Effect of Exercise on Muscles

Sustained, repeated overload of a group of muscles leads to muscle hypertrophy and strengthening of that muscle group.

LEARNING OBJECTIVE

By the end of this section, you will be able to:

  • Differentiate between the short-term and long-term effects of exercise on muscles

KEY POINTS

  • Aerobic exercise, or physical activity under conditions of high oxygen availability, involves extended periods of exercise at levels below maximal contraction strength, and uses a high percentage of Type I muscle fibers.
  • Anaerobic respiration, or physical activity with a low availability of oxygen, involves high intensity muscle contractions for short periods of time, utilizing primarily Type II muscle fibers.
  • Anaerobic exercise involves short, high-intensity bouts of exertion that utilize little or no oxygen and lead to increased levels of lactic acid.
  • Muscle metabolism differs due to exercise type, primarily the availability (aerobic) or lack of availability (anaerobic) of oxygen. The rate at which energy is needed determines the relative contributions of these energy systems.
  • Muscle metabolism differs due to exercise type, primarily the availability (aerobic) of lack of availability (anaerobic) of oxygen.

GLOSSARY

muscle hypertrophy: Increase in muscle mass due to exercise, particularly weight training; a noticeable long-term effect of exercise.

lactic acid: Also known as milk acid, is a chemical compound that plays a role in various biochemical processes. During power exercises such as sprinting, when the rate of demand for energy is high, glucose is broken down and oxidized to pyruvate, and lactate is produced from the pyruvate faster than the tissues can remove it, so lactate concentration begins to rise.

Type II muscle fibers: Fast-twitch muscles fibers for short, high intensity contractions. Muscles prioritize the use of readily-available ATP, glucose and glycogen for these muscle contractions, which results in a build-up of lactic acid.

The Effects of Exercise

Exercise involves a series of sustained muscle contractions, of either long or short duration, depending on the nature of the physical activity. Effects of exercise on muscles can be considered short-term or immediate, both during and shortly after exercise; as well as long-term, lasting effects.

IMMEDIATE, SHORT-TERM EFFECTS

The effects of exercise on muscles varies with the type and duration of the activity.Aerobic exercise is typical of activities requiring endurance and sustained muscle contractions. Such activities rely mainly on Type I (slow-twitch muscles) which sustain maximal contraction for extensive periods of time. This use of slow-twitch muscles, and the availability of oxygen, prevents the buildup of lactic acid, and typically does not result in substantial muscle fatigue in the short-term. Sustained aerobic respiration tends to shift the metabolic pathways of muscle to favor the use of fat as the primary source of ATP, and glycogen is generally avoided.

Anaerobic respiration, typical of sprinting and weight lifting , prioritizes the use of Type II (fast-twitch) muscles fibers for short, high-intensity contractions. Muscles prioritize the use of readily-available ATP, glucose and glycogen for these contractions, which results in a build-up of lactic acid. Though traditionally viewed as the cause of muscle fatigue, recent research indicates ion shortages, particularly of calcium, during an aerobic exercise, causes such muscle fatigue. However, lactic acid inhibits further ATP production, indirectly causing fatigue.

Close-up photograph of weightlifter's bicep and shoulder

Muscle Hypertrophy: Shoulder with deltoid and bicep hypertrophy

Muscle soreness, once thought to be due to lactic acid accumulation, has more recently been attributed to small tearing of the muscles fibers caused by eccentric contraction.

LONG-TERM EFFECTS

Muscle hypertrophy, or the increase in muscle mass due to exercise , particularly weight training, is a noticeable long-term effect of exercise. Exercise of specific muscles can often result in hypertrophy in the opposite muscles as well, a phenomenon known as cross education.

Experts and professionals differ widely on the best approaches to specifically achieve muscle growth, as opposed to focusing on gaining strength, power, or endurance. It was generally considered that consistent anaerobic strength training will produce hypertrophy over the long term, in addition to its effects on muscular strength and endurance.