The Science Muscle of Growth
What makes muscles grow? The obvious answer would be intense exercise and good nutrition, with enough rest and recuperation to maximize size and strength gains. The reason lifting weights produces greater gains in muscle size and strength is that it places more stress on the muscles than other exercise, such as stretching or aerobics. The muscles respond to the stress through adaptation, involving upgraded muscle protein synthesis.
That’s the general picture of what causes muscle growth. What happens in the muscle after exercise is a much more complex picture. On a molecular level, muscle growth is a precise symphony involving the immune system, inflammation, hormone release and structural changes. While the knowledge of what’s happening in a muscle during and after training may seem superfluous to anyone except a research scientist, a rudimentary understanding of the internal workings of exercised muscles can tell you what constitutes correct training and rest cycles for gains in size and strength.
What Is Muscular Hypertrophy?
The term hypertrophy means “excessive growth,” and in reference to muscles, that means enlarged muscles usually acquired through exercise. An ongoing debate in physiology is whether muscles get bigger through the addition of new fibers—a process called hyperplasia through which existing muscle fibers split to form new fibers—or whether muscles grow by thickening existing fibers. The fiber-thickening scenario is the generally accepted view.
Some studies comparing world-class bodybuilders to untrained college students showed that both groups’ muscle fibers had similar dimensions when viewed under a microscope, though the bodybuilders clearly had much larger muscles. Later studies showed that the bodybuilders had far more muscle fibers than untrained college students. The speculation is that years of intense, heavy training promote hyperplasia of muscle fibers.
Muscle size is related to the cross-sectional area of muscle fibers, or their thickness. As the muscle fiber thickens from a compensation effect induced by heavy exercise, the muscle gets bigger and stronger. Big muscles aren’t always stronger muscles, however. What determines muscle strength is a combination of factors, including favorable leverage and connective tissue. Most important is the increase in muscle contractile proteins, specifically actin and myosin. Some pathological conditions feature large but, paradoxically, weak muscles. An example is acromegaly, usually the result of a small tumor in the anterior pituitary gland that causes the release of huge amounts of growth hormone. People suffering from the disease from an early age wind up very tall, with larger but weaker muscles.
Indeed, the majority of studies examining the athletic use of growth hormone injections conclude that the drug promotes larger muscle size but without an accompanying increase in strength. GH promotes connective tissue increase in muscle but doesn’t affect the muscle contractile proteins that are the cornerstone of muscular strength.
Satellite Cells: The Inner Space of Muscles
Satellite cells are so named because of their location on the outer surface of the muscle fibers, between the muscle cell membrane, or sarcolemma, and uppermost layer of the basement membrane, or basal lamina. Satellite cells are muscle precursors, or a type of stem cell, that usually lie dormant outside existing muscle fibers. They become activated when any form of trauma, such as damage or injury, occurs to a muscle fiber.1 Resistance exercise, as exemplified by weight training, causes damage to muscle fibers, which deal with it by marshaling adaptation mechanisms, the most significant being activation of satellite cells.
The damage causes satellite cells to multiply, and various other factors, as we’ll see, cause them to migrate toward the injured area. The satellite cells then fuse to the injured area, while adding a nucleus to the existing fiber, which aids the regeneration process. That doesn’t add new muscle fibers but instead leads to an increase in the amounts of contractile proteins—the actin and myosin—within the fiber. The net effect is muscular growth and strength. The process peaks at 48 hours but continues for four days after the initial trauma (exercise) occurs. That’s why you need time to let a muscle recover after a training session.