Kinesiology THURSDAY – Why Hamstrings Tear (Part 4)

Assumptions are often made that hamstrings tear because they are tight. But tightness has been shown to be inconsistent as a risk factor for hamstring strain injuries (HSI). Studies repeatedly show that hamstrings fail due to weakness under eccentric load, with the bicep femoris long head (BFlh) being the most vulnerable. Here’s a short list of muscle imbalances that can make the BFlh weak:

Hamstring to quad ratio: A H:Q ratio of less than 0.6 was shown to increased risk of hamstring injury by 17 times.
Gluteus maximus weakness: Gluteus maximus is a functional synergist of the hamstrings. If this muscle is weak in decelerating hip flexion, the BFlh has to work harder and is at greater risk of overload
Weakness of the medial hamstrings: The medial hamstrings work in concert with the lateral hamstrings to decelerate hip flexion and knee extension. Weakness of the medial hamstrings have been associated with overload of the BFlh. In a study that compared medial to lateral hamstring activation during specific exercises, it was shown that the medial hamstring was more active with a kettle bell swing (pictured above) and deadlifts while the lateral hamstring (BFlh) was more active with supine leg curl and hip extension (open chain).

Why does this matter?

That list is a lot to chew on so let’s break it down for the real world. This means in the case of HSI, it is more important to figure out what is making the BFlh weak instead of worrying about stretching it.

A key goal should be to strengthen the gluteus maximus and the medial hamstring to prevent bicep femoris overload. And it appears performing exercises that require eccentric lengthening with your foot on the ground are great ways to activate these two synergists of the bicep femoris versus the conventional open chain exercises of prone leg lifts and leg curls.

The next time you encounter someone with repeated bicep femoris injury, listen to the BFlh and then look somewhere else for the solution. Remember, hamstring strains occur due to overload and weakness, not tightness. And it’s not just about strength, but strength at long muscle lengths. We can help the bicep femoris out.

Because nobody has time to be in pain.

Until next time…