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.