RUNNING
Cramp: every runner's nightmare
Posted Tue, 30 May 2000

Dr Lindsay Weight

If there is one thing every runner dreads for its devastating potential to reduce winners to also-rans and silver medals to bronze, it's cramp. So everybody has their favourite cramp theory and their favoured remedy but lets consider some facts first.

1. Cramp is more common in males than in females
2. Cramp is associated both with the duration and intensity of exercise: the further and the faster the more likely cramp is to occur.
3. Onset of cramp is usually after 30km in a distance race, regardless of running speed.
4. Cramp during exercise is almost always in the large muscles that span two joints: the hamstrings, quadriceps and calf (gastrocnemius) muscles.
5. Cramps are often initiated by a change in speed, gradient or running surface. A classic example is cresting a hill and then accelerating downhill or running on to grass after hours on the tar.
6. Some people are simply more cramp-prone than others.

The most widely held belief is that cramp is due to dehydration and the loss of minerals such as sodium, potassium and magnesium. However, if this were so, then presumably all muscles would be affected and not just the large leg muscles, as is generally the case. A recent study on Two Oceans marathon finishers showed that the fluid and electrolyte balance of those who cramped and those who did not, was entirely normal. However what was abnormal in the crampers was the level of electrical activity in the muscle both during and after a muscle cramp. This suggests that some people have hyper-excitable muscle which is more likely to malfunction during prolonged weight-bearing activity. Receptors in the muscle which co-ordinate muscle contraction and relaxation via the spinal cord start to misfire with the result that muscle remains in a contracted state: this is cramp. Whether electrolyte imbalances contribute to this loss of receptor function in working muscle is not yet known.

The electrolyte deficit most commonly blamed for cramp is magnesium. Magnesium plays a role both in muscle function and energy production but during prolonged exercise it appears that magnesium gets relocated in the body as opposed to lost via sweat or urine (reason not known). Hence magnesium imbalance relative to other electrolytes (sodium and potassium) could be one of the factors involved in cramping in fatigued muscle. Ingesting magnesium supplements might serve to correct this temporary deficit and maintain muscle function, but they will not compensate for basic lack of training or injudicious running.

Another misconception is that lactic acid (melksuur) build-up causes cramp. This is patently incorrect. High levels of lactic acid in the muscle contribute to fatigue and the inability to sustain a particular exercise intensity by interfering with muscle contraction. On the contrary cramp is a sustained muscle contraction. Lactic acid is generated during high intensity exercise where oxygen delivery to the muscle is insufficient to meet the demand. In endurance events oxygen transport is not a limiting factor, so lactic acid production is minimal. Also, lactic acid does not stay in muscle for very long - it is shunted off to the liver to be recycled into more glucose. It does not form crystals, either in the muscle or anywhere else, so this additional explanation for cramp and also muscle pain is also without physiological foundation.

What can you do then to prevent muscle receptor fatigue and consequent cramp ?

1. Train: you have to teach muscle what you want it to do at a certain pace, over varying gradients for long periods of time. Ensure that you train on terrain similar to that you plan to race on.

2. Get strong: the stronger the muscle, the more fatigue resistant and therefore possibly less cramp prone. Do hill work and if you have the opportunity, specific leg strengthening workouts in the gym.

3. Stretch regularly: increased muscle flexibility might help to counteract cramp. When cramp strikes, stretch it out. You have to unlock the muscle from its contractile circuit.

4. Be very cautious when changing speed or gradient in the latter stages of a race. Fatigued muscle cannot readily adapt to the changing demands made upon it.

5. If you have a formula or preparation that is legal, has no side effects and works for you, use it.