The two main categories of muscle fibers – type I and type II – vary in their chemical and mechanical properties (Wilson et al., 2012). Type I muscle fibers are slower to produce maximum tension and more resistant to fatigue, which is why they are often referred to as “slow-twitch” muscle fibers. Conversely, type II are quick to produce tension but fatigue quickly, which is why they are often referred to as “fast-twitch” muscle fibers (Joumaa et al., 2015).
Type I muscle fibers are smaller in size than type II, but contain a large number of capillaries, mitochondria and myoglobin, which allows for a better delivery of oxygen and leads to fatigue more slowly. Myoglobin is similar to hemoglobin – the red, oxygen-carrying pigment in red blood cells – which is why type I muscle fibers are often referred to as “red fibers” (Wilson et al., 2012). Type I fibers are important for muscles that must produce sustained contractions necessary for stabilization and postural control (e.g., the deep musculature of the spine) (Moore et al., 2018; Wilson et al., 2012).
Type II muscle fibers are larger in size and produce maximum tension faster than type I fibers. In general, type II fibers contain fewer capillaries, mitochondria, and myoglobin. Due to limited myoglobin, they have a low oxidative capacity, are easily fatigued and are often referred to as “white fibers” (Wilson et al., 2012). These fibers are important for muscles that produce movements and require high levels of strength and endurance, such as the quadriceps during sprinting (Brunner et al., 2007).
It is important to note that all muscles have different combinations of both type I and II fibers. Concentrations of each will vary depending on muscle function and genetics. For example, within the calf complex, it has been shown that the soleus muscle is composed mostly of type I muscle fibers, while the gastrocnemius has a majority of type II fibers (Balius et al., 2013; Edgerton et al., 1975).