Your heart pumps blood to your entire body.
But getting that blood back up from your lower legs is a different challenge.
Gravity pulls blood downward. Your heart cannot reach down and pull it back up.
So your body has another solution: a muscle that acts like a second heart.
It is called the soleus.
Short answer: The soleus is called the second heart because it acts as a venous pump, helping push blood upward from your lower legs toward your heart. This function is part of what is known as the calf muscle pump. When you stand or walk, this happens continuously. When you sit for hours, this pump slows down.
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The soleus is a muscle in your calf, located beneath the larger gastrocnemius. When it contracts, it compresses the deep veins in your leg. One-way valves ensure that blood flows only upward — toward your heart. This mechanism is called the calf muscle pump or venous pump, and the soleus is the primary driver of this process.
The soleus muscle circulation function is so important that researchers have described it as performing the role of a peripheral heart. The term second heart appears in published scientific literature, including the NIH paper "Calf pump activity influencing venous hemodynamics in the lower extremity" (PMC3699225).
The idea that a single muscle influences circulation during prolonged sitting is one of the foundations of what we call Soleus Health.
Despite its importance, the soleus receives far less attention than muscles such as the biceps, chest, or abs. Why? Because its primary role is endurance and circulation rather than visible movement or appearance. Health conversations have historically focused on what people can see — weight loss, muscle size, athletic performance. The soleus works quietly in the background, which is why most people have never heard of it.
Read more: Why have you never heard of the soleus? →
When you stand or walk, your soleus is continuously active. It works all day without you noticing. But when you sit with your knees at 90 degrees, the soleus becomes substantially less active. The calf muscle pump slows. Blood can pool in your lower legs, which may contribute to heaviness, swelling, or fatigue.
Understanding why the soleus is called the second heart helps explain what happens to your body during prolonged sitting — and why activating this muscle may matter. Learn more about soleus push-ups →
Read the complete guide to the soleus muscle →
The term second heart is descriptive, not literal. Your heart remains the primary pump. The soleus supports circulation but does not replace your heart. If you searched for "what is the second heart" because of a medical device like a pacemaker or VAD, this page is not about that. The second heart in medical literature refers to the soleus muscle.
Educational purpose only. This information is not medical advice. If you have concerns about your circulation or leg health, consult a doctor.