Soleus

What it is

A broad, flat muscle lying underneath gastrocnemius, named for its resemblance to a sole — the fish, not the shoe. It arises from the back of both bones of the lower leg, below the knee, and joins gastrocnemius in the calcaneal tendon.

Because it originates below the knee, it crosses the ankle only. Nothing the knee does changes its length or its leverage.

It is composed largely of slow-contracting, fatigue-resistant fibres — one of the more consistently slow-twitch muscles in the body — which fits what it spends its day doing.

What it does

Plantarflexes the ankle. That is its only action, and it is a strong one.

Its less obvious job is postural. When you stand still, your body's weight line falls slightly in front of the ankle joint, so you are continuously tipping forward by a small amount. Soleus is what quietly resists that, all day. It is sometimes called the second heart, on the basis that its contractions help return venous blood from the leg — a real effect, though the nickname oversells it.

What loads it

Any plantarflexion with the knee bent, which removes gastrocnemius from the equation: seated calf raises, and cycling in a low position. Also plain standing, walking, and running, where it works throughout.

The calf raise — and the bent-knee version specifically, which is the only common way to load this muscle with gastrocnemius taken out.

What is commonly got wrong about it

That it is the lesser calf muscle. It is the larger of the two by cross-section in most people, and it produces more force in plantarflexion than gastrocnemius does. It is simply underneath, so it is not the one people see.

That seated calf raises are a soft option. They are the only common way to load this muscle in relative isolation, because bending the knee is precisely what takes gastrocnemius out.

That soleus tightness limits squat depth. Ankle range does limit squat depth for many people — that part is sound, and it is why a heel lift transforms some people's squat. But whether the restriction is muscular, capsular or bony varies from person to person, and it is not something to diagnose from a website.