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28. 6. 2012.

Muscle hypertrophy

How does a muscle size increase? Two types of hypertrophy can occur: transient and chronic. Transient hypertrophy is the increased muscle size that develops during and immediately following a single exercise bout. This results mainly from fluid accumulation(edema) in the interstitial and intracellular spaces of the muscle that comes from the blood plasma. Transient hypertrophy, as its name implies, lasts only for a short time. The fluid returns to the blood within hours after exercise.
Chronic hypertrophy refers to the increase in muscle size that occurs with long-term resistance training. This reflects actual structural changes in the muscle that can result from an increase in the size of existing individual muscle fibers(fiber hypertrophy), in the number of muscle fibers(fiber hyperplasia), or both. Controversy surrounds the theories that attempt to explain the underlying cause of this phenomenon. Of importance, however, is the finding that the eccentric component of training is important in maximizing increases in muscle fiber cross-sectional area. A number of studies have shown greater hypertrophy and strength resulting solely from eccentric contraction training as compared to concentric contraction or combined eccentric and concentric contraction training. Further, higher-velocity eccentric training appears to result in greater hypertrophy and strength gains than slower-velocity training. These greater increases appear to be related to disruptions in the sarcomere Z-lines. This disruption had originally been labeled as muscle damage, but is now thought to represent fiber protein remodeling. Thus, training with only concentric actions could limit muscle hypertrophy and increases in muscle strength.

Fiber hypertrophy

Early research suggested that the number of muscle fibers in each of a person’s muscles is established by birth or shortly thereafter and that this number remains fixed throughout life. If this were true, then wholemuscle hypertrophy could result only from individual muscle fiber hypertrophy. This could be explained by:

Intense resistance training can significantly increase the cross-sectional area of muscle fibers. For example, take a man who did not train two years, and returns to normal program and trains six months. His fiber size will be significantly bigger, and more cross-bridges for force production will appear in his legs. Assume is that hypertrophy will be caused by increased numbers of myofibrils and actin and myosin filaments, but this will not be the cause every single time.
Individual muscle fiber hypertrophy from resistance training appears to result from a net increase in muscle protein synthesis. The muscle’s protein content is in continual state of flux. Protein is always being synthetized and degraded. But the rates of these processes vary with the demands placed on the body. During exercise, protein synthesis decreases, while protein degradation apparently increases. This pattern reverses during the postexercise recovery period, even to the point of a net synthesis of protein. The provision of a carbohydrate and protein supplement immediately after a training bout can create a more positive nitrogen balance, facilitating protein synthesis.
The hormone testosterone is thought to be at least partly responsible for these changes, because one of its primary functions is the promotion of muscle growth. For example, males experience a significantly greater increase in muscle growth starting at puberty, which is largely due to a 10-fold increase in testosterone production. Testosterone is a steroidal hormone with major anabolic functions. It has been well established that massive doses of anabolic steroids coupled with resistance training markedly increase muscle mass and strength.

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