Muscle cramps happen when a muscle is "stuck" in a contracted state, unable to relax. For a muscle to cramp, two things need to happen first:
1. The muscle must be under prolonged partial-contraction.
2. The muscle must be starved of energy (ATP) and/or electrolytes
Swimming lends itself to both of these prerequisites. Here's why...
Regardless of the stroke, a person's position in the water is horizontal with their legs behind them and their arms in front of them. This posture forces the muscles of the shoulder and posterior chain (muscles of the back and back of the legs) to remain in partial contraction to counteract gravity. This strain remains throughout the entire stroke, so satisfies the first point: the muscle must be under prolonged partial-contraction.
Swimming can create a strain on the muscles, but it also burns a lot of calories. That means that, after your first few laps, you've already burnt up almost all of your breakfast as calories and have very little energy left. That fulfils the second requirement of a cramp - energy (ATP) and electrolyte starvation.
To ensure that you've got enough energy, all you have to do is eat. Food takes roughly 90min to turn into a fuel that the body can use (ATP), so make sure you eat about 90min before getting in the pool. Low-GI carbohydrates are the best type of food to have to conserve energy. Sports drinks are good to have between laps to ensure your electrolytes are kept replenished.
For more information on stretching, email me directly via the contact page. For specific information on an injury you have, or for treatment, contact one of my clinics directly. You can find that information here.
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Dr Mitch Clark