Pain in the upper leg muscles that affects walking is often due to a muscle strain or overuse, but it can also be caused by a deeper injury if severe. Rest the leg, avoid putting too much weight on it, and use a cold compress for the first day or two, then a warm compress to relax the muscle. Gentle stretching can help once the pain starts improving. If you are unable to walk properly or the pain does not settle, it is better to consult your doctor to rule out any serious problem.