Types of Muscle Fibers

There are two main types of muscle fibers: type I ("slow twitch") and type II ("fast twitch"). In this post I will discuss the differences between these and why it is important to be aware of the differences when doing resistance training.

Type I or slow twitch muscle fibers are smaller in size, but contain more capillaries thus allowing for increased oxygen to be received by the muscle fiber. These muscle fibers do not produce a large amount of force when contracted but they are slow to fatigue. Slow twitch muscle fibers are utilized for long-term contractions and stabilization - for example, when we need to stand or sit upright for a long period of time, the muscle fibers that are used to keep our limbs in proper postural alignment are primarily type I slow twitch muscle fibers.

Type II or fast twitch muscle fibers are larger in size, but contain fewer capillaries. This means there is less oxygen that is delivered to these muscle fibers. This type of muscle fibers can produce a large amount of force when contracted, but fatigue quickly. Fast twitch muscle fibers are used primarily for short periods, when we require a large amount of force or power - for example, throwing a baseball or lifting a heavy object.

All of our muscles contain both types of muscle fibers - but in differing proportions, based on the primary use for each muscle. Some muscles that are primarily used for stabilization but not used often for generating a lot of force, are not surprisingly primarily composed of slow twitch muscle fibers. For the average person, most major muscle groups are fairly evenly split between the amount of slow twitch and fast twitch muscle fibers. Thus when doing resistance training, it is beneficial to do different types of exercise - exercises with a lighter weight, slow tempo and higher number of repetitions to train the slow twitch muscle fibers, and exercises with a heavier weight, fast tempo and lower number of repetitions to train the fast twitch muscle fibers.