What really causes muscle cramps?
Muscle cramps have long been attributed to electrolyte depletion and dehydration, especially when they occur in athletes. However, research does not support these theories, and instead points to altered neuromuscular control and muscle fatigue as the culprits.
In this post, I’ll summarize the research and explain why muscle cramps actually occur, and discuss the most effective way to reduce and eliminate muscle cramps.
What is a muscle cramp?
Muscle cramps are short-lived, intense, involuntary muscle contractions. They most often occur in the legs—most of us have had a “charley horse” at one time or another—but can occur in any muscle that is overworked.
Muscle cramps are most often caused by physical activity, and go away with rest or stretching. These types of muscle cramps, while painful and frustrating, are harmless.
However, certain medical conditions including nerve compression, inadequate blood supply, and mineral depletion can cause muscle cramping. If your muscle cramps fit any of the following criteria, you should see your doctor:
- Cause severe discomfort
- Are associated with leg swelling, redness, or skin changes
- Are associated with muscle weakness
- Happen frequently
- Don’t improve with rest or self-care
- Aren’t associated with an obvious cause, like exercise
The electrolyte-depletion and dehydration hypotheses of muscle cramping
Dr. Martin Schwellnus published a thorough review of the research on muscle cramping titled “Cause of Exercise Associated Muscle Cramps (EAMC)—Altered Neuromuscular Control, Dehydration or Electrolyte Depletion?” in the British Journal of Sports Medicine. I’ll be referring to his article throughout these two sections, and you can read his article here.
The association between electrolyte depletion, dehydration, and muscle cramps dates back more than 100 years. The first recorded reports of muscle cramping resulting from physical activity were from people working in hot, humid conditions on steamships and in mines. These early reports of muscle cramping were attributed both to the hot work environment and to profuse sweating.
While exercise associated muscle cramps (EAMC) are also known to occur in athletes exercising in moderate, cool, and extreme cold conditions, the term “heat cramps” persists today.
Both electrolyte depletion and dehydration can result from intense exercise and sweating, and it has long been believed that these are the causes of muscle cramping in athletes. However, there are no published studies showing that electrolyte concentrations are abnormal in athletes at the time of cramping, nor are there any published studies showing that athletes with EAMC are more dehydrated than controls.
In fact, there are four prospective cohort studies that show no correlation between electrolyte concentrations and muscle cramping in marathon runners and triathletes. Likewise, prospective cohort studies consistently show that athletes at the time of acute cramping are not more dehydrated than non-cramping athletes.
As Dr. Schwellnus explains, EAMC tends to occur locally in working muscle groups, so the theory that a systemic imbalance—like electrolyte depletion or dehydration—is the cause does not make sense. A systemic imbalance would affect all the skeletal muscles in the body. However, the majority of cases of EAMC occur locally in muscle groups that are being repeatedly contracted.
The theory of altered neuromuscular control and muscle fatigue
Research supporting altered neuromuscular control and muscle fatigue as the causes of muscle cramps dates back more than 60 years. In 1957, a study of 115 college students showed that sustained maximal muscle contraction before a period of exercise (and thus, before any potential electrolyte depletion or dehydration had occurred) resulted in muscle cramping in 18% of the students. The study was one of the first to use electromyography to show that exercise induced muscle cramps were electrically active, and that these cramps could be treated with passive stretching.
These findings have been confirmed by subsequent research, and in addition, laboratory studies show that repetitive electrical stimulation of motor nerves can reliably induce muscle cramping in humans. So, the first element of muscle cramping is repetitive voluntary muscular contraction. The effects of this can be exacerbated by muscle fatigue: Evidence shows that muscle fatigue is associated with increased excitatory signals and decreased inhibitory signals to motor nerves. So as muscles become fatigued by intense or extended periods of exercise, muscles that are being contracted repeatedly will be even more susceptible to cramping.
The theory that muscle cramps are a result of altered neuromuscular control and muscle fatigue also explains why non-athletes experience muscle cramps. Wearing high heels, for example, causes muscle cramps in the calves and feet because those muscles are kept in a shortened, contracted state all day long.
As you may already know from reading about the stretch reflex and my constant harping on about the inefficacy and dangers of static stretching, pulling a muscle past the point to which it can voluntarily lengthen causes it to reflexively contract in order to protect itself from being torn: This is the stretch reflex or myotatic reflex.
However, we have another reflex called the reverse myotatic reflex, which can override the myotatic reflex. This reflex kicks in when a muscle is under extreme tension (like holding something very heavy) and causes the muscle to automatically release completely so that the muscle and attached tendons don’t get torn.
The reverse myotatic reflex is one reason why passive stretching alleviates muscle cramps. Pulling on the already tight muscle increases tension in the muscle to the point that the reverse myotatic reflex kicks in, automatically releasing the cramping muscle.
Another reason why passive stretching alleviates muscle cramps is reciprocal inhibition. This is an automatic reaction that occurs in our nervous system when we voluntarily contract a muscle group: Reciprocal inhibition kicks in and releases the opposing muscle group, allowing for full contraction of the working muscles. When we passively stretch a cramping muscle, we often instinctively contract the opposing muscle group, so reciprocal inhibition releases the cramping muscle.
However, it’s important to know that passive stretching is only a temporary fix for muscle cramps. Passive stretching does not change the baseline level of muscle tension in a non-working muscle that is set by the nervous system. And as it turns out, athletes tend to experience cramps in muscles that have increased EMG activity while at rest. This is referred to as a “cramp prone state.”
A study of triathletes found that baseline EMG activity was higher in cramping muscles than non-cramping muscles in between periods of acute cramping. In these athletes, muscle cramping was not associated with electrolyte depletion or dehydration. This finding makes a great deal of sense when you understand how the nervous system develops increasing levels of baseline muscle tension as a result of repetitive activities. It also offers an effective solution for people who experience muscle cramps on a regular basis: pandiculation.
Stop getting muscle cramps by reducing your baseline level of muscle tension
Whether you’re an athlete or not, if you experience muscle cramps on a regular basis you can reduce and even eliminate them by reducing your resting level of muscle tension.
When we repeat a movement over and over, our nervous system gradually makes it automatic so that we don’t have to consciously think about it anymore. This innate process of developing muscle memory is a survival mechanism: It helps us act quickly in dangerous situations.
Muscle memory is a wonderful thing, because it helps us move through our daily lives efficiently. The downside is that we tend to build up elevated levels of tension in the muscles that we use most often. This results both from messages being sent by our brain to contract the muscles, and from a feedback loop called the gamma loop. The gamma loop operates between the spinal cord and the muscles, quickly and automatically regulating the level of tension in our muscles.
When we repeatedly choose to contract certain muscles, the gamma loop adjusts, gradually increasing the baseline level of tension in these muscles. Muscles with elevated levels of tension are in the “cramp prone state;” it takes fewer muscular contractions and less fatigue to push them over the edge into a cramping state.
The most effective way to reduce the baseline level of muscle tension is with pandiculation. Pandiculation is our nervous system’s instinctive reaction to excess muscle tension. If you’ve ever seen a cat or dog arch their back when they get up from a nap, you’ve witnessed the pandicular response.
Thomas Hanna developed exercises that use the pandicular response to reduce muscle tension (I sometimes refer to his technique as “voluntary pandiculation”). Voluntary pandiculation is a specific way of contracting and very slowly releasing muscles that sends accurate biofeedback to the nervous system about the level of tension in the muscles. The technique of pandiculation allows you to retrain your own nervous system, reducing baseline muscle tension and changing habitual posture and movement patterns.
If you want to learn Thomas Hanna’s technique of pandiculation, the best place to start is with the online Level One & Two Courses.