Anaerobic effort and maximal capacity for anaerobic exercise

The most common methods for estimating anaerobic effort involve the examination of either the excess postexercise oxygen consumption(EPOC) or the lactate threshold.

Postexercise oxygen consumption

The matching of energy requirements during exercise with oxygen delivery is not perfect. When aerobic exercise begins, the oxygen transport system(respiration and circulation) does not immediately supply the needed quantity of oxygen to the active muscle. Oxygen consumption requires several minutes to reach the required(steady state) level at which the aerobic processes are fully functional, even though the body’s oxygen requirements increase the moment exercise begins.

Because oxygen needs and oxygen supply differ during the transition from rest to exercise, the body incurs an oxygen deficit, as shown in the figure below. The deficit occurs even at low exercise intensities. The oxygen deficit is calculated simply as the difference between the oxygen required for a given exercise intensity(steady state) and the actual oxygen consumption. Despite the insufficient oxygen delivery at the onset of exercise, the active muscles are able to generate the ATP needed through anaerobic pathways.

During the initial minutes of recovery, even though muscle activity has stopped, oxygen consumption does not immediately decrease. Rather, oxygen consumption remains temporarily elevated. This consumption, which exceeds that usually required at rest, traditionally has been referred to as the “oxygen debt”. The more common term today is excess postexercise oxygen consumption(EPOC). The EPOC is the volume of oxygen consumed above that normally consumed at rest. Everyone has experienced this phenomenon at the end of an intense exercise bout: A fast climb up several flights of stairs leaves one with a rapid pulse and breathing hard. These physiological adjustments are serving to support the EPOC. After several minutes of recovery, the pulse and breathing return to resting rates.

For many years, the EPOC curve was described as having two distinct components: an initial fast component and a secondary slow component. According to classical theory, the fast component of the curve represented the oxygen required to rebuild the ATP and phosphocreatine(PCr) used during exercise, especially in its initial stages. Without sufficient oxygen, the high-energy phosphate bonds in these compounds were broken to supply the required energy. During recovery, these bonds would need to be re-formed, via oxidative processes, to replenish the energy stores, or repay the debt. The slow component of the curve was thought to result from removal of accumulated lactate from the tissues, by either conversion to glycogen or oxidation to CO₂ and H₂O, thus providing the energy needed to restore glycogen stores.

According to this theory, both the fast and slow components of the curve reflected the anaerobic activity that had occurred during exercise. The belief was that by examining the postexercise oxygen consumption, one could estimate the amount of anaerobic activity that had occurred.

However, more recently researchers have concluded that the classical explanation of EPOC is too simplistic. For example, during the initial phase of exercise, some oxygen is borrowed from the oxygen stores(hemoglobin and myoglobin). That oxygen must be replenished during recovery. Also, respiration remains temporarily elevated following exercise partly in an effort to clear CO₂ that has accumulated in the tissues as a by-product of metabolism. Body temperature also is elevated, which keeps the metabolic and respiratory rates high, thus requiring more oxygen; and elevated levels of norepinephrine and epinephrine during exercise have similar effects.

Thus, the EPOC depends on many factors other than merely the rebuilding of ATP and PCr and the clearing of lactate produced by anaerobic metabolism. The physiological mechanisms responsible for the EPOC are not yet clearly defined.

Lactate threshold

Many investigators consider the lactate threshold to be a good indicator of an athlete’s potential for endurance exercise. The lactate threshold is defined as the point at which blood lactate begins to accumulate substantially above resting concentrations during exercise of increasing intensity. For example, a runner might be required to run on the treadmill at different speeds with a rest between each speed. After each run, a blood sample is taken from his or her fingertip, or from a catheter in one of the arm veins, from which blood lactate is measured. As illustrated in the picture below, the results of such testing can be used to plot the relationship between blood lactate and running velocity. At low running velocities, blood lactate concentrations remain at or near resting levels. But as running speed increases, the blood lactate concentration increases rapidly beyond some threshold velocity. The point at which blood lactate appears to increase disproportionately above resting values is termed the lactate threshold.

The lactate threshold has been thought to reflect the interaction of the aerobic and anaerobic energy systems. Some researchers have suggested that the lactate threshold represents a significant shift toward anaerobic glycolysis, which forms lactate. Consequently, the sudden increase in blood lactate with increasing effort has also been referred to as the “anaerobic threshold”. However, blood lactate concentration is determined not only by the production of lactate in skeletal muscle or other tissues but also by the clearance of lactate from the blood by the liver, skeletal muscle, cardiac muscle, and other tissues in the body. Thus, lactate threshold is best defined as that point in time during exercise of increasing intensity when the rate of lactate production exceeds the rate of lactate clearance or removal.

The lactate threshold is usually expressed as the percentage of maximal oxygen uptake( % VO₂max) at which it occurs. The ability to exercise at a high intensity without accumulating lactate is beneficial to the athlete because lactate accumulation contributes to fatique. Major determinants of successful endurance performance are VO₂max and the percentage of VO₂max that an athlete can maintain for a prolonged period. The latter is probably related to the lactate threshold, because the the lactate threshold is likely the major determinant of the pace that can be tolerated during a long-term endurance event. So the ability to perform at a higher percentage of VO₂max probably reflects a higher lactate threshold. Consequently, a lactate threshold of 80% VO₂max suggests a greater aerobic exercise tolerance than a threshold at 60% VO₂max. Generally, in two individuals with the same maximal oxygen uptake, the person with the highest lactate threshold exhibits the best endurance performance, although other factors contribute as well.