Overview
Regular exercise is a component of maintaining good health. Cardiac (heart) muscle and skeletal muscles, which are voluntary muscles used for movement and posture, work harder during exercise. Not only are muscles impacted during exercise, they also make demands on the body. The body has a complex process to manage the demands of working muscles.
Muscle Contraction of Skeletal Muscles
Muscles contract through a process called the sliding filament mechanism, as described by Dr. Elaine Marieb and Dr. Katja Hoehm in “Human Anatomy and Physiology.” To initiate contraction, a nerve impulse releases calcium throughout the muscle. The presence of calcium allows actin and myosin to bind to each other. Actin and myosin are two proteins that contribute to muscle contraction and are situated in rows opposite from one another. The binding of actin and myosin generates ATP (adenosine-5′-triphosphate), which is an energy source for cells. Myosin detaches from actin and reattaches to another binding site further down the row of the actin molecule, which causes muscle tension.
Energy for Muscles
Muscles use energy to generate movement and force during exercise. ATP is the energy source for muscles. During exercise, muscles need to continue making ATP. More strenuous exercising requires more energy for the muscles to continue working. Fatigue and exhaustion result if energy needs are not met during exercise.
Oxygen and Muscles
Muscles need oxygen when exercising longer than a few minutes, which they receive from oxygen-rich blood. During exercise, muscles take oxygen out of the blood at a higher rate than at rest. Muscles cease to work when not enough oxygen is supplied to them. Working muscles receive an increase of oxygen-rich blood by increasing blood circulating to muscles and redirecting blood flow from other body parts to the needed muscles. The heart also pumps more rapidly, sending blood more quickly through the body, including the muscles at work. Faster and deeper breathing also helps increase the amount of oxygen in the body.
Muscular Hypertrophy
Muscle hypertrophy occurs when muscle cells increase in size, as described in “Science and Practice of Strength Training.” In sarcoplasmic hypertrophy, fluid inside the muscle cell increases with no improvement in muscle strength. Sarcoplasmic hypertrophy typically occurs in the muscles of bodybuilders. During myofibrillar hypertrophy, actin and myosin increase in number, which adds to muscle strength and size. Myofibrillar hypertrophy is usually seen in the muscles of weightlifters. Strength training typically yields both types of hypertrophy. Microtrauma, or small damage to the muscles, may contribute to muscle hypertrophy. Exercise can cause microtrauma to occur. In reaction to microtrauma, the body overcompensates by replacing additional tissue to the damaged area in an attempt to reduce the risk of repeated injury.
Cardiac Muscle
The cardiac, or heart, muscle works more intensely during exercise, as it attempts to get more blood to the working skeletal muscles. In comparison to rest, the heart beats faster during exercise. As a result of the increased heartbeat, blood flows more rapidly through the heart and throughout the body.
