Fast-Twitch, Slow-Twitch Muscle Fibers: What's the difference and does it matter?
Fast-Twitch, Slow-Twitch Muscle Fibers: What's the difference and does it matter?
There are two main types of human skeletal muscle fiber types, type I and type II, or slow-twitch and fast-twitch, respectively. Fast-twitch are further classified into type IIa and type IIx. They differ in both their contractile and biochemical properties.
Slow-twitch, Type I
These muscle fibers have high concentrations of mitochondria and myoglobin, and although they are smaller than the fast-twitch fibers, are surrounded by more capillaries (1,2). This combination supports their capacity for aerobic metabolism and fatigue resistance, particularly important for prolonged submaximal exercise activities. Type I fibers produce less force, are slower to produce maximal tension (lower myosin ATPase activity) compared to type II fibers, but they are able to maintain longer-term contractions, key for stabilization and postural control.
Fast-twitch, Type II
Type IIx: These fibers produce the most force, but are incredibly inefficient based on their high myosin ATPase activity, low oxidative capacity, and heavy reliance on anaerobic metabolism (1,2).
Type IIa: These fibers are also known as intermediate fibers, a mix if you will, of type I and type IIx, with comparable tension. Able to use both anaerobic and aerobic energy systems, these fibers have a higher oxidative capacity and fatigue more slowly than type IIx.
Our fast-twitch, type II muscle fibers are further divided into type IIx and type IIa. Typically, these have lower concentrations of mitochondria, myoglobin, and capillaries compared to our slow-twitch fibers and are quicker to fatigue. These larger-sized fibers are also able to produce a greater and quicker force, an important consideration for power activities.
What's my type?
Now that we've covered the different types, are you wondering what type you are? For the power athlete, there's a higher ratio of fast-twitch fibers (e.g., sprinters 70-75 % type II), whereas for the endurance athlete there are more slow-twitch fibers (e.g., marathon/distance runners 70-80 % type I). Of course, muscle fiber type is not the only factor in an athlete's success.
As we age, there's a loss in lean muscle mass, with a decline in our fast-twitch fibers, especially the type IIx, but there is also an increase in our slow-twitch fibers (2-4). Recall that the fast-twitch fibers are larger in size than the slow-twitch, metabolically efficient fibers.
Resistance training increases the size of both type I and type II muscle fibers, with greater growth (i.e., hypertrophy) occurring in the type II fibers with an increase in actin and myosin filaments, which also results in an increased ability to generate force (2). An increase in type IIx to type IIa, but not increase type I can also be seen in prolonged resistance training (2).
Muscle Type Training
Strength training targets the type II fibers. Resistance training increases the size of both type I and type II muscle fibers, with greater growth (i.e., hypertrophy) occurring in the type II fibers with an increase in actin and myosin filaments, which also results in an increased ability to generate force. An increase in type IIx to type IIa, but not increase type I can also be seen in prolonged resistance training.
Tapering during training programs (e.g., reducing volume and intensity) can also improve the strength and power of type IIa fibers, without a decrease in type 1 performance. In a study investigating muscle fiber changes in recreational runners training for a marathon, after 13 weeks of increasing mileage and a three week tapering cycle, it was found that not only did the functions of type 1 and type IIa fibers improve, but that type IIa continued to improve significantly during the tapering cycle
There are two main types of human skeletal muscle fiber types, type I and type II, or slow-twitch and fast-twitch, respectively. Fast-twitch are further classified into type IIa and type IIx. They differ in both their contractile and biochemical properties.
Slow-twitch, Type I
These muscle fibers have high concentrations of mitochondria and myoglobin, and although they are smaller than the fast-twitch fibers, are surrounded by more capillaries (1,2). This combination supports their capacity for aerobic metabolism and fatigue resistance, particularly important for prolonged submaximal exercise activities. Type I fibers produce less force, are slower to produce maximal tension (lower myosin ATPase activity) compared to type II fibers, but they are able to maintain longer-term contractions, key for stabilization and postural control.
Fast-twitch, Type II
Type IIx: These fibers produce the most force, but are incredibly inefficient based on their high myosin ATPase activity, low oxidative capacity, and heavy reliance on anaerobic metabolism (1,2).
Type IIa: These fibers are also known as intermediate fibers, a mix if you will, of type I and type IIx, with comparable tension. Able to use both anaerobic and aerobic energy systems, these fibers have a higher oxidative capacity and fatigue more slowly than type IIx.
Our fast-twitch, type II muscle fibers are further divided into type IIx and type IIa. Typically, these have lower concentrations of mitochondria, myoglobin, and capillaries compared to our slow-twitch fibers and are quicker to fatigue. These larger-sized fibers are also able to produce a greater and quicker force, an important consideration for power activities.
What's my type?
Now that we've covered the different types, are you wondering what type you are? For the power athlete, there's a higher ratio of fast-twitch fibers (e.g., sprinters 70-75 % type II), whereas for the endurance athlete there are more slow-twitch fibers (e.g., marathon/distance runners 70-80 % type I). Of course, muscle fiber type is not the only factor in an athlete's success.
As we age, there's a loss in lean muscle mass, with a decline in our fast-twitch fibers, especially the type IIx, but there is also an increase in our slow-twitch fibers (2-4). Recall that the fast-twitch fibers are larger in size than the slow-twitch, metabolically efficient fibers.
Resistance training increases the size of both type I and type II muscle fibers, with greater growth (i.e., hypertrophy) occurring in the type II fibers with an increase in actin and myosin filaments, which also results in an increased ability to generate force (2). An increase in type IIx to type IIa, but not increase type I can also be seen in prolonged resistance training (2).
Muscle Type Training
Strength training targets the type II fibers. Resistance training increases the size of both type I and type II muscle fibers, with greater growth (i.e., hypertrophy) occurring in the type II fibers with an increase in actin and myosin filaments, which also results in an increased ability to generate force. An increase in type IIx to type IIa, but not increase type I can also be seen in prolonged resistance training.
Tapering during training programs (e.g., reducing volume and intensity) can also improve the strength and power of type IIa fibers, without a decrease in type 1 performance. In a study investigating muscle fiber changes in recreational runners training for a marathon, after 13 weeks of increasing mileage and a three week tapering cycle, it was found that not only did the functions of type 1 and type IIa fibers improve, but that type IIa continued to improve significantly during the tapering cycle
