Many exercisers use the concept of target heart rate to determine how hard they should work when doing cardio, but this methodology is wrought with assumption. The best way for apparently healthy individuals to determine where they should work along the spectrum from total inactivity to maximal effort is by understanding the physiological responses that define the levels they'll encounter along the way. Researchers in the field of oxygen uptake kinetics have identified specific zones, which have been termed "intensity domains." Understanding how you will feel when you are operating within each of these is essential if you wish to determine your optimal exercise pace.
Beginning at the bottom of the spectrum, the moderate-intensity domain comprises all workloads from complete rest up to a delineation point known as the "lactate threshold." To understand this breakpoint, we must consider how energy is transferred from fuel to usable form within the body. The body utilizes the movement of hydrogens and associated electrons that are contained in food we consume much like a water wheel harnesses water flow to make electricity. The end result is energy release that is available to power muscular contraction and other cellular functions.
The two main players that supply electrons for transfer are sugar and fat. Both of these are used to form acetyl CoA, an intermediary that is degraded through a series of reactions (the tricarboxylic acid cycle and electron transport chain). Fatty acids enter directly at the level of acetyl CoA, whereas sugar in the form of glycogen must be broken down through another reaction sequence (glycolysis) first. The end product of glycolysis is pyruvate. Once pyruvate is formed, it has two fates: It can be oxidized (have electrons removed) to form acetyl CoA for further oxidation in the aforementioned reaction series or reduced (have electrons added) to form a temporary hydrogen disposal product known as lactate.
Rate of energy transfer is the main determinant of which way pyruvate will go. During benign efforts, little lactate is formed and that which is produced does not accumulate: It quickly moves back in the opposite direction for oxidation or is shuttled in the bloodstream to a tissue that is more capable of using its electrons as fuel. At complete rest, blood lactate concentration is low. When we increase our metabolic rate to one well within the capacity of our aerobic energy system (the system that uses oxygen as the final electron acceptor once all potential energy has been stripped), lactate production increases, but this elevation is compensated for by increased removal/use so that other than a transient spike, normal blood concentration is maintained. This is important because an increased presence of lactate within the blood is associated with a shift toward acidity, which presents a challenge to homeostatic regulation.
Glycolysis can transfer energy at accelerated rates, but only when pyruvate is converted to lactate. Consequently, when we work somewhat hard, the presence of lactate within the blood will exceed resting level. The lactate threshold is the point above which resting blood lactate levels no longer prevail. The lactate threshold can be identified in an exercise laboratory in a number of ways. By gradually increasing the work rate, an exerciser encounters on an ergometer (an exercise device that precisely measures work being performed) blood samples are taken at various stages.
A less invasive way to pinpoint the lactate threshold is by measuring expiratory gases. When lactate begins to accumulate in the blood, the corresponding increase in acidity requires bicarbonate buffering. This generates carbon dioxide, which stimulates ventilation. So, when you're working above the lactate threshold, your expired gases will reveal a ratio of carbon dioxide produced versus oxygen consumed that skews toward the former. In addition, your ventilatory rate will increase disproportionately. That is why the lactate threshold is synonymous with both the gas exchange and ventilatory thresholds.
You can't complete a sentence comfortably when you're working above the lactate threshold. And you will experience some discomfort both systemically and spot specifically in the muscles you are using due to the increased acidic presence. Does that mean you should adhere to the "talk test" criteria and not perform cardio at this level? Absolutely not! And it also doesn't mean you are only working anaerobically when you transcend the moderate domain. Aerobic energy transfer is present in varying amounts from the lowest all the way to the highest workload you can tackle, and even though you are above the lactate threshold, you might not be near the point above which anaerobic metabolism begins to play a predominant role. To identify the dividing line up to which the aerobic system is still the main player, you must delineate the intensity domains that have been identified in the supra-lactate threshold region of the metabolic spectrum.
Fred Dimenna, a Certified Strength and Conditioning Specialist and lifestyle and Weight Management Consultant is a two-time Natural Mr. United States and a WNBF drug-free professional bodybuilder. Visit him at www.freddimenna.com or email him: firstname.lastname@example.org