Michael Jordan played in the NBA into his late 30s, a feat that he attributes to a rigorous stretching routine. Stretching will not only prepare your muscles for exercise, it aids in healing, blood flow to tissue, and even affects the function of your heart.
Muscle Oxygen Levels During Stretching
According to the 2010 article "The Influence of Passive Stretch on Muscle Oxygen Saturation," the gastrocnemius and quadriceps muscles show a decrease in oxygen saturation during maximal stretching. The article states that passive stretching may result in significant hypoxia, or low oxygen concentration, in certain muscle groups. Consequently, skeletal muscle will resort to anaerobic respiration during hypoxic periods, which results in the increased production of lactic acid by these tissues.
Changes in Blood Flow
Lactic acid is the end product of anaerobic metabolism, which occurs in tissues when oxygen demand exceeds supply. Blood vessels are highly adaptive tissues, capable of responding to small changes in blood characteristics such as pH. As the acidity of the blood in a stretching muscle increases, the blood vessels in that area will respond by dilating the supplying arteries to allow for sufficient oxygen delivery. These adaptations prime the skeletal and cardiovascular system for an efficient workout.
Changes in Venous Return
Your skeletal muscles act as a secondary pumping system; the alternating contraction and relaxation of major muscle groups during stretching and exercise will squeeze the blood out of muscle capillaries and into the venous system. By a similar mechanism, changes in respiration will also supplement the heart's efforts to pump blood throughout your system. During stretching, as your breaths become deeper and larger, blood is pulled into the chest as its diameter increases. The negative pressure generated by your expanding rib cage will aid in the return of blood to the heart.
Effects on the Heart
Changes in blood circulation can have significant effects on heart function, particularly pulse rate. As more blood returns from the periphery through the venous system, the heart will automatically respond by increasing the rate and strength of contraction. This response allows the body to avoid accumulating fluid in peripheral tissues, a phenomenon known as edema. Patients suffering from heart failure, for example, may experience pulmonary edema, or shortness of breath, with such changes in venous return because the heart is unable to increase its output, which leads to fluid accumulation.
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