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Ouch! What Causes Muscles to Cramp
Rogelio Realzola, M.S. and Len Kravitz, Ph.D.

Muscles cramps are abrupt, harsh, involuntary muscle contractions that can cause mild-to-severe agony and immobility (Minetto et al.,2013). Minetto and colleagues add that muscle cramps usually self-extinguish within seconds to minutes but may be accompanied with a knotting of the affected muscle. They occur in healthy people during exercise, sleep, pregnancy or after vigorous physical exertion. There is no gender difference with skeletal muscle cramps. However, they appear to occur more frequently with endurance athletes and in the elderly (Giuriato et al., 2018). According to Giuriato, the occurrence rate of muscle cramping is 50-60% in a healthy population. During endurance exercise, muscle cramps are correlated with long duration workouts, as well as harder workout intensities. While they are widely discussed by fitness pros, until recently, little has been known about the actual physiology of cramps, which we will discuss in this research column.

What are the Types of Muscle Cramps?
Muscle cramps are multifactorial that Giuriato and colleagues (2018) have categorized into three groups: (1) nocturnal cramps, which occur during sleep without any clear causal mechanism; (2) pathological cramps, which are a consequence of having diabetes, nerve dysfunctions, or metabolic disorders in the body; (3) exercise-associated muscle cramps (EAMC), the muscle cramps that occur while exercising or post-exercise. It is interesting to note that the first scientific confirmation of EAMC dates back to 1908, when cramps were described with mineworkers who were required to do physical work in a high-humidity, hot environmental condition (Giuriato).

What are the Risk Factors Associated with Muscle Cramps?
With marathon runners, Schwellnus et al. (1997) summarize research that shows certain risk factors are more associated with the occurrence of a muscle cramp. These risks include a longer history of running (i.e., running years), older chronological age of the individual, higher body mass index, shorter daily stretching time, irregular stretching habits and a positive family history of cramping. With marathon runners, Schwellnus summarize that the two most important observations from the research are that EAMC are associated with longer running conditions (which lead to muscle fatigue), and poor stretching habits.

What are the Early Theories of the Causes of EAMC?
The following is an analysis of three early theories on the causes of EAMC.
Serum electrolyte theory: Blood plasma contains all electrolytes, such as sodium, potassium, chloride, bicarbonate, calcium, phosphate and more. Although a depletion of electrolytes is often labeled as a theoretical cause of cramps in the lay literature, there currently is no solid explanation of a mechanism whereby abnormalities of serum electrolyte concentrations can result in skeletal muscle cramping. Schwellnus and colleagues (1997) summarize two studies that measured various serum electrolyte concentrations with endurance runners at pre-race, immediate post-race and 60-minutes post-exercise recovery. Interestingly, in both investigations there was a complete dissociation between the post-race recovery, EAMC and changes in serum electrolyte concentrations. Therefore, research findings do not support the hypothesis that EAMC being associated with abnormalities in serum electrolyte concentrations (Schwellnus).

Dehydration theory: Early studies, which did not directly measure hydration in workers and fireman experiencing cramps, treated study participants with fluids and electrolytes to manage muscle cramps. However, more recent studies, which have estimated measures of blood volume and plasma volume, do not support the hypothesis of a direct relationship between dehydration and EAMC (Schwellnus et al, 1997).

Environmental theory: The environmental theory comes from the term and condition referred to as 'heat cramps.' Physical exercise performed in a hot, humid environment may be correlated with the development of EAMC. However, there is no evidence that an increase in core body temperature is positively be linked to EAMC (Schwellnus et al, 1997).

What is the Current Theory on the Cause of Muscle Cramps?
The newest concept of muscle cramps is a neuromuscular theory (Giuriato et al., 2018). Currently, this theory has evolved to have two different origins: a central (i.e., spinal column) and a peripheral (i.e., neuromuscular junction) origin which are discussed below.

The central, or spinal, origin theory suggests that the involuntary contraction of a muscle cramp occurs when nerve messages to the spinal column are altered, perhaps due to muscle fatigue (See Figure 1). This results in an imbalance of excitatory (from muscle spindles) and inhibitory spinal messages (golgi tendon organs) to muscles (See Side Bar 1 on muscle spindles and golgi tendon organs). This neural signaling imbalance to muscle leads to enhanced muscle cell excitability and a muscle cramp.

With the peripheral origin theory, scientists submit there is abnormal excitation of the motor nerves terminal branches to the muscle, leading to a cramping in the muscle.
The scientific evidence of a neuromuscular theory is mounting (Giuriato et al., 2018). The research appears to show that in some incidences, fatigued muscle is unable to fully relax. This condition leads to the imbalance of excitatory signals in relation to inhibitory messages to the muscle. Thus, current research findings appear to suggest that the central origin theory is the primary mechanism responsible for the muscle cramp (Giuriato et al., 2018).

Are There Any Cautions to Protect Against Cramps from Occurring?
It is clear that intense, extremely long duration workouts (for the level of fitness of the exerciser) leads to more skeletal muscle cramps. As well, lack of training and/or training in a hot, humid environment predisposes a person to muscle fatigue and possible muscle cramping. Research also shows there is a greater incidence of muscle cramps with the elderly, a phenomenon which needs more research, but important for fitness pros to be aware. Although the research shows that poor or inadequate stretching may predispose a person to muscle cramps, we do not have an evidence-based recommendation for what type of stretches and how much stretching should be done to reduce cramps. However, encouraging clients to stretch regularly, with proper bodily alignment, after a training session seems quite relevant.

Take Home Messages and Muscle Cramp Reflections
From a health and teaching perspective, the results of this new research show that we no longer have the evidence to state that a cramp is due to electrolyte imbalances or water depletion in muscle. Further, recommending particular supplements in the hopes that it impedes cramps also appears not to be rooted in any current literature. Being restrained about taking clients through too intense and/or long-duration workouts they are not properly prepared to do is imperative. Teaching and encouraging proper stretching exercises, particularly of the limbs is also essential. Although studying cramps is complicated and the experimental research is difficult to produce, more studies need to be done to hopes to understand the mechanisms of a cramp better, and then to develop evidence-based strategies to combat them from occurring.

Side Bar 1: What is a Muscle Spindle and Golgi Tendon Organ?
Muscle spindles and glogi tendon organs are referred to as proprioceptors. A proprioceptor is a sensory receptor which receives stimuli from within the body, particularly signals that are related to bodily 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 mechanism describes when a muscle is stretched quickly to its end-point of movement. The muscle spindle sends a rapid message to the spinal column which returns a protective message to the muscle to contract (i.e., protecting the muscle from any damage from over stretching).

Golgi tendon organs, also called Golgi organs, are a neurotendinous sensory organ which sense changes in muscle tension within a muscle. The golgi tendon organ lies at the origin and insertion of skeletal muscle fibers into the tendons (of that skeletal muscle). If there is too much muscle 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), thus protecting the muscle from being injured.

Figure 1. The Neuromuscular Theory of Skeletal Muscle Cramps
Adapted from Schwellnus et al. 1997

Rogelio Realzola, M.S, CSCS, ACE-CPT, USA-W is a strong advocate that exercise is the best medicine a person can get with his highly effective personal training. He is pursuing his doctoral degree in Exercise Science at the University of New Mexico.

Len Kravitz, PhD, CSCS, is the program coordinator of exercise science at the University of New Mexico, where he received the Outstanding Teacher of the Year and Presidential Award of Distinction. He just released his third book, HIIT Your Limit: High-Intensity Interval Training (Amazon).

Giuriato, G., Pedrinolla, A., Schena, F., and Venturelli, M. (2018). Muscle cramps: a comparison of the two-leading hypothesis. Journal of Electromyography and Kinesiology. 41: 89-95.
Minetto, M.A, Holobar, A., Botter, A., and Farina, D. (2013). Origin and development of muscle cramps. Exercise and Sport Science Reviews, 41(1), 3-10.
Schwellnus, M.P., Derman, E.W., and Noakes, T.D. (1997). Aetology of skeletal muscle 'cramps' during exercise: A novel hypothesis. Journal of Sports Science, 15(3), 277-285.