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What Are the Main Adaptations to Aerobic Training?

This is an excerpt from Practical Guide to Exercise Physiology by Robert Murray & W. Larry Kenney.

Muscles use oxygen to produce much of the ATP required for contracting muscle cells and fueling other tissues throughout the body. At rest, you breathe slowly but at a rate sufficient to expose the lungs to ample oxygen and flush out the carbon dioxide resulting from energy metabolism. Oxygen molecules enter the bloodstream, bind to hemoglobin molecules in red blood cells, and are transported through arteries, arterioles, and capillaries for delivery to individual cells. Once inside cells, the oxygen molecules enter the mitochondria for use in the electron transport chain for the continuous production of ATP. During exercise, all those events accelerate: Breathing rate and depth increase, the heart beats faster, the left ventricle fills with more blood, cardiac output increases, arterioles dilate, and more capillaries fill with blood. Inside muscle cells, the increased oxygen delivery is matched by increases in the rate of glycogen breakdown, fatty-acid catabolism, glycolysis, lactate production, the Krebs cycle, and the electron transport chain. All of those events are reflected by one measurement: O_2max. The higher the O_2max, the faster the muscles can produce the ATP required for contraction.

An increase in O_2max is one of many adaptations that occur with endurance training. Because it is one of many adaptations, a high O_2max is not necessarily a good predictor of successful endurance performance. For example, there is no doubt that a person with a O_2max of 55 ml/kg/min has a competitive advantage over someone with a O_2max of 40 ml/kg/min, but the advantage may not hold true when competing against someone with a O_2max of 50 ml/kg/min. In essence, O_2max is simply a measure of the body's ability to extract oxygen from inhaled air and deliver it into the mitochondria in active muscle cells. There is an upper limit to that ability, and that upper limit is in large part determined by the heart's capacity to pump blood, the cardiac output. The upper limit for cardiac output determines the capacity not only for endurance exercise but also for the ability to live independently later in life. Not all people aspire to be endurance athletes, but all people do value the freedom associated with being able to take care of themselves. People engage in training to enhance functional capacity, be it to improve physical performance or simply to improve quality of life.

Mitochondria contain more than 1,000 different proteins. Training increases the number of mitochondria, antioxidants within mitochondria, and a variety of proteins that protect the muscle cells against stress.

The most important adaptation for athletes is improved performance. Better performance is also an interest of exercise scientists because improving the capacity for exercise is important not only for athletes but also for everyone because improved aerobic fitness is related to a reduced risk of noncommunicable diseases such as heart disease, obesity, and diabetes; improved recovery from surgery; and all the other health-related issues listed in figure 9.1.

Regular exercise and the improved fitness that results help reduce the risk of many diseases and disorders.

Why Is O_2max So Important for Endurance?

Aerobic capacity - as measured by O_2max - reflects the capacity of muscles to produce ATP from the aerobic metabolism of carbohydrate (glucose) and fat (fatty acids). To improve fitness and endurance performance, the ability to produce ATP aerobically has to increase. That ability is reflected in O_2max. The higher the O_2max, the greater the ability to produce - and continue to produce - ATP.

As mentioned, the athlete with the highest O_2max does not always finish first. Many other factors interact to determine overall athletic success; in endurance sports, O_2max is just one of those factors. However, it is one of the major factors in the ability to complete endurance events.

Athletes and clients who want to improve their endurance have to complete the right amount and type of training in order to reach their performance goals. Figure 9.2 summarizes the main physiological changes that underlie an improvement in O_2max. Training programs - including nutrition, hydration, and rest - should improve all of these responses.

Factors that determine O_2max.

What Factors Determine How Much Aerobic Capacity Improves?

An obvious answer to this question is training. Adhering to an endurance training program is perhaps the single most important factor that determines how much aerobic capacity (O_2max) improves. However, individual response to an aerobic training program can vary. Following are key factors that combine to determine the overall improvement in O_2max:

• Initial fitness. An athlete with a high O_2max before training will have a smaller improvement than an athlete who begins training with a low O_2max.
• Heredity. Genes establish the upper limit of improvement in O_2max as the result of training.
• Sex. O_2maxin women is typically 10% to 15% lower than that in similarly trained men.
• High or low responder. Genetics (heredity) also determines the extent to which people respond to training. High responders improve quickly and to a greater extent than low responders.
• Training. Even after O_2max plateaus with training, endurance performance can still improve in terms of movement economy and anaerobic threshold.

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