Aerobic Exercise Part 2
Last month, I’m sure I opened many readers’ eyes by suggesting that the most important muscle to train is one that never sees the light of day. It is easy to take your myocardium (the muscle of your heart) for granted . . . at least until it reminds you how important it is. Unfortunately, by then, it might be too late. So, in addition to the many exercises that are done to work the showy skeletal muscles, it’s essential to always make sure that sufficient time is devoted to the strongest muscle of them all.
With regard to exercise training, the principle of specificity suggests that different forms of exercise will bring improvements that are unique and highly dependent on the particular exercise stressor that is encountered. This principle is simple to grasp when training your skeletal muscles: If you challenge a specific muscle by making it contract against a load to which it is unaccustomed, it will adapt to a higher level of development (i.e. get stronger and bigger). Resistance training accomplishes this feat rather handily, so that is why we do curls for biceps, squats for hamstrings, glutes and quads, and weighted crunches for abs.
Training the myocardium is not quite that straightforward. The heart contracts to pump blood and it is well known that myocardial contractions (a.k.a. heart rate) increase in conjunction with an elevation of metabolic demand that mandates greater blood distribution to contracting muscles (for example, during aerobic exercise). This has lead to the use of heart rate as a convenient gauge of cardiovascular training intensity. However, it is important to realize that heart rate also goes up for other reasons, so the fact that your heart is beating more times than it normally does at rest doesn’t mean that it is being trained. A perfect example is during resistance training. When you’re lifting weights, your heart rate will be elevated in association with the need to recruit muscle fibers. However, circulation will not be enhanced and may actually be impeded because movement will be slow and your muscles will be contracting vigorously. This type of contraction squeezes nearby blood vessels and momentarily prevents blood from returning to the heart. This is the exact opposite of the specific overload that brings cardiovascular adaptations.
The cardiovascular system’s primary purpose is to circulate blood, so the forms of exercise that are ideal for inducing positive adaptations in cardiovascular function are those that require rapid movement of blood to and from the body’s skeletal muscles. This is effectively accomplished when those muscles contract less vigorously, but more rapidly. That is why endurance exercise that involves repeated rhythmic contractions of multiple muscle groups for an extended period (for example, jogging, rowing or cross-country skiing) is the ideal means for keeping your cardiovascular system healthy and strong. Some of the most important adaptations that this type of training brings are those that affect the heart muscle itself.
When blood flow back to the heart is elevated for prolonged periods during endurance exercise sessions, the chambers of the heart grow bigger and more capable of accommodating blood. This structural change is very important because a given amount of blood can be circulated with the heart beating less times. That is why endurance athletes have very low heart rates at rest and during submaximal exercise. This greater contractile efficiency results in the heart working less during all physical activities.
Endurance exercise makes the heart bigger from the inside out. This is called eccentric ventricular hypertrophy. Conversely, concentric ventricular hypertrophy is characterized by thickening of the heart muscle’s contractile tissue. This is often problematic because a thickened heart’s chambers can become less compliant. What is more, blood vessels that feed oxygen to myocardial tissue can lose touch with some of the new-formed regions. This is a recipe for disaster because oxygen-deprived myocardial tissue is on borrowed time. Concentric ventricular hypertrophy is often associated with hypertension because the heart adapts to the constant challenge of expelling blood against elevated systemic pressure.
There has been longstanding debate regarding the degree to which concentric ventricular hypertrophy occurs due to exercise training. Initial beliefs were that physical activities that predominantly involved static-type contractions where high systemic pressures were encountered for transient periods (heavy resistance training and wrestling, for example) would bring similar deleterious change. However, it has subsequently been shown that the degree of concentric ventricular hypertrophy that occurs due to these types of physical challenges is modest compared to that which is present in the diseased state. Furthermore, there is now evidence to suggest that myocardial tissue is also thickened in endurance athletes who don’t frequently face the challenge of blood expulsion against heavy systemic resistance. However, this thickening (referred to as “the athlete’s heart”) appears to be roughly proportional to the enlargement of the ventricular chamber (i.e. the degree of eccentric hypertrophy) that is also present, which differentiates endurance exercise-induced concentric hypertrophy from that which is pathological in nature. In fact, the athlete’s heart appears to be another exercise-induced positive cardiovascular adaptation because an increase in myocardial muscle mass ensures a stronger ventricular contraction that facilitates better chamber emptying per contractile effort.
This article was originally published in New Living Magazine, which can be accessed on-line at www.newliving.com.
