Damaging the muscle fibers is critically important to have in order for a muscle to grow. This damaging of the muscle fibers is most effective when you take the muscle to failure during an exercise. Muscle failure means that the muscle is unable to apply enough force in order to overcome the workload needed to perform a movement.
There are two reason for the muscle to experience soreness after training to failure. The first reason for the muscle soreness is the build up lactic acid within the muscle. Lactic acid causes irritation and discomfort within the muscles after exercise. However the lactic acid is not the major reason for the muscle soreness. The main reason for the muscle soreness is the muscle swelling. Once the muscle fibers have been damaged the body begins the healing process. This healing process causes a surge in nutrients, leukocytes (white blood cells) and other body fluids that rush to the damaged areas to begin the repair process.
Once the muscle fiber is damaged the body begins the process of repairing the injured fibers through a process in which it bonds muscle fibers together to make new fibers. The result of this bonding of fibers is the fiber will increase in size thus resulting in muscle growth. This muscle growth will only happen when the rate of protein synthesis is greater that the rate of muscle breakdown. Muscle growth is highly reliant upon rest and nutrition intake.