The soleus, located in the lower part of the calf, is one of those multifaceted organs that not only keeps us upright but also contains two important veins inside that play a fundamental role in blood circulation.

Better known as our "second heart", part of what makes the soleus muscle special is its composition, as doctor Carles Pedret, a specialist at the Faculty of Sports Medicine at the University of Barcelona in Spain, explains to BBC News Mundo.

"First of all, it's very big," he says: "it has a lot of muscle mass. And it's mainly made up of pure muscle tissue, not as much connective tissue as other muscles." This has to do with its function. You'll see below.

Stability
"The soleus is essential for any standing or walking activity," explains doctor Marc Hamilton, from the University of Houston in the USA. Depending on their function, the body's muscles are made up of different types of fibers.

For the muscles that maintain the structure of the body - such as those on the inside of the back, which keep the spine upright - the body uses slow twitch fibers, fibers that, although they are not designed to make sudden movements, have great endurance and can remain contracted for hours with few symptoms of fatigue. They are what allow us to stand or walk for long periods of time.

On the other hand, there are the muscles of the hands, legs and arms, which contain fast-acting fibers - that is, fibers that contract and relax almost instantaneously to perform the movements we are capable of.

The soleus, as a structural muscle that helps keep us upright, has a large composition of slow twitch tissue, which makes it capable of generating large amounts of energy without becoming fatigued.

"The soleus has a large amount of muscle fiber, and muscle fiber has an element that is fundamental for generating energy, the mitochondria. Due to the large number of mitochondria, we see that when we stimulate it, it generates a large amount of energy," explains Pedret.

It is this density of fibers that makes this muscle, which represents just 1% of body weight, have a much greater energy capacity than many other organs in the body.

Pumping system
The soleus also has a very particular function: it helps the heart in its work of pumping blood through the body.

"The anatomy of the soleus is different from that of other muscles," says Hamilton. "Inside your calves, there are some large veins that are inside your soleus, and those veins are there for a good reason."

"If you think about it, gravity is causing blood to pool in your calves, ankles and feet. It's a problem that affects adults, but even young people."

"However, wise nature has placed these veins inside the soleus so that they compress when the muscle contracts. When it compresses, these veins fill and empty, and send this fluid back to the heart," he explains.

Basically, with every step we take, we push the blood in our legs back to the heart. This system, which also includes various veins in the foot and gastrocnemius muscle, is known as the popliteal pump.

Good maintenance
Like all muscles in the body, the soleus needs to work to stay healthy, but unlike the activities we subject fast-fiber muscles to, the soleus needs to work more slowly and steadily.

For Pedret, walking is ideal:

"Everyone tends to believe that by strengthening and exercising the soleus muscle a lot, you'll be healthier. And precisely because of its characteristics, this muscle needs something that is the opposite of all that: sustained activity, but without too much stress"

"So what it needs is simply to work. You can't leave it idle. Rest and a sedentary lifestyle are very bad for it, as is excessive exposure to strength training."

It's the golden rule when it comes to our muscles, which scientists are increasingly linking to the proper functioning of our bodies in general.