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Functions of the biceps femoris muscle

Functions and anatomy of the biceps femoris muscle shown with 3D model animation.

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Hi everyone! This is Joao and in this tutorial, we will cover the functions of the biceps femoris muscle. You can see that this muscle is now isolated on the screen. The biceps femoris is a two part fusiform or spindle-shaped muscle. It is located in the lower limb in the posterior compartment of the thigh – essentially, this is the back of your thigh as you can see on the screen. The biceps femoris muscle has two heads – a long head and a short head. The long head of the biceps femoris muscle belongs to the hamstring muscle group. Underneath the long head of the biceps femoris, we find then the short head.

When we're trying to identify this muscle in exam, a usual tip is to then note that the long head is a portion of the biceps femoris that can be readily seen as it lies on the top and only a small portion of the short head of the biceps femoris can be seen on the lateral side of the posterior thigh peeking out from underneath the long head. Each head has a different origin and innervation but the same insertion. The muscles of the posterior thigh are innervated by this nerve – the sciatic nerve – which has two divisions – the tibial division and the common fibular division. The long head of the biceps femoris is innervated by the tibial division of the sciatic nerve which you can see now highlighted on the screen and the short head of the biceps femoris is then innervated by the common fibular division of the sciatic nerve.

Useful for practical exams are the root values of these nerves. Both the tibial division and the common fibular division of the sciatic nerve have the same root values namely L5, S1 and S2.

The long head of the biceps femoris has its proximal attachment – or what we call origin – on the ischial tuberosity. The short head of the biceps femoris has its proximal attachment, also as origin, on the linea aspera and the lateral supracondylar line of the femur. The long head and short head of the biceps femoris muscle come together distally into the biceps femoris tendon which inserts onto the lateral side of the head of the fibula.

In terms of actions of the biceps femoris muscle, this muscle acts on both the knee and the hip joints. The short head of the biceps femoris acts only on the knee joint whereas the long head of this muscle acts on both the knee and the hip joints. If we consider the attachments of each of these sections of the muscle, we can see why that would be the case. The short head and the long head of the biceps femoris definitely influence the actions that the muscle undertakes in different ways.

We will consider the action of the long head of the biceps femoris first and we can see this head now isolated on the screen. If we begin with the hip in the anatomical position, then extension of the hip occurs when the distance between the torso and the anterior thigh increases. While acting as part of the hamstring group muscles, the long head of the biceps femoris is instrumental in this action.

Looking at the action of the long head of the biceps femoris, we can consider then the gait cycle. In the swing phase from mid to terminal swing, the hip is extended by the action of the long head of the biceps femoris muscle. This action and by consequence, the long head of the biceps femoris muscle is therefore crucial to an individual's ability to walk on flat ground and to run. The long head of the biceps femoris also works when performing exercises such as lunges.

Now looking at the next movement caused by the muscle, if we look at the biceps femoris on the screen now, we can see that the short head of the biceps femoris is now isolated along with the long head of the biceps femoris. If we now consider the knee joint in an extended position as shown on the screen, we can see that the flexion of the knee joint bends the lower limb and then decreasing the angle between the posterior thigh and the leg. This is the action that both the long and short heads of the biceps femoris muscles are involved in – flexion of the leg at the knee joint.

Now, let's consider the rotation of the knee joint when it's in a flexed position. The insertion of the biceps femoris is instrumental to this movement. Now recall that the tendon of the biceps femoris muscle inserts onto the head of the fibula – we can see this on the screen right now. Now when the knee is flexed, contraction of the biceps femoris muscle causes lateral rotation of the knee joint of approximately a forty degree angle. This lateral rotation of the flexed knee is very useful especially when you're dancing such as the twist if that's the kind of dance that you like to do and also when skiing.

Now it is important to note at this point the limitations of the biceps femoris. The two main actions of the muscle – hip extension and knee flexion – cannot be performed to a maximum level by the biceps femoris simultaneously. And this is because the biceps femoris muscle requires so much shortening or contraction to extend the hip that it cannot then be contracted any further to flex the knee. Similarly, for knee flexion, the muscle contracts so much to maximally flex the knee that it cannot contract any further to then extend the hip. As a result, the biceps femoris is known as a weak knee flexor when the hip is fully extended and a weak hip extender when the knee is fully flexed.

So, in summary, the biceps femoris muscle is responsible for extension of the hip via the long head of the muscle and flexion of the leg at the knee joint as well as lateral rotation of the leg when the knee is flexed via both the long and short heads of the muscle. The biceps muscle helps facilitate movements such as walking and running. It also performs lateral rotation of a flexed knee which is essential when undertaking activities such as dancing and skiing. The muscle cannot perform the main actions of maximum hip extension and maximum knee flexion at the same time.

And this concludes our tutorial on the functions of the biceps femoris. Hope you liked it and I'll see you next time.

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