muscles (see “What are Muscle Spindles and Golgi Tendon Organs?,” below). This neural signaling imbalance leads to enhanced muscle cell excitability and cramping.

With the peripheral origin theory, scientists suggest there is abnormal excitation of the motor nerves terminal branches to the muscle, causing cramping.

The scientific evidence of a neuromuscular theory is mounting. The research appears to show that, in some cases, fatigued muscle can’t fully relax. This condition leads to an imbalance between excitatory signals and inhibitory messages to the muscle. So the most recent research appears to support the central origin theory of the muscle cramp (Giuriato et al. 2018; Scwellnus, Derman & Noakes 1997).

Diagram on Neuromuscular Theory for Muscle Cramps

See also: How to Handle Exercise-Related Muscle Cramps

What Are Muscle Spindles and Golgi Tendon Organs?

Muscle spindles and Golgi tendon organs are referred to as proprioceptors. A proprioceptor is a sensory receptor that receives stimuli from within the body, particularly signals related to body position and movement. The neuromuscular theory of muscle cramps suggests that muscle spindle and Golgi tendon organ signaling are involved in the development of a muscle cramp (Giuriato et al. 2018).

Muscle spindles are stretch receptors within a muscle that serve to detect changes in the length of the muscle and/or the speed of length change. They convey muscle length information to the spinal column via specialized afferent nerve fibers. The muscle spindle activates the stretch reflex. This happens when a muscle is stretched quickly to its endpoint of movement. The muscle spindle sends a rapid message to the spinal column, which tells the muscle to contract, keeping the muscle from overstretching.

Golgi tendon organs, also called Golgi organs, are neurotendinous sensory organs that sense changes in tension within a muscle. The Golgi tendon organ lies at the origin and insertion of skeletal muscle fibers into the tendons. If there is too much tension (i.e., too much force) placed upon a muscle, the Golgi tendon organ will inhibit the muscle from creating any more force (via a reflex arc), protecting it from injury.

MUSCLE SPINDLE (CROSS SECTION)



MUSCLE SPINDLE (CROSS SECTION)

It is surrounded by a capsule and includes fibers with nuclear bag and nuclear chain fibers.

MUSCLE SPINDLE (LONGITUDINAL SECTION)

MUSCLE SPINDLE (LONGITUDINAL SECTION)

It is located between normal or extrafusal muscle fibers. Two types of fiber are distinguished in its interior: nuclear chain fibers and nuclear bag fibers.

Muscle Cramp Prevention

Intense, extremely long workouts (relative to the fitness of the exerciser) clearly lead to more skeletal muscle cramps. Lack of training and training in a hot, humid environment also predispose a person to muscle fatigue and potential cramping. As mentioned previously, research also shows muscle cramps are more common in the elderly, a phenomenon that needs more research but is important for fitness pros to be aware of.

Although studies show that poor or inadequate stretching may spark muscle cramps, there is no evidence-based recommendation for what type and how much stretching should be done to ward off cramps. But encouraging clients to stretch regularly and with proper body alignment after exercise seems logical.

See also: Treating Muscle Soreness: Heat or Cold?

Research Takeaways

From a health and teaching perspective, the latest research shows no evidence that muscle cramping is due to electrolyte imbalances or water depletion in muscle. Studies also fail to support the use of particular supplements to impede cramps. What is imperative is avoiding intense or long workouts for which clients are not properly prepared. Teaching and encouraging proper stretching exercises, particularly of the limbs, is also essential. Although studying cramps is complicated, more research needs to be done to better understand the mechanisms of cramps and develop evidence-based strategies to prevent them.

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Len Kravitz, PhD

Len Kravitz, PhD, is the program coordinator of exercise science and a researcher at the University of New Mexico in Albuquerque, where he recently won the Outstanding Teacher of the Year award. Len was also honored as the 2006 Fitness Educator of the Year by the American Council on Exercise.

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