A hamstring strain happens when you overstretch, or pull, the muscles that run down the back of your thigh. It can happen when you exercise or lift something or if you’re injured in an accident.
You may feel pain and tenderness that’s worse when you move your injured leg. The back of your thigh may be swollen and bruised. If you have a bad strain, you may not be able to move your leg normally.
While a minor strain often heals well with rest and other treatment, a severe strain may require medical treatment. If a severe strain isn’t treated, you may have long-lasting problems.