By Stacey Penney, MS, NASM-CPT, CES, PES, FNS
Looking to build endurance? What about power? Do dreams of being an all-star hitter or marathon runner need to be dashed if twitch ratios aren’t ideal? Not necessarily. The types of muscle fibers targeted in different types of training programs can impact performance goals.
As most of us may recall from our physiology studies, 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 biochemical and contractile 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 (1,2).
Fast-twitch, Type II
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 (1,2). These larger-sized fibers are also able to produce a greater and quicker force, an important consideration for power activities (1,2).
- 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 aerobic and anaerobic energy systems, these fibers have a higher oxidative capacity and fatigue more slowly than type IIx (1,2).
What’s my type?
So now that we’ve covered the different types, are you wondering what type you are? Short of having a muscle biopsy, and make that multiple biopsies since not all muscles in the body will be the same, we are a mix of both fast and slow in all of our muscles (1). Nonathletic individuals have close to a 50/50 balance of fiber types. When you start looking at highly skilled, top-performing athletes, some differences may begin to appear. 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) (2). Of course, muscle fiber type is not the only factor in an athlete’s success! There are plenty of other variables that take an athlete from good to great.
Age is also a factor for our muscle fibers. 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. This loss of lean muscle mass can contribute to age-related metabolic dysfunctions, body composition changes, even an increased risk of falls (2-5). Resistance training can help combat this decline.
Fiber types can be modified to some degree by exercise. Type I fibers are targeted with endurance training, such as lower resistance with higher repetitions, or longer duration with a lower intensity, as seen in OPT ™ Phases 1 and 2 (7,8). 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 (2). An increase in type IIx to type IIa, but not increase type I can also be seen in prolonged resistance training (2). Fast-twitch fibers can be slow-twitch recruits: endurance training and high-intensity intervals can be effective in improving aerobic power (2,6).
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 (9). For example, 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 (9).