If the cramps occur during or immediately after exercise, they may be a sign of atherosclerosis, when the arteries are clogged by fatty deposits, limiting the supply of blood to the muscles.
When you exercise your leg muscles need more oxygen. But because the arteries are narrowed, they can't get enough and so switch to anaerobic metabolism. This results in the build-up of chemicals that can trigger pain and spasm, usually in the calves. When you rest the pain gets better. This is called intermittent claudication.
In mild cases, you may notice your legs are cold, with dry skin and few hairs. As it gets worse the limbs may become blue, ulcerated and even gangrenous. Smokers, people with high cholesterol or diabetes and those with a family history of heart disease are all at particular risk. Intermittent claudication needs thorough investigation, so see your GP.
Cramps may also occur when a muscle is injured. Although the cramp or spasm may be felt in the legs, the underlying problem may be elsewhere, such as the back or knee.
Very rarely leg cramps are caused by medical conditions, such as an underactive thyroid gland (hypothyroidism), Addison's disease and kidney problems.
When cramps persist it's important you see a doctor, who'll be able to check for these underlying causes.
Many people find that leg cramps cause particular trouble at night, when the pain stops them from sleeping or wakes them up. These night cramps are rarely serious, but can cause a lot of discomfort.
Avoiding cramp
Before exercise, warm up gently and stretch the muscles out. You should also stretch after exercise too. Stretching also helps relieve the spasms of cramp.
Dehydration upsets calcium and phosphorous levels in the body, which may trigger cramps. So make sure you drink plenty of fluid before and during activity. Non-phosphate calcium supplements may also help reduce cramps.
If night cramps are a problem, sleep under a light, loose duvet. Heavy or tight covers can force the toes downwards, straining the leg muscles.
Sometimes quinine is used to prevent cramps. However, its use is controversial and potential side effects include dizziness, blurred vision and blood problems. Tonic water contains very small amounts of quinine, which may be enough to relieve night cramps.
Some people recommend vitamin E creams for cramp, but there's little evidence that they have any effect other than that of a good massage.
If cramps persist, get advice from your pharmacist about simple over-the-counter treatments. If these don't help, talk to your doctor.