The hamstrings are a group of muscles on the back of the thigh that connect, or attach to, the back of the hip and the back of the knee. This makes hamstrings one of the few muscle groups in the body that cross two joints, as hamstrings influence extension of the hips and flexing of the knees, says Lucas Theodore Buchler, MD, a Northwestern Medicine orthopaedic surgeon specializing in sports medicine.

Dr. Buchler says that, in general, these injuries happen when the muscle is trying to contract while the force against the muscle is actually trying to stretch it out. This occurs most often in athletic settings during high-force movements, such as sprinting or jumping.

However, you can also get pulled into a muscle strain. This is most common when you have a straight-locked knee and are pulled or pushed into a bending motion at the hip.

No matter the cause, you'll likely experience one or more of the following symptoms after the strain occurs:

• Swelling, especially in the hours following the injury
• Bruising or discoloration of the back of your leg
• Weakness in your hamstring

Treatment and Prevention

If you suspect a hamstring injury, you should see a medical professional for treatment. The majority of strains within this muscle group are nonsurgical and treated with physical therapy.

However, the biggest part of treatment for hamstring strains can likely be prevention, notes Dr. Buchler.

"You want to make sure to warm up well before exercising and to progress gradually in your levels of physical activity," he says. "If you haven't exercised vigorously in a while, it's important to condition your muscles to tolerate stress."