Learning Objectives
Relate the connections between exercise and muscle performance
By the end of this section, you will be able to:
- Describe muscle hypertrophy and muscle atrophy
- Explain how endurance and resistance exercises affect muscle
- Explain how performance-enhancing substances affect muscle
Physical training can alter the appearance of skeletal muscles and produce changes in muscle performance. Conversely, a lack of use can result in decreased muscle mass and performance. Although muscle cells can change in size, new cells are rarely formed when muscles grow. Instead, structural proteins are added to a muscle fiber in a process called muscle hypertrophy, resulting in an increase in fiber diameter. The reverse, when structural proteins are lost and muscle mass decreases, is called muscle atrophy.
Endurance Exercise
Slow fibers are predominantly used in endurance exercises that require limited force generation but involve numerous repetitions. The aerobic metabolism used by slow oxidative fibers allows them to maintain contractions over long periods. Endurance training modifies these slow fibers to make them even more efficient by producing more mitochondria and synthesizing more myoglobin, both which lead to an increase in ATP production by increasing the rate of aerobic metabolism.
The training can trigger the formation of more extensive capillary networks around the fiber, a process called angiogenesis, to supply oxygen to the fibers and remove metabolic waste. To allow these capillary networks to supply the deep portions of the muscle, muscle mass does not greatly increase in order to maintain a smaller area for the diffusion of nutrients and gases.
The proportion of slow oxidative muscle fibers in muscle determines the suitability of a muscle for endurance, and may benefit those participating in endurance activities (Figure 10.6.1). Postural muscles have a large number of slow oxidative fibers as they are continually contracting to keep the body erect. Endurance athletes benefit greatly from having muscles containing a larger proportion of slow oxidative fibers compared to fast oxidative fibers. Studies suggest that genetics play a critical role in determining the overall fiber proportions of slow oxidative to fast glycolytic fibers in muscles with repetitive training having its greatest influence on the fast oxidative fibers.
Resistance Exercise
Resistance exercises, as opposed to endurance exercise, target fast glycolytic fibers by focusing on short, powerful movements that are not repeated over long periods. The high rates of ATP hydrolysis and cross-bridge formation in fast glycolytic fibers is responsible for such powerful muscle contractions. Thus, muscles used for power often have a higher ratio of fast glycolytic fibers compared to slow oxidative fibers. Resistance exercise affects muscles by increasing the formation of myofibrils, thereby increasing the diameter of muscle fibers (Figure 10.6.2). Because this muscular enla