Hamstring Injury – Role Biceps Femoris plays

Hamstring injury happens frequently in running-based sports such as athletics, football, rugby etc. Over 80% of these occur in the outer hamstring when the leg is swinging through.

It is often thought that hamstrings are injured from changes in direction, pushing off and explosive movements but in reality, most hamstring injury happens when the leg is swinging through, just before the foot touches down.

Which Muscles are the Hamstrings?

Here is a quick few stats and anatomy refresher to ground you:

Hamstring injury

The hamstrings are made up of three muscles

  • The Biceps Femoris, which has two parts to it. The long head which cross’ both the hip and then knee joint and the short head which only crosses one joint
  • Semitendinosus that helps with hip and knee rotation as well as knee flexion
  • And Semimembranosus at the inner thigh that helps with hip extension and knee flexion

What is the cause of Hamstring injury?

Hamstring injury happens when any of these muscles get injured. It usually happens during a sudden strenuous movements which impact one of the hamstring muscles or any of the tendons.

Most hamstring injuries are thought to happen in late swing phase of running, just before the foot lands.

Hamstrings’ Function:

Check out the video below to see how the hamstring works in walking:

YouTube player

As you can see in the video, the hamstrings fire into action before, during and after the foot lands. At this point when the knee is extended, the muscle is working while at it’s peak length and at maximal force development working hard eccentrically to slow leg swing down.

What is eccentric contraction?

An eccentric contraction is where the muscle controls lengthening out, which is far harder on the muscle than a concentric contraction where it contracts to push-off.

Biceps Femoris’ role in Hamstring Injury

The Biceps Femoris long head (BFlonghead) is involved in almost 80% hamstring injuries.(1). So what is the link between the mechanism of hamstring injury described above and BFlonghead taking the brunt of injuries?

Recent studies have shown that the Biceps Femoris is more active when the hip is extending, rather than when the knee is flexing. Which means that BFlonghead has to work harder with eccentrically slowing the leg down as compared to other hamstring muscles.

This also means that exercises performed using knee flexion do not often strengthen Biceps Femoris as much as say Semitendinosus, which is more active in knee flexion where it works to bend the knee

nordic curls - hamstring rehab and strengthening exerciseA lot of hamstring strengthening is done at the knee (nordic curls, hamstring curls etc) which has been shown to be more the work of the medial hamstrings than Biceps Femoris.


Hamstring chair bridges
Credit irunfar.com

So there you have it, the BFlonghead of the hamstrings works harder eccentrically slowing down the momentum of the leg swinging forward and often gets missed in strengthening sessions – Stuck between a rock and a hard place!

This gives athletes and health professionals better guidance as to what rehab exercises to add in post injury and also in injury prevention programs depending on injury, leading to decreasing the nearly 30% re-injury rate.(1)

Click here to learn more about hamstring injury and find information about healing faster and stronger from it.

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