Why is the soleus called the second heart?

The soleus muscle is often referred to as the "second heart" because of its crucial role in assisting the cardiovascular system, specifically in pumping deoxygenated blood from the lower extremities back up to the heart. Unlike the primary heart, which is a continuous, involuntary pump, the soleus's pumping action is activated by movement.

The Calf Muscle Pump: Your Body's Venous Return System

The term "second heart" primarily refers to the calf muscle pump, a system that includes the soleus and gastrocnemius muscles. These powerful muscles, located in the lower leg, play a vital role in overcoming gravity and returning blood to the chest.

How the Soleus Assists Blood Circulation

The soleus muscle is particularly significant within this pump system due to its unique characteristics:

• Deep Location: It lies deep beneath the gastrocnemius and has a large volume, allowing it to exert significant pressure on the deep veins of the calf.
• Sustained Contraction: The soleus is rich in slow-twitch muscle fibers, making it highly resistant to fatigue. This allows it to sustain contractions during activities like walking or standing, providing a continuous pumping action.
• Venous Compression: As the soleus contracts, it squeezes the deep veins running through it. This compression propels blood upwards, against gravity, preventing it from pooling in the feet and ankles.

The Mechanism of Venous Return

The process by which the soleus and other calf muscles facilitate blood return is often called the "skeletal muscle pump":

1. Muscle Contraction: When the soleus muscle contracts (e.g., during walking, running, or even subtle movements like ankle flexing), it tightens and expands.
2. Vein Compression: This contraction applies pressure to the surrounding deep veins within the calf.
3. Valve Function: Veins contain one-way valves. When compressed, blood is pushed upwards past a valve, which then closes to prevent backflow.
4. Blood Propelled Upwards: This sequential action, moving blood from one segment to the next, effectively propels deoxygenated blood towards the primary heart.

Why Movement is Crucial for Your "Second Heart"

The "second heart" only begins to pump when your legs are in motion. This critical dependency on physical activity means that the soleus pump is inactive when you are stationary. This becomes a significant issue for many individuals who spend prolonged periods sitting or standing still, such as during long work hours or extended travel.

Risks of Prolonged Inactivity

When the soleus pump is not activated, blood flow from the lower legs can become sluggish, leading to several health concerns:

• Blood Pooling: Deoxygenated blood tends to pool in the lower legs and feet due to gravity.
• Swelling (Edema): Increased fluid retention in the ankles and feet.
• Deep Vein Thrombosis (DVT): A serious condition where blood clots form in the deep veins, often in the legs, due to stagnant blood flow.
• Reduced Overall Circulation: Decreased efficiency in returning blood to the heart can impact the entire circulatory system.

Activating Your Soleus Pump: Practical Tips

To ensure your "second heart" functions optimally and supports healthy circulation, incorporating regular movement is key.

• Take Frequent Breaks: If your job involves prolonged sitting or standing, take short breaks every 30-60 minutes to walk around, stretch, or do simple exercises.
• Perform Ankle Pumps: While seated or standing, repeatedly flex your ankles up and down (pointing toes up and then down) to engage the calf muscles.
• Calf Raises: Stand up and slowly raise yourself onto the balls of your feet, then lower back down. Repeat this motion regularly.
• Walk More: Integrate walking into your daily routine, whether it's during commutes, lunch breaks, or for leisure.
• Use a Standing Desk: If you have a standing desk, incorporate movement by shifting your weight, performing subtle calf raises, or even marching in place.

Comparison: Primary vs. Secondary Heart Function

Understanding the distinct yet complementary roles of the primary heart and the soleus pump can highlight the soleus's importance.

The soleus muscle, therefore, serves as a crucial muscular assistant to the cardiovascular system, actively helping to circulate blood back to the heart and prevent circulatory issues in the lower body.