The Science of Hamstring Injuries


by Dr. Reed Ferber, Ph.D. CAT(C)

The hamstring injuries of high-profile athletes received a lot of attention at last year’s World Track and Field Championships in London. While these injuries were related to sprinting, the mechanics of the injury itself are quite similar to distance running. Moreover, hamstring strains are one of the top 10 injuries for distance runners, especially for recreational runners.

At the Worlds, Andre De Grasse was aiming for a chance to dethrone Jamaica’s Usain Bolt as the fastest man in the world. But the day before the start of the 100-metre competition, the Canadian sprinter announced that a hamstring tear was forcing him to withdraw. A few days later, Bolt had his own dramatic hamstring injury in what was likely his final race. During his leg of the 4×100-metre relay, the Olympic champion pulled his hamstring muscle. So, what is science behind a hamstring strain?


The hamstring is actually a group of three muscles at the back of your thigh that play an important role in running. They function to control both your knee and hip during both the swing phase (when your foot is off of the ground) and the stance phase (when your foot is on the ground).

Injuries occur when these three muscles undergo too much strain and they cramp up or develop tears. A cramp is when the muscle stops working altogether as it simply contracts and cannot relax. A tear occurs when the muscle fibres come apart, and the injury is classified according to its severity. A first-degree tear involves minor damage while a second-degree tear is a partial tear of the muscle itself. A third-degree tear would be a complete rupture of the muscle and a catastrophic career-ending injury.

At the knee, the hamstring muscles act to slow down your leg as it swings forward very quickly in preparation for foot strike and the beginning of stance. It’s this action that commonly leads to injuries because the muscle is trying to shorten and contract, but the knee is extending very quickly, which pulls on the hamstring muscle and creates a significant amount of strain.

At the hip, these powerful muscles generate force just prior to the foot coming off of the ground as they extend the hip backwards. Injuries can occur at this point of the running cycle because these muscles are generating tremendous amounts of force to maintain, or increase, forward running velocity.

Treatment for a cramp requires rest, massage, rehydration and stretching. A cramping hamstring is often an overworked hamstring, so rest is  usually the best medicine. However, a hamstring cramp can often be misdiagnosed and could, in fact, be a first-degree tear.

A hamstring muscle tear demands a much more complex treatment approach. De Grasse experienced a second-degree tear, which was later confirmed by ultrasound. An audible “pop” was heard due to the lightning-fast tearing of the muscle, and this injury demands complete rest and directed treatment. First, the tear must be allowed to heal up, similar to the way a cut on your skin needs time to develop a scab and scar tissue. Once the scar tissue has developed, exercise therapy can help increase muscle strength so the scar tissue is properly replaced with new muscle tissue. If the muscle doesn’t fully recover, injury recurrence is very probable. Unfortunately, hamstring re-injuries are very common and are most often due to improper rehabilitation and not finding out the root cause of the injury.

The route I highly recommend is to determine the root cause of an injury. Since increased strain is the mechanism, you need to understand why there was increased strain within the hamstring muscles. For example, during the 2016 Rio Olympics, in a Globe and Mailarticle, I discussed the asymmetry of De Grasse’s sprinting style. More recently, Bolt’s running asymmetry was revealed during a biomechanical analysis, and it is this asymmetry that places increased demands on one set (or side) of the hamstrings. While getting stronger, resting and allowing the body to recover allows for tissue healing and the ability to deal with increased hamstring strain, finding the root cause of the strain is a biomechanical mystery that must be solved to help prevent future injuries.



Dr. Reed Ferber is the director of the Running Injury Clinic, a world leader in running-related research and 3D gait analysis technology. For more information, visit

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