Myofibrils or muscle fibrils are basic rod-like units of a muscle. Muscles are composed of tubular cells called myocytes, also known as muscle fibers, and these cells in turn contain many chains of myofibrils. They are created during embryo development in a process known as myogenesis.
Myofibrillar hypertrophy leads to increased levels of actin and myosin which are contractile proteins in myofibrils. In more simple terms, when you lift heavy weights and force new muscle to stress (progressive overload – increase of the weights during every workout is made for myofibrillar hypertrophy), your body will respond by improving the contractility of the muscles.
Using the muscle fibers (motor units) in exercises increases the hypertrophy of myofibrils, which increases the power of muscle contraction, increased strength and allows progressively increasing load, resulting in even greater hypertrophy of myofibrils.
The stronger your fast-muscle fibers are, the stronger is the neuromuscular response and thus you can lift heavier weights that you can overcome intense exercises.
