Type I vs Type II Muscle Fibers
Your body is composed of two types of muscle fibers that have very specific functions for fundamental movements. These fundamental movements require stabilizers and prime movers.
Starting with type 1 muscle fibers, also known as slow twitch muscles fibers, whose main functions reside in stabilizing the body. They’re composed of smaller motor neurons and are thinner, which makes them produce less force in a given time frame. However, they are highly dense with capillaries and mitochondria which provide lots of oxygen and ATP for better endurance. When you take a look from a training perspective, type 1 muscle fibers are mainly utilized for low intensity, endurance based exercises, and rely on receiving energy mostly through the aerobic system pathway, which cycles after 2 minutes of activity and beyond. A 50 meter sprinter cannot rely on type I muscle fibers to win; he or she will have to focus mainly on explosive power output done by type II fibers. However a 5K runner will mostly rely on slow twitch fibers and needs a lot of endurance training to effectively get their type I fibers fit for performance.
Type II muscle fibers, also called fast twitch fibers, mainly reside in the prime movers and large muscle groups. The fast twitch fibers are the source of maximal power and strength output. The fast twitch fibers have bigger motor neurons and are thicker. Once you understand the Energy system pathways (ATP-CP, Anaerobic Glycolysis, and Aerobic), you can easily break down how the muscle fibers are recruited for use. Fast twitch fibers have low endurance capacity due to the fact that they rely on receiving energy through the ATP-CP system that runs cycles every 5-8 seconds. That system is fast at generating force but also has a low ATP production rate. Thus when doing an explosive exercise like plyometrics or when doing a strength based exercise, the fast twitch fibers are the ones generating all the force needed. That is also why the repetition range stays low when you’re training for maximal strength.