The term "second heart" refers to the soleus muscle, a deep muscle in the calf. It is called the second heart because it plays a major role in supporting venous return — the movement of blood from the lower legs back to the heart. When the soleus contracts, it compresses deep veins, helping push blood upward against gravity.
This term appears in published scientific literature, including the NIH paper "Calf pump activity influencing venous hemodynamics in the lower extremity" (PMC3699225). It is not a marketing term; it is a physiological description.
Read more: Why is the soleus called the second heart? →
If you searched for "second heart" because of a medical device — such as a pacemaker, ventricular assist device (VAD), or artificial heart — this page is not about that. The second heart in medical literature refers to the soleus muscle, not a device.
The term is descriptive, not literal. Your heart remains the primary pump. The soleus supports circulation but does not replace your heart.
Understanding that the soleus is called the second heart helps explain what happens to your body during prolonged sitting. When you sit with your knees at 90 degrees, the soleus becomes substantially less active. The calf pump slows. Venous return decreases.
This may contribute to sensations of heaviness, swelling, or fatigue in the legs after a long day of sitting.
What happens when the second heart stops moving? →
Learn more about heavy legs after sitting →
The term "second heart" appears in peer-reviewed literature, including a 2013 paper in the International Journal of Angiology titled "Calf pump activity influencing venous hemodynamics in the lower extremity." The paper describes the soleus as performing the role of a peripheral heart.
This is not a claim made by 2HEART. It is a description used in published scientific literature.
Educational purpose only. This glossary entry is for informational purposes and is not medical advice. The term "second heart" refers to the soleus muscle, not a medical device.