The Real Cause of Muscle Cramps
It feels as if you’ve been blind-sided. When a muscle painfully seizes up during training or competition, your forward motion comes to a screeching, and sometimes screaming, halt until the cramp subsides and you gingerly resume. Nearly everyone has experienced muscle cramps- a painful, involuntary, and sustained contraction of one or more muscles that can lead to the abrupt cessation of your physical activity. You may experience a lingering soreness, reduction in normal muscle function, and even signs and symptoms of muscle damage. You know the pain but what about the root cause of a cramp? You know the popular home treatments: bananas, salt pills and electrolytes. But what is the hard science of prevention and treatment? Scientists have started to solve the mystery of a condition as old as competition itself.
Cramps Are Nothing New
Not surprisingly, written accounts of muscle cramps date back at least a century1. One thing is certain in history- muscles don’t cramp on their own.
The normal chain of events is that when you decide to move, your brain sends signals to nerve cells in your spinal cord that connect to your muscles. The stimulated muscles contract and off you go. When the nervous input to muscles ceases, so does muscle contraction. This simplified explanation leaves out many details, including the important fact that as muscles contract, a lot of information is sent from the muscles and their tendons back to the spinal cord to keep the central nervous system informed about joint position, muscle length, tendon tension, muscle temperature, and the surrounding chemical environment. Your body is a complex circuit board, constantly adjusting to new inputs.
Cramps Are A Failure of Neuromuscular Performance
When the neuromuscular system (your nerves plus muscles) works in sync, your exercise routine can continue for hours. You feel unstoppable. But when the system is perturbed by low blood glucose, muscle glycogen depletion, dehydration, accumulated muscle damage, high body temperature, severe salt loss, accumulation of metabolites, or reduced muscle blood flow, fatigue will gradually or suddenly sets in. The neuromuscular system becomes unstable. Muscle cramps are often associated with fatigue, a painful example of a failure in neuromuscular performance.
As a fuller picture of the etiology (cause) of muscle cramps is emerging through studies by researchers such as Dr. Rod MacKinnon, a Nobel Prize-winning neuroscientist, there are new insights that will help prevent or minimize cramps. First, the cramp is not your fault. Most likely, your cramp had nothing to do with preparation or fitness. As we’re learning, the breakthrough in solving the cramp mystery is in understanding the root of the problem. It’s not the muscle; it’s the nerve.
It’s Not The Muscle; It’s The Nerve
Skeletal muscle cells, as opposed to cardiac muscle cells in the heart and smooth muscle cells in the lining of blood vessels and the intestine, are under voluntary control. You will a muscle to contract and it obeys.
But all those hours you spent training can be for naught with just one ill-timed muscle cramp. You can’t finish a race or complete a training session. Regardless of the conditions that provoke it, muscles cramp because of hyper-excited nerves, alpha motoneurons to be more precise, the nerve cells that project from the spinal cord directly to many skeletal muscle fibers. The motoneuron and the connected muscle cells go haywire, the motor unit malfunctions. In the laboratory, muscles can be made to cramp by electrically stimulating motoneurons, a simple way to illustrate the point that your muscles are slaves to your nervous system.
Maintaining Neuromuscular Performance
Some important clues have surfaced over the past few years that lend support to the notion that “calming” hyper-excited nerves will prevent muscle cramps. An initial finding is that cramp-prone subjects require less electrical stimulation to produce a cramp, suggesting that their neuromuscular systems are more sensitive to cramping2. This observation helps explain why some athletes are haunted by cramps, while others rarely have a problem.
There have been many proposed “cures” for muscle cramps, including eating mustard and drinking pickle juice. At first, the effectiveness of pickle juice baffled scientists, but laboratory studies confirmed its benefit, at least on small muscle groups in the foot that were electrically stimulated to cramp3. Those results led scientists to speculate that pickle juice might activate sensory fibers in the mouth and throat and send signals to the nervous system that calm down hyper-exitable motoneurons and reduce the duration of a cramp.
This mouth-to-spine-to-muscle connection is not as far-fetched as it may sound. We have all experienced how the nervous system reacts to icy-cold drinks, acidic solutions such as pickle juice, and hot spices. For example, “brain freeze” often occurs as a result of quickly drinking ice-cold beverages because of rapid cooling of the sphenopalatine ganglion, a cluster of nerves adjacent to the roof of the mouth. For similar reasons, certain spices and other natural ingredients may be good anti-cramp candidates because spices such as capsaicin in red peppers activate specific membrane channels in sensory nerves called TRP channels that are found in the oropharyngeal region (mouth and throat) and esophagus that project to the spinal cord and indirectly inhibit hyper-excited alpha motoneurons.
This theory came to MacKinnon, who is an endurance athlete and sea kayaker himself, after a bout of debilitating cramps on the open ocean. The experience reminded him that, aside from being a painful nuisance, muscle cramps could mean the difference between life and death in some situations. MacKinnon won the 2003 Nobel Prize in chemistry for his work in understanding how channels in cell membranes work to move ions such as potassium from one side of a membrane to the other. Building on this, he reasoned that the right combination of natural TRP channels activators could switch on nerves that could in turn inhibit the over-active alpha motoneurons that cause cramping.
MacKinnon’s original idea has triggered a wave of laboratory and field research, with recent results demonstrating that the frequency and duration of cramps can be reduced when subjects ingest a specially formulated spicy beverage before exercise.
As a result of this research, we are coming closer to the understanding the true cause of cramping. So, while electrolytes, hydration, and fitness certainly matter for performance, athletes should not be looking to those elements to cure their cramping issue. As we continue to learn more about the root cause of cramps, we will also begin to understand how we can prevent them.
Minetto MA, A Holobar, A Botter, D Farina. (2013) Origin and development of muscle cramps. Exerc Sports Sci Rev 41(1):3-10.
Minetto MA, A Botter. Elicitability of muscle cramps in different leg and foot muscles. (2009) Muscle Nerve 40:535-544.
Miller KC, GW Mack, KL Knight, JT Hopkins, DO Draper, PJ Fields, I Hunter. (2010) Reflex inhibition of electrically induced muscle cramps in hypohydrated humans. Med Sci Sports Exerc 42(5):953-961.