Richard Watterston, physiotherapist, replies: While dehydration and a lack of mineral salts in the body do cause muscle cramps, extremes of temperature, fatigue, poor blood supply or metabolic abnormalities of the muscle can also cause them. Unless you're working extremely intensely for the half hour, it's unlikely that you're fatiguing your muscles enough for tiredness to be the problem, so it's possible that the cramps are caused by overly tense muscles. Tension restricts the blood flow to the muscles, which leads to a build up of lactic acid, a cause of cramps. To help relieve the problem, check your style and stroke and make sure you stretch and warm up well. Some people find drinking tonic water several hours before exercise helps as it contains quinine that is thought to relax muscles.