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Functions and anatomy of the soleus muscle shown with 3D model animation.
Today, I’m going to begin with a question for all you gym rats, I mean, gym enthusiasts out there! Today is a leg day for our friend here, and as you can see, he’s working hard on his calves right now. Later on in his workout, our friend decides to do his calf raises from a seated position, rather than the initial standing position. Why is this? Is he just feeling a bit tired and perhaps needs to sit down for a while? Could there be another reason for his decision to work his calf muscle from a seated position? Some of you may already know the answer to this question, but for those who don’t, stay with me now and we’ll find out the answer to this question and lots more as we discuss the functions of the soleus muscle.
As you already know, the soleus is one of the muscles of the posterior compartment of the leg. It is a relatively superficial muscle with just one other muscle overlying it, which is of course, the gastrocnemius muscle. Together, these two muscles form a larger round compound muscle known as the triceps surae, which most of us commonly know as the calf muscle.
Like all muscles, the soleus muscle needs a connection to the complex electrical network that is our nervous system. That connection, or innervation, comes in the form of the tibial nerve, which we know is one of the two terminal branches of the sciatic nerve. The tibial nerve also supplies the rest of the muscles of the posterior compartment as well as several smaller muscles of the foot.
It’s useful to note the root values of the nerves associated with the muscle that we’re learning. In this case, the root values of the tibial nerve, specifically, the muscular branches to the soleus are S1 and S2.
To understand the function of any muscle, we always need to know its attachment points and what joints it acts upon. In the case of the soleus, it has an aponeurotic or flat tendinous origin. This is found along the upper most corner of the shaft of the fibula reaching out to the fibular head.
From here, it forms a tendinous band known as the arch of the soleus which reaches across to the tibia where it has attachments to the soleal line and the middle third of the medial border of the tibia. The muscular belly of the soleus muscle has many different parts, with muscle fiber bundles passing in different directions.
For example, the long fibers contained within the posterior portion of the soleus muscle tend to pass inferiorly to the primary tendon of insertion found on the posterior surface of the muscle. Other fiber bundles found within the anterior portion of the muscle tend to be shorter and sometimes bipennate in arrangement, converging on a more anterior tendon seen here. Both of these tendons merge distally and join the tendon of the overlying gastrocnemius forming the calcaneal tendon which many of us commonly know as the Achilles tendon. The calcaneal tendon inserts into the posterior surface of the calcaneus bone.
Let’s continue and have a look at this muscle from a functional point of view. Unlike the overlying gastrocnemius, the soleus muscle only crosses a single joint making it what’s known as a monoarticular muscle. The joint in question here is, of course, the ankle joint, also known as the talocrural joint, which is formed by the medial and lateral malleoli of the tibia and fibula, which articulate with the talus bone of the foot.
Having refreshed our memory on the anatomy of the soleus, we can easily deduce what its primary function is going to be, which is, of course, plantarflexion of the foot at the ankle joint. Put simply, plantarflexion is caused when the posterior part of the foot is pulled towards the leg causing the toes or digits of the foot to move inferiorly in the direction of the plantar aspect or the sole of the foot.
Plantarflexion is important in the push off stage of walking or the gait cycle, and is essential when trying to stand on your tiptoes. I also want to draw your attention to a major difference between the soleus and gastrocnemius in their roles as plantarflexors of the foot. When the leg is extended at the knee joint, the gastrocnemius and soleus work collectively as a powerful plantarflexor of the foot. However, when the leg is flexed at the knee joint, the role of the gastrocnemius in plantarflexion is greatly reduced causing the soleus to act as a primary plantarflexor of the foot in these conditions.
That’s why our friend at the beginning of the video was doing his calf raises from both the standing and seated position. In the seated position, with his knees flexed, he’s able to isolate the soleus in his movement and give this specific muscle a good workout.
Another important role of plantarflexion specific to the soleus is actually simply to keep us vertical while standing. Plantarflexion is an extremely important movement needed to maintain balance as our center of gravity when standing is slightly anterior to the bones of the leg as well as the ankle joint. This creates a natural tendency for our body to lean forward by means of dorsiflexion at the ankle joint. The soleus counteracts this by maintaining the sustained state of plantarflexion of the foot which basically means that when we’re standing, the soleus is continuously pulling posteriorly on the bones of the leg to keep us upright, and is preventing this from happening all the time.
How can the soleus be constantly contracting, you ask? Well, this is possible due to the slow twitching type I muscle fibers of which the soleus is composed of. Type I muscle fibers are built for endurance rather than quick movements and are able to contract for long periods of time if necessary. This makes the soleus a slow, yet impressively strong and fatigue-resistant muscle.
And with that, we’ve discussed the major functions of the soleus muscle. Before I leave you though, let’s recap quickly what we’ve learned. We established that the primary function of the soleus was plantarflexion of the foot which occurs at the ankle joint. We also mentioned that the soleus becomes the primary plantarflexor of the foot when the knee is in a flexed position.
After that, we also explored the role of the soleus muscle in balance standing upright where it works constantly to pull posteriorly on the leg and keep us in an upright position.
And that’s it for the tutorial. I hope you enjoyed exploring the functions of the soleus muscle with me today, and please be sure to check out our website for more videos about all things anatomy. We’ll see you next time!