The output from your heart - known as cardiac output - is completely dependent on the venous return to your heart. When a person is lying down, there is a minimal gravitational influence on the blood in the body, so adequate venous return can be ensured by low levels of venous pressure. However, when sitting or standing upright, the venous blood in our lower limbs must be returned to the heart against the force of gravity, requiring pressures well above the usual pressures in the veins.
To overcome this challenge, the human body relies on skeletal muscle pumping to return pooled venous blood, as well as interstitial fluid, back to the heart. Three lower limb skeletal muscle pumps exist, the foot pump, the calf muscle pump, and the thigh pump. The foot pump serves primarily to "prime" the calf muscle pump, which provides about three-quarters of the fluid return from the lower body to the heart, while the thigh pump provides the remaining quarter.
In the absence of movement though, for example, while seated, muscle pumping is created almost exclusively through the activity of the soleus muscles. These are specialized postural muscles in the calf of the leg which contain large venous sinuses that store blood until a reflex response activates contraction of the muscle. This results in the production of sufficient pressure to return venous blood, as well as lymphatic fluid, back to the heart.
The soleus muscles are so critical in returning blood to the heart that they have come to be called the "secondary hearts" of the body. The cardiovascular system in the upright person should, therefore, be viewed as a two-pump system where the cardiac muscle circulates blood, and the soleus muscles return pooled blood and lymphatic fluid back to the heart.
In the absence of adequate fluid return to the heart, the cardiac muscle cannot operate effectively. When the cardiac muscle cannot pump sufficient blood to provide nutrients and oxygen to all the tissues of the body the condition is referred to as heart failure. Inadequate fluid return from the lower body is indicated by symptoms such as swollen ankles, painful joints, varicose veins and venous insufficiency, nighttime leg cramps, venous ulcers, and poor wound healing, and osteoporosis.
In addition, insufficient fluid return to the heart causes lowered cardiac output and blood pressure, resulting in conditions such as weight gain, chronic fatigue, as well as memory and attention deficits. Secondary heart insufficiency is remarkably common. We estimate that 40 percent of women, or more, have symptoms associated with insufficient venous return to the heart. This is largely a result of our modern lifestyle.
When our ancient ancestor wanted to rest, they squatted on their haunches, an activity that requires extensive soleus muscle activity. Today, we sit in chairs. Fortunately, it is possible to protect your soleus muscles from weakening, or rebuild them if they are failing you. Performing toe-stands throughout the day is very helpful, as is squatting instead of sitting. Certain forms of exercise, like Tai Chi and yoga, typically involve substantial use of the soleus muscles.
Of course, postural muscles such as the soleus muscles require several hours per day of exercise to stay in shape, or to rebuild, and as this can be difficult for many people to fit into their daily schedule. To assist you, numerous types of exercise equipment have been developed. These include, for example, the Revitive Medic, which electrically stimulates your foot muscles while providing a convenient means to exercise your soleus muscles through a rocking action.
Similarly, the Advanced Foot Energizer by Northwest Essentials relies on electrical nerve stimulation to enhance circulation in the feet. Alternatively, the HeartPartner offered by Sonostics utilizes mechanical stimulation of nerve endings on the feet to activate a reflex arc that produces soleus stimulation. This approach allows the user to keep their socks and shoes on during therapy.
Whichever approach you use, the important point is to keep your soleus muscles in shape. Your heart, as well as the rest of your body, is critically dependent on the health of your secondary heart.
About the Author
Dr. Kenneth McLeod: Dr. McLeod, an MIT scholar, is recognized internationally for his work on muscle activity and its effect on skeletal adaptation, as well as the link between muscle activity of the lower extremities and cardiovascular function. In the last three decades, he has pioneered awareness of the second heart that resides within every person.
There has been so much validity built around Dr. McLeod's findings that a product specifically designed to assist the functioning of the "second heart" was created called the HeartPartner, which is sold to consumers throughout the United States. HeartPartner employs proprietary technology to non-invasively enhance the performance of the Soleus muscles, also known as "the secondary hearts" of the human body.