Each muscle in your body contains thousands of microscopic fibers that work together during movement. In fact, leg muscles contain over 1 million muscle fibers. During exercise, utilized fibers sustain damage that must repair themselves while at rest. Instead of growing new fibers, your body heals damaged fibers -- causing increased strength and muscle size.
Muscle Breakdown During Exercise
Muscle fibers attach and pull against each other during contraction, or movement. A positive relationship exists between exercise intensity and level of subsequent damage incurred by fibers. For example, fibers slide against each other slowly during slow, careful movements and quickly during fast, forceful actions. Forceful activity, such as weight lifting, causes portions of your muscle fibers to tear under pressure. Although exercise-induced fiber damage might seem harmful, fiber damage and repair are a normal and unavoidable outcome of an active lifestyle.
Hypertrophy vs. Hyperplasia
Hypertrophy occurs when a damaged muscle fiber becomes thicker after healing. Conversely, hyperplasia defines the creation of new muscle fibers in response to exercise. Although both options could increase muscle size and strength, it is generally accepted that human muscle fibers perform hypertrophy instead of hyperplasia -- meaning you cannot grow new muscle fibers. Although muscular hypertrophy has been confirmed, investigation into the possibility of hyperplasia within human muscle continues.
Steps of Hypertrophy
Successful muscle-fiber repair, or hypertrophy, relies on numerous physiological responses. Immediately after exercise, satellite cells activate and begin a healing process that can last up to 48 hours. Satellite cells, located toward the outside of every muscle fiber, fuse together and form new contractile proteins called actin and myosin -- the same muscle-fiber proteins that can tear away during physical activity. Greater levels of actin and myosin enhance attachment between fibers and promote superior strength production within an affected muscle.
Significance for Fitness
Inability to grow new muscle fibers implies that genetic factors determine the quantity and quality of your muscle fibers. Therefore, individuals with more muscle fibers can grow larger muscles than someone with fewer muscle fibers. Although the process of hypertrophy starts immediately after exercise, visible muscle changes require anywhere from four weeks to two months to occur. Understanding muscle-fiber healing after exercise enables informed program design and a better understanding of how your muscles grow.
- Dr. Len Kravitz: Mystery of Skeletal Muscle Hypertrophy
- Exercise Physiology: Human Bioenergetics and Its Applications; George Brooks et al.