The Mystifying Muscle Cramp

The Mystifying Muscle Cramp

First, let's get this out of the way.  Cramping during or after exercise is not caused by fluid/electrolyte/sodium/magnesium/potassium depletion.  Furthermore, Gatorade or bananas won't help.  It has been shown in two large scale studies that runners who cramp are less dehydrated and have normal and similar electrolyte levels to noncramping control groups (Martin Schwellnus, British Journal of Sports Medicine, December, 2008).   
When you sweat you lose more fluid than sodium, creating a 'saltier' internal environment.  Sweating cannot, and does not, lower your electrolyte levels; the sodium concentration will rise as you sweat.  Therefore, there is another mechanism responsible for cramping. 

Now that that's settled, what does cause exercise induced cramping?  The current theory is that fatigue and reflex dysfunction are the causes.  

Muscle Fatigue

Only active muscles cramp.  This means that the act of muscle contraction is most likely the cause.  In fact, muscle cramps often happen during racing, not training.  Studies have found that racing on hills and racing above a level that you've trained for are predictive for cramping.  In other  words, those who cramp race harder and start faster than their training has prepared them do.  

Muscle Reflex Dysfunction

Your muscles are stimulated to contract by motor nerves. These nerves are regulated by reflex activity.  There are two reflexes, or circuits, that are important to consider.  The first circuit is known as the muscle spindle reflex.  Examples of this include the knee-jerk reflex and the  tendo-achilles reflex.  Simply put, when the doctor taps your knee or achilles tendon with a small hammer your knee or foot jumps out.  This action causes  the muscle to stretch.  When a muscle is stretched, this circuit is then activated, causing the muscle to contract.  This is a protective action that prevents overstretching of the muscle.
The second circuit, the Golgi tendon reflex, performs a different role than the muscle spindle reflex.  When this circuit fires, it causes a reduction in contraction.  This is protective because it prevents the muscle from taking on too much load.  

The two reflexes work together to regulate muscle activity

The muscle spindle and Golgi tendon reflexes are stimulated by load inside the muscle.  If the strain is too high, one of them activates to tell the muscles to chill out and relax so they won't pull too hard and tear (diminishing contraction).  The other does the opposite and tells the muscle to pull harder to protect itself (increasing contraction).  They work together in a feedback loop to keep your movement smooth and precise.  

So what causes cramping?

When muscles fatigue, the firing rate of the muscle spindle increases (causing an involuntary, reflex contraction of the muscle).  Fatigue has the opposite effect on the Golgi tendon organ, decreasing the firing rate (causing the muscle to contract even more).  Now we've connected the dots: fatigue and it's associated reflex dysfunction, not electrolyte depletion or dehydration, are the main causes of muscle cramps. 

Treatment for cramps

We know that the most effective treatment for cramps is passive stretching.  The temporary elongation of the muscle leads to a reduction in the firing rate of one of the circuits and an increase in the firing rate of the other circuit (opposite to what occurs during fatigue).  The protective circuits come to an agreement, "resetting" the muscle, causing muscle relaxation.   
It's the same concept of warming up before exercise.  After some light movement and easy stretching, the muscle resets and allows better movement.  

You may have experienced extreme pain when putting a cramped muscle in a stretched position.  It may seem as if you're getting worse before you get better.  This is because the initial stretch causes an increase in the firing rate of the muscle spindle, increasing contraction.  Eventually, however, the spindle firing rate decreases, the muscle relaxes and the cramp is alleviated.
These concepts hold for some types non-exercise induced cramping, as well.  During sleep, you may stretch and activate the circuit causing contraction and cramping.  Slapping the foot down and stretching resets the circuits.

Stretching is not Stretching

Lastly, the stretching we do is not a big enough dose to physically elongate the muscles or increase the blood supply.  Research has shown that the forces necessary to deform or stretch connective tissue are impossible to create with hands, elbows or foam rollers (Schleip Chaudry, Journal of the American Osteopathic Association, August, 2008).  We think we are lengthening the muscle  by stretching, but the increased range of motion is caused by the nervous system's tolerance to stretch.  As we have seen, what we're actually doing is resetting the dysfunctional reflexes, or improving our software.  

Though many people swear by salt tablets, electrolyte gels, etc., no study has shown their effectiveness.  It may be a placebo effect, a sign that we don't know the whole story, or that no absolute answer may exist for the mystifying muscle cramp. 

 
References:  Jay  Dicharry, Running Rewired, 2017; Todd Hargrove, A Guide To Better Movement, 2014;  Ross Tucker, Jonathann Dugas and Matt Fitzgerald, The Runner's Body, 2009.