Muscle cramps are common during very intense exercise and occur far less often during less-intense training, because the most common cause of muscle cramps in exercisers is muscle damage from all-out pressure on the muscles.
Cause: Muscle Damage
The leading theory is that most cases of muscle cramps in serious exercisers and athletes are caused by an exaggerated "stretch reflex". When you stretch a muscle, it pulls on its tendon. Stretch reflex nerves in that tendon send a message back to the spinal cord (not the brain), and then the "stretch reflex" in the spinal cord sends a message along nerves from the spine to cause the muscle to contract. During sustained extreme pressure on the muscles, the muscle retains its contraction to form a cramp. A study from South Africa showed that the most likely cause is muscle fatigue or tearing of the muscle itself (Medicine & Science in Sports & Exercise, July 2005).
If this is true, muscle cramps during endurance events can be prevented by slowing down when you feel excessive soreness in one muscle group or straining in a muscle. Of course, competitive athletes will not do this, and they pay for it with muscle cramps.
The researchers studied triathletes and found that most of the time, the muscles cramps were not caused by dehydration, thyroid disease, blocked blood flow, nerve damage, or mineral abnormalities of calcium, sodium, magnesium or potassium. The athletes with cramps had normal electrolytes, and did not lose more fluid during exercise than those who did not suffer cramps. The researchers showed that the most likely cause is muscle fatigue or tearing of the muscle itself. Electromyograph (EMG) studies at one to five minutes showed markedly elevated electrical activity of the nerves controlling the cramped muscles. A review of the scientific literature shows the most common cause of muscle cramps appears to be muscle damage (Journal of the American Academy of Orthopaedic Surgeons, July 2007).
What to Do When a Cramp Strikes
Almost all racers who experience exercise- associated muscle cramps do not suffer from disease and can usually prevent cramps by slowing down when one muscle group starts to feel tight or excessive soreness. You do this by switching pressure from the cramped leg to the uncramped one. A bicycle racer moves most of his pressure to the pedal of the uncramped leg. A runner shortens the stride of the cramped leg. Continuing to put pressure on the cramped muscle can rupture the muscle.
Dehydration or Lack of Minerals Less Common
Some cramps are caused by low mineral or fluid levels (The Japanese Journal of Clinical Pathology, November 2007). However, for the vast majority of people who suffer exercise- associated muscle cramps, blood levels of sodium, potassium, calcium and magnesium are normal. Research in athletes after they ran in 52-mile races showed that the runners who suffered cramps had the same level of dehydration and blood mineral levels as those who did not get muscle cramps.
Lack of Salt
The most common mineral cause of muscle cramps in exercisers is lack of salt, according to a report from the University of Oklahoma (Sports Medicine, April-May 2007). The authors cite studies of tennis and football players, showing that crampers tend to be salty sweaters; and of triathletes, where those who cramp lost more salt during a race than their peers who did not cramp. They found that intravenous saline can reverse cramping, and that more salt in the diet or in sports drinks can help to prevent heat-associated cramping.
For many years I have recommended eating salted peanuts or other salty foods during heavy exercise, but other doctors believe that extra salt may raise blood pressure. If this is a concern for you, get a wrist cuff monitor and check your blood pressure every day. You are likely to find that your blood pressure goes down, not up, with regular exercise even when you add salt.
Nobody has shown consistent benefit from any of the most common treatments: multivitamin pills; mineral pills with calcium, zinc, magnesium, salt and/or potassium; massage or chiropractic manipulation; drinking large amounts of water; dietary manipulations; or bio-mechanical stretching and strengthening.
Non-causes in Serious Exercisers
Known medical causes of muscle cramps are extremely rare. If you suffer from recurrent muscle cramps, you may need special tests for pinched nerves, Parkinson's disease, low thyroid, diabetes, narrowed arteries from arteriosclerosis, low blood mineral levels, metabolic diseases that cause muscle damage, or side effects of drugs used for high cholesterol, high blood pressure or diabetes, diuretics, oral contraceptives or alcohol (Neurology 2010; 74: 691-96).
Quinine should not be used because it can damage blood cells. Some studies show that gabapentin (an anticonvulsant), diltiazem ( a blood pressure medication), or B-complex vitamins may help to relieve muscle cramps in some people (Journal of Clinical Pharmacology, 1998;38:1151).
Before a cramp develops, you will probably feel the muscle pulling and tightening. If you slow down, the pulling lessens, but if you continue to push the pace, the muscle goes into a sustained cramp and you have to stop exercising to work the cramp out. Further evidence that muscle damage is the cause of the cramp is that the muscle often hurts for hours or days afterwards. You may be able to prevent cramps by exercising more frequently but less intensely and for shorter periods of time, but most racers do not want to do this.
There is some evidence that taking sugared drinks or foods during prolonged exercise helps to maintain endurance and muscle integrity which helps to prevent cramps. Take a source of sugar every 30 minutes or so during vigorous workouts or races, and back off if you feel a group of muscles pulling or tightening during exercise.
Lack of Vitamin D
A leading cause of muscle damage, soreness and slow- healing injuries in athletes is lack of vitamin D. If you suffer frequent cramping and your muscles feel sore or you keep on being injured when you exercise, get a blood test called D3. If it is below 75 nmol/L, your problems may be caused by lack of vitamin D and be cured by getting some sunshine or taking at least 2000 IU each day of the very inexpensive vitamin D3.
Occasional Cramps Not Harmful
Most racers and serious exercisers accept that occasional cramps will occur, and will cause no long-term harm.