- Slow muscle fibers
- Fast muscle fibers
Slow muscle fibers (often referred to as type I) are slow-shrinking fibers that are very durable, i.e. They have a high resistance to fatigue. Type I muscle fibers play a role in maintaining posture and are predominantly activated during lower-intensity activities, such as running. Why is this so? One of the reasons lies in the fact that such fibers have more mitochondria, so they have a high oxidative ability and release ATP more slowly. In addition, slow muscle fibers have a lower ability to store glycogen. Because of the mitochondria and the aerobic way of providing energy and high blood flow, these muscle fibers appear red, and are often referred to as red muscle fibers. But the amount of force these muscles can produce is less than that of fast-twitching muscle fibers.
Fast muscle fibers are divided into IIA and IIB. Type IIA is also called oxidative-glycolytic, because they are muscle fibers that receive energy from both oxidative (aerobic) and glycolytic (anaerobic) energy mechanisms. IIA fibers are activated predominantly during fast, repetitive and slightly lower intensity movements. IIA fibers are activated after type I. This type of fiber possesses a relatively large amount of mitochondria and recovers quickly after activity.
Type IIB are fibers that contract very quickly and are activated during high-intensity activities, such as certain athletic disciplines and Olympic weightlifting. Low mitochondrial numbers, lower oxidative and higher glycolytic capacity are characteristics that describe type IIB fibers. It is important to understand that these fibers get tired very quickly and their recovery is somewhat slower, which happens mainly after physical activity. However, unlike Type I, Type IIB fibers have the ability to generate great force and are almost exclusively responsible for overcoming maximum loads. When considering the composition of certain muscles, it is important to understand that they are not exclusively slow or exclusively fast muscle fibers, but a combination of both components. The exact ratio depends on a number of factors, but in general, certain muscle groups have more of one component or another. Thus, in the leaf muscles, m. soleus possesses slower muscle fibers than gastrocnemius. Therefore, the primary functions of the m.soleus are walking and maintaining an upright body position, i.e. Activities that may last longer, but less intense. In contrast, the m.gastrocnemius is composed predominantly of fast-twitching muscle fibers, so it is most activated during running, jumping and other rapid movements of the legs.
