Aerobic Exercise Part 3
If you’re up to date with my last two columns, you’re aware that I’ve taken a little detour from my usual course. Sure, I’m still talking about exercise designed to make your muscles stronger; however, in this case, I’m referring to a muscle that you might not usually think of as such. Every exerciser knows how important their heart is, but far less realize that it is a muscle and even fewer are aware of how to train this muscle properly.
In previous installments, I’ve explained that the stimulus required to make your heart grow bigger (specifically, growth that occurs from the inside out so that it is capable of accommodating and expelling more blood each time it contracts) is a prolonged period for which venous return (blood coming back to the heart) is elevated. This means that it’s not enough to simply elevate your heart rate to train this muscle because in some cases, your heart can be beating rapidly even though blood flowing back to it is restricted (during activities that involve more static-type contractions like resistance training, for example).Therefore, the age-old practice of monitoring heart rate to infer a cardiovascular training stimulus must be used with a grain of salt. The key thing to consider is whether your muscles’ need for oxygen is sufficiently elevated in conjunction with the heart rate elevation you measure. If the large muscles of your body are contracting rhythmically against a relatively light load (for example, if you’re jogging, stairclimbing, cycling or cross-country skiing), a large volume of oxygen will be needed to allow aerobic energy transfer to prevail and as a consequence, a lot of blood will have to be distributed to your exercising muscles.
Your cardiovascular system is a closed network and there is nary enough blood to go around. Consequently, the only way to get enough oxygenated blood to your contracting muscles to satisfy high demand during aerobic exercise is to get a lot of deoxygenated blood back to your heart quickly. That is why aerobic exercise overloads venous return and forces the heart to adapt. However, when you are exercising aerobically, blood vessels in the active musculature open up (vasodilate) to allow more blood to travel through needy tissue. With so much blood circulating through the body’s periphery, there is a serious risk of not being able to get enough back to the heart for redistribution. This is particularly problematic because the top-priority areas for blood leaving the heart are the brain and the heart muscle itself, and if these two organs don’t receive sufficient flow continuously, you’re in trouble. Fortunately, the cardiovascular system meets this challenge by strategically shunting blood from less essential areas (the stomach, spleen, pancreas, intestines and liver, for example) so that central venous pressure and venous return can be maintained. This acute adaptation during an aerobic exercise bout is complemented by a rapidly-occurring chronic one as plasma volume expands in short order due to regular endurance training.
In addition to oxygen-delivery related competition for blood flow during exercise, thermoregulation also requires adaptive circulatory measures. Muscle contraction is relatively inefficient because a great deal of the energy that is transferred to power the process is lost as heat. Consequently, to keep the exercising body cool, fluid from the bloodstream must be expelled as sweat to facilitate evaporative cooling. A greater portion of blood is also sent to the skin (i.e., closer to a cooler environment) to keep the exercising body from overheating. This makes blood distribution during aerobic exercise even more challenging for the cardiovascular system.
There are a number of important repercussions attributable to the relative shortage of blood that characterizes intense aerobic exercise. During such efforts, less blood flow to the stomach and intestines means that the ability to absorb fluids is compromised. This can make it difficult to maintain a sufficiently hydrated status even if you continue to consume fluids while you work out. A sports beverage designed to improve absorption can be of benefit under these circumstances. Less blood to the splanchnic region also means that digestion will be impaired, so pre-exercise meals should be planned accordingly and any carbohydrate replenishment attempted during prolonged aerobic bouts should involve easily-digested products specifically made for that purpose.
The specifics of blood distribution during aerobic exercise also influence what happens once the exercise bout is terminated. During exercise performed in the upright position, getting blood back to the heart for redistribution is further complicated by the fact that the pull of gravity is not your ally. An important mechanism that helps to return deoxygenated blood when active muscle blood vessels are vasodilated is the pumping action of the contracting muscles. Consequently, if exercise is stopped abruptly and this muscle pump is removed with a large quantity of blood perfusing the periphery, venous return will be insufficient and cardiac output will drop. Light headedness/fainting (insufficient flow to the brain) and cardiac contractile irregularities (insufficient flow to the heart) can result. These dangers can be averted if exercise intensity is progressively reduced to a low level of exertion (a “cool down” is performed) prior to stopping aerobic exercise completely.
This article was originally published in New Living Magazine, which can be accessed on-line at www.newliving.com.
