There are over 620 muscles in the body, all contracting to perform a variety of actions at various joints. Normal muscles consist of many thousands of muscle fibers that run in different directions along and across the lengths of a muscle. The muscles pull on the bone causing a series of actions at freely moveable joints. A single muscle consists of approximately 8,000 myofibrils. Myofibrils are pencil like structures that group together to form a bundle, making up a single muscle fiber. Myofibrils are further broken up into sacromeres that lie end to end. Each myofibril consists of approximately 4,500 sacromeres. Sacromeres are the functional unit of the muscle, and it is here that the muscle contraction takes place. The structure of a muscle fiber is similar to the construction of a brick wall.
When it comes to understanding our muscles, know that a muscle becomes large due to resistance training, it is said to occur either by hypertrophy or hyperplasia. Hypertrophy is an increase in the size and number of myofibrils, while hyperplasia is an increase in the size and number of muscle fibers, often referred to as fiber splitting. There is some debate as to what is the true explanation for increasing muscle size; however, hypertrophy appears to be the most widely accepted theory. In the case of hypertrophy, a muscle fiber, like a brick wall lays down more layers or myofibrils by building more bricks or sarcomeres.