The hamstring may be the single most challenging part of the body to sports medicine researchers. To athletes who have injured it, it’s simply the most frustrating.

Known as the “hammy” to many, the hamstring is essential to sprinting and acceleration. But the explosive contraction it provides has a downside. The hamstring is also frequently injured. A recent USA Today study showed that it was the cause of almost 10 percent of all NFL players’ injuries, more than any other muscle.

Worst of all, athletes and researchers can’t anticipate what kinds of movements will cause the hamstring to fail. Todd Heap of the Baltimore Ravens discovered this Sunday, when he tore his hamstring running a route on the first play of the game.

The hamstring is really made up of three separate muscles. All three work to bend the knee and straighten the hip, which are key motions in sprinting. Most athletes are using their hamstring muscles all the time, and they become especially important when accelerating or changing directions.

Typically, when the hamstring fails, it tears. Tears are graded on a scale of 1 to 3, with grade 3 tears being the most severe.

As muscles recover from tears, they don’t heal in the same way that a broken bone does. Bones heal by laying down new calcium and the bone ends up recovering to be at least as strong as it was before the injury. Muscles heal by laying down a new substance called collagen, which forms a scar. As the scar solidifies, it has less elasticity and strength than the native muscle. This has important implications.

First, athletes seeking to help their muscles recover should work to keep the collagen scar as strong and flexible as possible. Physical therapists can help with “active healing” by adding massage, limited stretching, and sometimes modalities like ultrasound to the recovery process.

Athletes also learn that their recovered hamstring is more prone to injury. Of all the injuries I treat, hamstring strains frustrate the athletes the most. They can take up to 12 weeks to recover, and it is not unusual for athletes to try to return to sports too quickly and re-injure themselves.

Eccentric hamstring contractions are the only thing that has been proven to help injured athletes return to sports sooner, and they also have been shown to reduce the rates of hamstring injuries. They’re so simple and effective that most major soccer teams in the world include them in their exercise routines.

Simply put, eccentric hamstring exercises are ones in which the muscle is put under load as it is lengthening — just like it is when we’re sprinting on the field. They’re performed in a kneeling position, like a backward sit-up. They’re also called Russian leg curls, and videos are available on the Internet so that readers can learn how to do them. All the evidence shows that athletes who do these exercises regularly can expect to cut the risk of hamstring injuries significantly.

For a single exercise, that’s not bad. Most athletes can expect to benefit from including them in their training routines. For those recovering from a hamstring injury, they are an essential part of any rehabilitation protocol.

Just ask Todd Heap. He, like many other athletes, is learning that his path to success runs right through the hammy.

Dr. James Glazer is a sports medicine physician for Coastal Orthopedics and Sports Medicine in Freeport. He serves as a consultant for the Portland Pirates and the U.S. Ski Team.