The heart we didn’t know we had: the soleus, the hidden muscle that works against gravity and reveals just how much movement is integral to our physical and mental health.
In a quiet museum gallery, a man is looking at a painting. He is motionless—or so it seems. His arms hang relaxed at his sides, his face turned toward the canvas, his weight distributed across his legs. Very little is happening around him: no obvious gestures, no effort, no movement capable of drawing attention.
And yet, inside him, nothing is truly still.
The heart continues to contract. The diaphragm moves with every breath. Blood vessels imperceptibly adjust their diameter. The nervous system receives, filters, and processes a ceaseless stream of information from within the body.
And further down, deep within the calf, hidden beneath the gastrocnemius, a muscle works unseen to enable that man to do something we consider so natural that we don’t even think about it: standing upright.
That muscle is called the SOLEO.
Someone has called it the “second heart.” It’s a striking expression, and—as long as it isn’t taken literally—it contains a grain of truth. The soleus, of course, is not a heart.
More precisely, it is the entire muscular pump of the calf—together with the venous valves and the other leg muscles—that helps return blood from the lower limbs to the chest.
The soleus, however, occupies a special position within this system.
It is deep-seated, resilient, and rich in oxidative fibers. It seems designed more for endurance than for show. It comes into play when we stand, when we walk, when we shift our weight from one leg to the other, and when the ankle performs its most ordinary movements. It supports our posture and participates in that silent battle our body wages every day, without demanding our attention: the battle against gravity.
Perhaps it is precisely its hidden nature that makes it so interesting.
Because the soleus, after all, resembles many of the essential functions of our biological and psychological: they support us continuously, long before we learn to recognize them.
When we stand upright, gravity tends to cause blood to pool in the lower parts of the body. The valves in the veins help prevent backflow, and the contraction of the leg muscles helps pump the blood back upward. Walking, therefore, is not merely a matter of moving from one place to another.
Every step is also a small circulatory event.
A muscle contraction alters pressure, blood flow, and energy expenditure. Behind what appears to us as the banality of walking lies an extremely sophisticated physiological choreography, in which the muscles, cardiovascular system, and nervous system are in constant communication.
And this is where one of the paradoxes of our age emerges.
Just as we are gaining a deeper understanding of how deeply movement is embedded in human physiology, we have organized much of contemporary life around its gradual disappearance.
Sitting to work. Sitting in a car. Sitting during a meeting. Sitting in front of a screen. Sitting while we study, talk, stay informed, shop, and have fun.
Then, perhaps, an hour at the gym.
We’ve ended up turning movement into an activity separate from life: something with a beginning and an end, to be slotted into the schedule alongside a meeting, a doctor’s appointment, or a business meeting.
But the human body wasn’t designed with this logic in mind.
Its original “grammar” consists of variations: standing, rising, bending, walking, carrying, changing position, stopping, resting, and then starting again.
The issue, however, isn’t just about metabolism.
It concerns something even more intimate: the way we’ve learned to inhabit our bodies.
In clinical practice, we often encounter a surprisingly common condition: many people become aware of their bodies almost exclusively when the body begins to protest.
We feel it when it hurts.
When our neck stiffens. When our jaw remains clenched for hours without us noticing. When our breath seems to get stuck too high up. When our heart races. When our stomach tightens. When a fatigue sets in that’s hard to explain, or, in the evening, we realize we’ve been in the same position for most of the day.
Yet the body didn’t start speaking to us at that moment.
It had already been speaking to us for hours.
We simply weren’t listening.
A significant part of contemporary neuroscience studies precisely this incessant dialogue between the brain and the body through the concept of interoception, the process by which the nervous system detects, integrates, and interprets signals coming from within the body.
Heart rate, breathing, visceral tension, temperature, energy levels, and numerous other pieces of information continuously contribute to the construction of our internal experience.
Interoception is also closely linked to affective processes and the regulation of emotions.
And this profoundly changes the way we can think about what we feel.
Emotions do not simply happen “inside the head.”
We feel fear in a body. We wait for someone in a body.
We love, desire, suffer, and remember through an organism that continuously alters our breathing, heart rate, muscle tone, posture, peripheral temperature, and even the way we direct our attention.
The mind, therefore, is not a tenant of the body.
It is one of its expressions. This is another reason why prolonged immobility cannot be considered simply an absence of activity.
From a metabolic standpoint, spending many consecutive hours sitting is, in fact, a different condition from simply “not exercising.” We can exercise regularly and, at the same time, spend most of the rest of the day almost completely immobile.
In recent years, several studies have observed that periodically interrupting long periods of sitting with brief moments of movement can produce favorable effects—at least in the short term—on the glycemic and insulin response after meals. Reviews of experimental studies have shown particularly interesting results when sedentary behavior is interrupted by brief walking intervals.
This doesn’t mean we have to turn every half-hour of our day into a new exercise regimen.
On the contrary. The interesting point is exactly the opposite: to stop thinking about health solely in terms of exercise regimens.
Our bodies seem to respond not only to the total amount of exercise we do, but also to how movement and inactivity are distributed throughout the day.
It’s a seemingly small difference, but it changes our perspective.
Because it restores importance to all those little actions we normally don’t even consider physical activity.
Getting up from a chair.
Taking a few steps.
Walking to another room.
Climbing a flight of stairs.
Walking while talking on the phone.
Moving your legs after sitting for a long time.
Simple actions without heroics or records to break. And perhaps that’s precisely why we’ve stopped valuing them.
Amid this renewed scientific interest in what happens to muscles during inactivity, the soleus muscle suddenly emerged from obscurity in 2022.
A team from the University of Houston led by Marc Hamilton studied a specific method of activating the soleus while seated, called the soleus push-up.
This wasn’t simply a matter of absentmindedly lifting one’s heel under a desk. The protocol involved a specific plantar flexion movement, designed to predominantly recruit the soleus and monitored in the lab via electromyographic activity.
The results sparked great interest.
Under the experimental conditions observed, the muscle was able to maintain high oxidative activity for prolonged periods while using relatively small amounts of muscle glycogen. Soleus activation was also associated with changes in the postprandial metabolic response, including reductions in glycemic fluctuations and hyperinsulinemia.
These are fascinating findings, but it is important not to demand more from the research than it can actually tell us.
The study involved a limited number of participants and a specific procedure carried out under controlled experimental conditions. It does not prove that lifting your heels slightly under your desk can replace a walk, regular exercise, a balanced diet, or, when necessary, medical treatment.
Its most interesting implication lies elsewhere.
It shows us just how metabolically active a muscle can be even when, viewed from the outside, the movement seems almost insignificant.
For a long time, we thought of muscle tissue primarily as a mechanical motor: something that contracts to move the skeleton.
Today we know that this view is incomplete.
Skeletal muscle is also a metabolically and endocrinologically active tissue. When it contracts, it engages in complex communication with other tissues through numerous mediators, including the so-called myokines.
It is one of the areas where physiology, metabolism, and immunology cease to appear as separate disciplines and instead reveal their profound interdependence.
Research on myokines is showing how muscle activity contributes to the regulation of numerous processes through signals capable of reaching different tissues. Of course, the responses vary depending on the type, intensity, and duration of exercise, and their biology is far more complex than the simplistic slogans with which they are sometimes described.
Simply saying that “exercise reduces inflammation,” for example, would be an oversimplification.
But the general principle remains important: the muscle is not just a tissue that receives commands. It is also an organ that communicates.
This changes the way we think about movement.
A muscle contraction is not just a movement in space. It is a biological event.
The nervous system modifies the muscle. The muscle influences metabolism. Metabolism interacts with the endocrine and immune systems. The cardiovascular system responds to activity, and the brain, meanwhile, continues to receive information about what is happening in the rest of the body.
Dividing the body into compartments helps us study it.
But the body itself knows nothing of these compartments. It functions as a network.
And this is precisely where the story of the soleus muscle inevitably intersects with psychology.
Stress, too, is not simply a negative thought.
It is a response of the entire organism.
When we perceive a threat, go through a conflict, anticipate something that worries us, or face uncertainty, the brain mobilizes ancient systems deeply rooted in our biology. Our autonomic tone, attention, and breathing change. Our heart rate and muscle tension shift. Energy resources are mobilized.
It’s physiology. The problem isn’t that this response exists. We need it.
The difficulty arises when a system designed to activate and then deactivate becomes the almost permanent backdrop of our daily lives.
From this perspective, contemporary life contains a singular contradiction.
We can spend hours almost completely motionless while our brain processes an enormous amount of stimuli.
A notification.
A deadline.
An argument.
A message we’ve been waiting for that never arrives.
A loss.
A photograph.
A news story capable of unsettling us.
A recurring thought.
On the outside, we’re sitting in front of a screen.
Inside, however, anything but stillness may be taking place.
Perhaps one of the great dissonances of modern life is precisely this: an organism constantly ready to respond within a body that we ask not to move.
This does not mean, of course, that getting up from a chair can “cure stress.” That would be a simplistic and scientifically untenable conclusion.
It means something more interesting: recognizing that psychological regulation does not occur in a realm separate from physiology.
Movement can interact with circuits involved in emotional regulation, and interoception plays a role in how we recognize, interpret, and attribute meaning to our emotional states.
The body does not resolve life’s conflicts for us.
But it is always present in the way we navigate them.
And there is another distinction, as simple as it is often overlooked.
Rest and immobility are not the same thing.
Rest means being able to reduce the demand for adaptation. To recover. To let down one’s guard. To feel safe enough not to have to constantly react to something.
We can be motionless for hours and not be at rest at all.
We can sit in front of a computer while our mind processes constant demands, our attention jumps from one stimulus to another, our muscles maintain a underlying tension, and part of us is already focused on everything we’ll have to face next.
And we can, on the contrary, walk slowly and feel deeply rested.
Anyone who has strolled unhurriedly down an unfamiliar street, walked along the seashore, or followed a path without an urgent destination intuitively knows this difference.
Movement isn’t always exertion.
Sometimes it’s restoration.
It restores rhythm to our breath.
It restores variation to our posture.
It restores depth to our gaze.
It restores our attention to what surrounds us.
And, above all, it can restore us to ourselves for a few moments, bringing us back into that body from which the daily rush so easily distances us.
Faced with every new scientific discovery, however, we face an almost irresistible temptation: to immediately turn it into a rule.
Ten thousand steps.
Eight hours of sleep.
A certain number of minutes of physical activity.
A predetermined amount of water.
We could now even add an ideal number of soleus contractions.
That would probably be the worst way to understand what this story can teach us.
Contemporary health is already at risk of becoming a second profession, made up of metrics to monitor, numbers to hit, apps to consult, and goals to check off.
Even well-being can turn into a source of anxiety when every behavior must be measured and every day evaluated based on its performance.
The message from the physiology of movement is perhaps much simpler. And even gentler.
It doesn’t necessarily ask us to do more and more.
Rather, it reminds us to spend a little less time completely still.
It’s different. It means reclaiming a more natural relationship with our bodies. Standing up every now and then.
Taking a few steps while we think.
Changing positions when we start to feel the need. Walking after eating.
Taking the stairs whenever possible.
Noticing fatigue before it turns into pain.
Feeling your breath become short before reaching the evening completely drained.
Above all, letting movement become a part of life again—and not just a task to be accomplished.
Not everything that protects our health has to be heroic.
And this is perhaps the most fascinating part of the soleus’s story.
An almost unknown muscle forces us to view the body from a different perspective.
We’re used to thinking about health in terms of major organs and major events: the heart, the brain, the lungs; a diagnosis, a surgery, a treatment, a workout.
Yet much of our biology is built through the repetition of tiny events.
One heartbeat after another.
One breath after another.
One contraction after another.
One meal after another.
One night after another.
The body doesn’t wait for major events to adapt.
It does so continuously.
And, if we broaden our perspective, perhaps something similar happens in our psychological lives as well.
A person’s emotional history isn’t made up solely of the traumas, loves, losses, or major turning points we can recount.
It’s also made up of thousands of seemingly insignificant experiences through which the nervous system, day after day, learns what to expect from the world.
Tension or calm.
Closeness or distance.
Danger or safety.
Presence or absence.
Movement or stillness.
It is through repetition that the body learns.
And it is often through new repetitions that it can slowly learn something different.
Let’s return, then, to the museum gallery.
The man is still standing in front of the painting.
Perhaps that canvas has awakened something that none of the other visitors could see.
A face.
A home.
A distant summer.
A person who is no longer here.
For a moment, his breathing changes. His heart imperceptibly alters its rhythm. His brain weaves the image before him with those stored in his memory.
A physiological change becomes an experience.
Experience meets memory.
And memory becomes emotion.
Then the man shifts his weight slightly onto his left leg. He takes half a step forward, perhaps just to get a better look at a detail on the canvas.
It’s a minimal movement. Almost invisible.
Yet, beneath the skin, much has already changed.
The muscles have contracted. The postural system has corrected his balance. The calf has contributed to venous return. The soleus has resumed its work against gravity.
No one in the room notices. Probably not even him.
Perhaps this is precisely the image worth preserving.
Because it tells us much more than just a small muscle hidden in the leg.
It reminds us that the body continues to support us even while we’re distracted by other things. That part of our experience arises from the ceaseless dialogue between brain and body. That movement, metabolism, emotion, and adaptation belong to the same biology, even when we’re forced to separate them in order to study them.
And perhaps it also suggests something even simpler: learning to connect with our bodies a little before pain forces us to listen to them.
The man continues to look at the painting. Before him, the image is still.
Inside him, nothing is truly still.
His heart beats in his chest.
His breath comes and goes.
His brain transforms sensations, memories, and emotions into experience.
And much lower down, hidden in his calf, a small muscle silently continues its work against gravity.
It’s not really a second heart.
But perhaps it reminds us of something we’ve forgotten amid the clamor of our daily lives:we are alive even in the movements we don’t see.
