How Muscles Become Stronger
Each muscle fiber is made of a series of blocks called sarcomeres that are lined up end to end. Each sarcomere is attached to the one next to it at a “Z line.” Muscle fibers do not contract equally along their lengths; they contract only at each “Z line”. To strengthen a muscle, you have to put enough force on the muscle to damage the Z-lines, as evidenced by bleeding and swelling into the Z-lines. You can tell you have damaged the Z-lines by the feeling of muscle soreness that begins 8 to 24 hours after you have lifted weights or done any form of resistance exercise. That is the time it takes for the swelling to occur in the Z-lines. This is called Delayed Onset Muscle Soreness (DOMS). Exercising your muscles intensely enough to damage them makes muscles stronger so they can withstand higher loads and be more resistant to injury.
When a muscle is damaged, your immune system sends to the damaged tissue large amounts of the same cells (lymphocytes) and chemicals (cytokines) that are used to kill germs when you have an infection. This causes inflammation, characterized by soreness (pain), increased blood flow to the injured fibers (redness), and increased flow of fluid into the damaged area (swelling). The immune cells release tissue growth factors to heal the damaged muscle fibers, and you should allow the muscle soreness to decrease or disappear before exercising intensely again. If you do not wait until the soreness goes away before exercising intensely again, the fibers can be torn, the muscles weaken and you can become injured.
