Why Calf Muscles Are Called the "Second Heart"
Calf muscles are referred to as the "second heart" because they play a crucial role in improving blood circulation by pumping blood back to the heart when exercised, particularly during walking or other physical activities. 1
Physiological Mechanism of the "Second Heart" Function
The calf muscles function as a secondary pumping mechanism in the circulatory system through several mechanisms:
Venous Return Enhancement: When calf muscles contract during activities like walking, they compress the deep veins in the legs, propelling blood upward against gravity toward the heart 2
Pressure Reduction: During walking at a normal pace (approximately 1.7 mph), venous pressure can be reduced from 80-90 mmHg (when standing still) to a mean of 22 mmHg 2
Valvular Assistance: The calf muscle pump works in conjunction with competent venous valves to prevent blood reflux and facilitate return of blood from the distal leg to the heart 2
Energy Storage and Release: The calf muscles and associated tendons store elastic energy during the stance phase of walking and release it during push-off, functioning like a spring mechanism 3
Clinical Significance
The "second heart" function of calf muscles has important clinical implications:
In Peripheral Arterial Disease (PAD)
- In patients with PAD, structured exercise training (SET) increases calf muscle pump function and improves lower limb circulation 1
- The American Heart Association recognizes that regular physical activity through SET programs leads to improved endothelium-dependent vasodilation, increased capillary density of the gastrocnemius muscle, and enhanced skeletal muscle metabolism 2
In Venous Insufficiency
- When calf muscle function is compromised or venous valves are damaged, ambulatory venous hypertension can occur, leading to symptoms of venous insufficiency 2
- Damage to venous valves impedes venous return to the heart, leading to venous hypertension and consequent leg pain and swelling 2
In Wound Healing
- The Centers for Medicare and Medicaid Services (CMS) now reimburses supervised exercise training sessions for patients with PAD to improve circulation through calf muscle activation 1
- Exercise that activates calf muscles may have potential benefits for chronic wound healing, though more research is needed 1
Biomechanical Efficiency
The calf muscles demonstrate remarkable biomechanical efficiency:
- During walking, they store energy in their series elastic components (tendons and connective tissues) and release this energy at high rates during push-off 3
- This "catapult action" allows for high power peaks with optimal efficiency 3
- The calf muscles contribute significantly to forward propulsion during walking and running 4
Common Pitfalls in Understanding Calf Muscle Function
Misunderstanding the primary role: The calf muscles' primary function during walking is not to propel the body forward but rather to restrain the body's forward momentum and maintain stability 5
Overlooking pathological conditions: Conditions like peripheral arterial disease and venous insufficiency can significantly impair the "second heart" function of calf muscles, leading to reduced circulation and symptoms 2
Neglecting exercise benefits: Failure to recognize the importance of calf muscle exercise in improving circulation can lead to missed therapeutic opportunities, especially in patients with circulatory disorders 1
The "second heart" designation highlights the vital role calf muscles play in maintaining efficient circulation, particularly in the lower extremities, and underscores the importance of regular physical activity for vascular health.