Each time you boost resistance, you break down muscle tissue.
Weight training builds muscle mass through progressive resistance. The movement of lifting a weight can cause small tears, or microtraumas, in your muscle tissue. Your body reacts by healing those tears. To prevent further injury to the muscle, your body's repair job makes the wounded muscle even bigger and more resilient. By gradually increasing resistance in your resistance workouts, you build your muscle tissue layer by layer.
The Power of Satellite Cells
Known as myofibrils, muscle cells have many nuclei, which participate in the process of synthesizing protein. For a muscle cell to grow, it has to multiply the number of nuclei that it contains. Surrounding a muscle cell are satellite cells, which function like stem cells for muscle tissue. Under the right conditions, these satellite cells donate their nuclei to muscle cells. When you lift weights, a muscle cell's wall is damaged, releasing growth factors that cause your satellite cells to proliferate. The satellite cells then fuse with your muscle cells, donating their nuclei and giving your muscle cells the opportunity to grow.
Successively Repairing Tears
Powerlifters and other athletes gradually add to the amount of weight they lift until they reach a resistance level that's impossible to handle. Once they reach that limit, they retreat a little and then push again to lift a heavier weight. As their muscles enlarge and strengthen, they can manage increasingly heavier weights. This process of progressive overload breaks down your muscle in incremental stages. You can achieve similar results by gradually adding volume -- reps and sets -- to an exercise. In a fatigued state, the muscle is more susceptible to microtraumas. Regardless of how much strength you build, you'll hit an ultimate limit on maximum lifting weight, which is determined by genetic inheritance, gender and age.
Hormones as Helpers
Two major hormones -- Insulin Growth Factor (IGF)-1 and testosterone -- largely contribute to the healing and growth of muscle cells by regulating the activity of satellite cells. The Insulin Growth Factor (IGF)-1 helps muscle development by facilitating protein synthesis and your body's consumption of glucose. This hormone also stimulates satellite cells to promote growth of muscle tissue and steers your body's uptake in amino acids -- the fundamental components of protein -- toward skeletal muscles, or the muscles you use when you lift weights. Testosterone enhances protein synthesis and kicks satellite cells and other anabolic hormones responsible for protein synthesis into action.
Leaving Muscles Alone
After lifting weights, you need to leave your muscles alone for 24 to 48 hours and give your body a chance to repair the microtraumas in muscle tissue. If you don't rest, your body can reverse the healing process and put your muscles into a destructive or catabolic state. Professional weightlifters work any single muscle group only once every four days. In addition, you should give your nervous system and joints -- shoulders, elbows, wrists, hips, knees, ankles -- a reprieve from the stress of resistance exercises. When you're not training at the gym, do moderate-to-light cardio and stretching exercises to improve flexibility.