Skeletal muscle fibers are broadly categorized into type I “slow twitch” and type II “fast twitch.” In truth, muscle fibers aren’t that binary, and there are diverse variations in subtypes. However, for the sake of explaining the principle differences, it helps to talk only about these distinctly different muscle fiber types.
The different muscle fiber types relate to muscle metabolism and actions, which you can find explained in the following article, muscle metabolism. Type I fibers are better suited to aerobic respiration and activity (walking, jogging, etc.). Type II fibers are better suited to anaerobic respiration and activity (sprinting, plyometrics, power training, strength training, etc.).
Alpha motor neurons activate muscle fibers. Each neuron and the muscle fibers it connects to is called a motor unit. A motor unit is composed of only one type of muscle fibers (e.g., type I or II). The number of muscle fibers it connects to varies. When a muscle creates force, and as that force increases, typically the type I muscle fibers are recruited first, followed by the type II muscle fibers.
The principle of motor unit (or muscle fiber) recruitment is one of the most important principles of physical exercise and health in general.
When designing an exercise program, it is essential to know the following. Only the motor units that are activated to create force will be recruited and positively adapt as a result of exercise. For anaerobic exercise (strength or power training), the higher the intensity, or force, the greater the number of muscle fibers recruited in the muscles used. Muscle fibers that aren’t recruited over time go into a hibernation state and wither away as you get older. This process begins in your thirties. So you need to lift heavy to obtain the most significant health benefits and limit the effects of aging.
Type II muscle fibers have more subtypes (IIa, IIax, IIx, etc.) which offers more opportunity for the transformation of these fibers with anaerobic exercise. The science gets nerdier than that but based on a need to know basis, I don’t think that we need to go there.