A hamstring injury typically causes a sudden, sharp pain in the back of your thigh. You might also feel a "popping" or tearing sensation. Swelling and tenderness usually develop within a few hours. You may also experience bruising or discoloration along the back of your leg, as well as muscle weakness or an inability to put weight on your injured leg.

The hamstring muscles are a group of three muscles that run along the back of your thigh from your hip to just below your knee. These muscles make it possible to extend your leg straight behind your body and to bend your knee. When any one of these muscles stretches beyond its limit during physical activity, injury can result.

Hamstring injury risk factors include:

• Sports participation. Sports that require sprinting or running, or other activities such as dancing that might require extreme stretching, make a hamstring injury more likely.
• Prior hamstring injury. After you've had one hamstring injury, you're more likely to have another one, especially if you try to resume all your activities at pre-injury levels of intensity before your muscles have time to heal and rebuild strength.
• Poor flexibility. If you have poor flexibility, your muscles may not be able to bear the full force of the action required during certain activities.
• Muscle imbalance. Although not all experts agree, some suggest that a muscle imbalance may lead to hamstring injury. When the muscles along the front of your thigh — the quadriceps — become stronger and more developed than your hamstring muscles, you may be more likely to injure your hamstring muscles.

Returning to strenuous activities before your hamstring muscles are completely healed might cause an injury recurrence.