It All Comes Down to O2
As we review how the body adapts to aerobic training (remember “aerobic” means “with oxygen”), remember that the body seeks to work more efficiently with oxygen in three ways:
Increase VO2MAX and Submaximal Endurance
Remember that VO2MAX (i.e. aerobic capacity) represents the maximum amount of oxygen that can be delivered AND utilized by the body. Most researchers find VO2MAX as the best indicator of cardiorespiratory endurance. According to the General Adaptation Syndrome, if the cardiovascular system is stressed through appropriate aerobic exercise the body will adapt and become more efficient at delivering and utilizing oxygen. There are many factors that affect how much aerobic capacity can be improved (ex: genetics, gender, age, etc.). A typical sedentary person who trains 3x/week, for 30 minutes a session at 75% of initial VO2MAX for 6 months can expect a 15% to 20% increase in VO2MAX.
Utilization VS Presentation: What’s the Limiting Factor for Improvement?
Aerobic capacity tends to increase proportionally with increases in training volume. With increased training, aerobic capacity will eventually reach an upper limit and VO2MAX will no longer increase even with harder and longer training sessions. This begs the question, is it the delivery of oxygen (presentation theory) that is the limiting factor or the ability of muscles to process the oxygen (utilization theory)? Both the presentation and utilization of oxygen improve with exercise and there is evidence that supports both theories. Most studies indicate that it is the presentation of oxygen (delivery) that is the limiting factor in increased endurance performance.
Increases in Muscle Utilization of O2 (QO2)
Endurance training increases both the size and number of mitochondria in muscles. With more (and bigger) mitochondria, there are more enzymes to allow for oxidative metabolism. The maximal amount of oxygen that a muscle can use is known as the muscle’s maximal respiratory capacity or QO2.
Increases in Muscle Presentation of O2
The delivery of oxygen is improved in many ways through endurance training. The amount of blood leaving the heart is increased (cardiac output) and the efficiency of blood flow to the working muscles is improved.
- Increased Cardiac Output
- Increase left ventricle size, leading to increased stroke volume
- Maximum heart rate stays or decreases (because heart is more efficient)
2. Increased Blood Flow
- Increase capillaries in trained muscles
- Increase capillary openings
- Increase blood volume
- Better blood distribution
Increase in Lactate Threshold or Anaerobic Threshold (Whatever you choose to call it)
Recall that at low intensities, aerobic metabolism dominates. At these lower intensities, increases in ventiliation parallels increases oxygen uptake. However, as intensity increases, anaerobic metabolism starts to contribute more to the energy needs of the body. Several changes begin take place at the point when anaerobic metabolism begins to dominate. First, lactate and H+ ions begin to accumulate – Lactate Threshold (LT). Second, ventilation abruptly increases disproportionately to the increase in oxygen consumption (this is needed to buffer the accumulating acid). This is known as the ventilatory breakpoint. When the ventilatory breakpoint coincides with a stable increase in CO2. This is known as the Anaerobic Threshold (AT).
So what’s the difference? Mainly, how the information is gathered. LT is objectively found by taking blood samples during various intensities of an incremental exercise test. AT is found through a ventilatory test where ventilation, VO2 and VCO2 are directly measured. It is a noninvasive estimate of lactate threshold. In most circumstances, AT represents LT.
So Why Should We Care About Anaerobic Threshold?
Anaerobic Threshold is a strong indicator of an athlete’s performance capability. Remember, aerobic capacity can be increased with exercise, but there is a limit to how much it can be improved. Athletes who maximize their genetic capacity for increasing VO2MAX can still increase aerobic performance. Remember that AT/LT is the point when lactate begins to accumulate in the blood. At this point lactate clearance from the blood cannot keep up with lactate production. This correlates to a decrease in muscle pH (acidosis). Think of AT as the point at which an athlete begins to “redline”. An athlete with a higher AT (as a % of VO2MAX) is able to maintain performance at a higher level than an athlete with the same VO2MAX but a lower AT.
Imagine two athletes with the same VO2MAX. One with an AT of 75% of VO2MAX, the other with an AT of 88% of VO2MAX. The athlete that reaches AT at 88% can maintain a higher race pace before “redlining”.
Muscle Fiber Adaptations
Aerobic training emphasizes usage of Type I (ST – slow twitch) muscle fibers. These muscle fibers will adapt to stress through muscle hypertrophy (growth in cross-sectional area). While these fibers have the capacity to grow (as much as 25%), they will not hypertrophy as much as fast twitch (FT) muscle fibers in response to anaerobic strength training. That’s why bodybuilders look different than marathon runners.
Increase Oxidative Capacity (increased fat metabolism)
With the improvements in muscles’ aerobic energy system with aerobic training, comes greater efficiency at using fat as an energy source during exercise. There is increased fat storage in the muscle fiber, enhanced ability to mobilize free fatty acids (FFAs) and improved capacity to oxidize fat.
What is RER and Why Should We Care?
RER (Respiratory Exchange Ratio) is the ratio of CO2 expired to O2 consumed. This ratio is an estimate of which type of energy substrate the body is using (carbohydrates vs. fat). Without going into too much science (you can thank us later), the lower the number, the greater percentage of fat being utilized. This type of testing shows that with aerobic training, RER decreases at submaximal levels (i.e. fat is being burned more efficiently).
Decreased Exercise HR and HR Recovery
One of the biggest myths that still persists in the fitness industry is that the goal of cardiovascular training is to get one’s heart rate higher and higher. Actually, at the same intensity, exercise heart rate should decrease over time. This makes sense as the real goal of cardiovascular training is to make the cardiovascular system more efficient. With proper training, even maximum heart may decrease.
Heart rate recovery also decreases over time. Heart rate recovery is the time it takes your heart to return to it’s normal resting rate. Basically, your body is switching from the sympathetic nervous system back to the parasympathetic nervous system. In general, a more fit person will recover quicker than an unfit person because the body is becoming more cardiovascularly efficient.
EPOC – Excess Postexercise Oxygen Consumption
What is happening during the initial recovery period when oxygen demand (and hence heart rate) is still high? From rest to the start of an aerobic exercise session, it takes time for the oxygen transport system to meet the new energy demands. Because of this, anaerobic metabolism dominates at the beginning of a workout (ATP-CP, anaerobic glycolysis). During recovery, anaerobic energy substrates must be replenished (ATP, PC and Glucose) and by-products removed (Lactate and H+). This is done aerobically. Energy demands are not just increased during exercise, but immediately after as well. Following light exercise, EPOC may last several minutes. For intense exercise (such as playing a football game), it may last up to 12 to 24 hours (or even longer after prolonged exercise such as a triathlon).
For more information on the science, see the NESTA Personal Fitness Trainer Certification program.