Today, we're at the ballet of all places, but why are we here? Well, take a moment and have a look at this graceful and polished ballet dancer. I want you to pay close attention to the many movements which require her to stand on point. To you and me, this means the movements which require her to stand on her tippy toes. You can see she's exceptionally good at doing this and if you look very closely at her legs, you might be able to identify one of the muscles of the leg which is helping her to do this so well. Yes, that's right. It's the large muscle of the posterior leg – the gastrocnemius muscle – and in this tutorial, we're going to be discussing its functions in 3D.
Before we learn about the functions of this large and powerful muscle, let's take a few moments to remind ourselves of the anatomy.
The gastrocnemius muscle is the most superficial muscle of the posterior compartment of the leg, meaning it lies on top of the other members of the posterior compartment. Being the most superficial muscle of this group, the gastrocnemius also presents itself as this well-defined region on the surface of the posterior leg especially for those of you who enjoy running, cycling or working out like our model here.
The gastrocnemius and underlying soleus muscle are sometimes collectively referred to as a composite three-headed muscle which is called the triceps surae. The triceps surae is also what is commonly referred to as your calf muscle which, in case you are wondering, has nothing to do with an actual calf. The term is actually believed to either have come from the Old Norse word "kálfi" or the Irish Gaelic word "calpa" both of which referred to the posterior region of the leg – an interesting fact for you today!
As you can see in our 3D model, the gastrocnemius muscle is a large fusiform or spindle-shaped muscle and has two heads – a medial and a lateral head. The medial head of the gastrocnemius is the larger of the two and extends further down the leg. The medial head has its proximal attachment or origin on the popliteal surface of the femur superior to the medial condyle of the femur. The lateral head has its proximal attachment or origin on the lateral side of the lateral condyle of the femur.
The two heads of the gastrocnemius muscle also act as important players informing the boundaries of the popliteal fossa. The lateral head forms the inferolateral margin and the medial head forms the inferomedial margin of the popliteal fossa. Both heads of the gastrocnemius muscle continue distally as two distinct muscular bellies which terminate midway down the leg by tapering into a flat aponeurosis. This aponeurosis continues as the calcaneal or Achilles tendon which we know to be the most powerful tendon in the body. This tendon inserts into the posterior surface of the calcaneus on the calcaneal tuberosity making it the distal attachment of the gastrocnemius muscle.
Now, to perform the functions of which we are about to learn, the gastrocnemius needs some form of innervation. All the superficial muscles of the posterior compartment of the leg are innervated by this nerve which is the tibial nerve and the gastrocnemius is no different in this respect. It is useful for practical exams to note the root values of the nerve associated with the muscle we are learning. In this case, the root values of the tibial nerve specific to the gastrocnemius are S1 and S2.
In terms of the functions of any muscle, it is important to know which joints are affected by it. The gastrocnemius is known as a two-joint or biarticular muscle because it acts on both the knee and the ankle joints. Let's take a look now at the functions of the gastrocnemius on these joints.
The main function of the gastrocnemius muscle is plantarflexion of the foot at the ankle joint. The triceps surae muscle mass, which we learned the gastrocnemius is a part of, is responsible for the vast majority or ninety three percent of total plantarflexion force generated. Repeating this action again but this time from a medial angle, you can see the plantarflexion of the ankle joint causes the heel to be pulled upwards forcing the foot to point downwards.
Plantarflexion is a very powerful movement and is essential in propelling the body forward and upwards. With that in mind, it's no surprise that plantarflexion is extremely important in the biomechanics of jumping and is most effective or powerful when initiated from a dorsiflexed position of the feet which is one reason why we tend to squat a little to get more height out of our jump.
The fibers of the gastrocnemius muscle are mostly type two muscle fibers which are also known as fast twitch muscle fibers. These lend themselves very well in producing fast and powerful movement during plantarflexion. As we saw at the beginning of our tutorial, plantarflexion is also an extremely important action for ballet dancers. When performing a relevé movement, standing on point, or on the tippy toes, ballet dancers are required to produce extreme plantarflexion of the ankle joint as we can see in the following clip.
An important thing to note is that the gastrocnemius muscle cannot perform plantarflexion when the knee is fully flexed. When flexed, plantarflexion is achieved by the other members of the posterior compartment of the leg namely the soleus muscle.
Moving our attention away from the ankle joint now and focusing on the knee joint, another function of the gastrocnemius is flexion of the leg at the knee joint. Looking at the movement again, this time from a medial view, we can see that flexion of the leg at the knee joint occurs when the leg moves posteriorly towards your hamstrings or posterior thigh. As we mentioned before, when the gastrocnemius muscle is involved in flexion of the knee, it is not able to produce the plantarflexion action at the same time. The muscle cannot effectively act on the ankle joint and the knee joints simultaneously. To get a better idea of how the two functions work and to give these movements a little more context, let's have a look at this short clip of a sprinter.
As our friend leaves the blocks, notice how the gastrocnemius of his right leg is working to flex the leg at the knee joint. The gastrocnemius of his left leg is also contracting, however, this time causing plantarflexion of the extended leg to occur.
So, in summary, the gastrocnemius muscle acts on both the knee and the ankle joints, however, it cannot work on both joints at full power simultaneously. The main function of the gastrocnemius muscle is to plantarflex the ankle joint. This is an important action for ballet dancers when standing on point. The gastrocnemius muscle also acts on the knee joint and is responsible for flexion of the leg at the knee joint. This is an important function and is used when jumping.
This concludes the video tutorial on the functions of the gastrocnemius muscle. Thanks for watching and see you next time!