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Charley horse is another name for a muscle spasm. Charley horses can occur in any muscle, but they're most common in the legs. These spasms are marked by uncomfortable muscle contractions. If the contracting muscles don't relax for several seconds or more, the pain can be severe. Severe Charley horses can result in muscle soreness that lasts anywhere from a few hours to a day. This is normal, so long as the pain isn't prolonged or recurring. If you have been in pain for a month, you need to see a doctor.