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 sports performance training goals. Show
In this article, we explore the two types of muscle fibers in detail and discuss how to train each type according to athletic goals. What are muscle fibers?Skeletal muscles are made up of individual muscle fibers. And like muscles themselves, not all muscle fibers are the same. There are two types of skeletal muscle fibers, fast-twitch and slow-twitch, and they each have different functions that are important to understand when it comes to movement and exercise programming. Slow-Twitch Muscle FibersSlow-twitch muscle fibers are fatigue resistant, and focused on sustained, smaller movements and postural control. They contain more mitochondria and myoglobin, and are aerobic in nature compared to fast-twitch fibers. Slow-twitch fibers are also sometimes called type I or red fibers because of their blood supply. Fast-Twitch Muscle FibersFast-twitch muscle fibers provide bigger and more powerful forces, but for shorter durations and fatigue quickly. They are more anaerobic with less blood supply, hence they are sometimes referred to as white fibers or type II. Skeletal muscles contain both types of fibers, but the ratios can differ depending on a variety of factors including muscle function, age and training. Skeletal muscles contain both types of fibers, but the ratios can differ depending on a variety of factors, including muscle function, age and training. If you are a sports performance specialist, it's crucial to remember the differences between the two muscle types. Slow Twitch vs. Fast Twitch Muscle Fiber TypesThe two types of skeletal muscle fibers are slow-twitch (type I) and fast-twitch (type II). Slow-twitch muscle fibers support long distance endurance activities like marathon running, while fast-twitch muscle fibers support quick, powerful movements such as sprinting or weightlifting. Muscle Fiber Type Comparison Chart
Slow-Twitch, Type ISlow-twitch muscle fibers have high concentrations of mitochondria and myoglobin. Although they are smaller than the fast-twitch fibers, they are surrounded by more capillaries (1,2). This combination supports aerobic metabolism and fatigue resistance, particularly important for prolonged submaximal (aerobic) exercise activities. Type I fibers produce less force and 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). Remember:
Fast-Twitch, Type IIFast-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, which means they are quicker to fatigue (1,2). These larger-sized fibers are also produce a greater and quicker force, an important consideration for power activities (1,2). Type IIX (also known as Type IIB) 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 fibers, also known as intermediate muscle fibers, are a mix 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). Remember:
What’s Your Muscle Fiber Type?So now that we’ve covered the different types, are you wondering what type you are? What type of muscles are in your hands, biceps, legs, chest and arms? You and your muscles are not comprised of one type of muscle fiber. All of your muscles are a mix of fast-twitch and slow-twitch muscle fiber types (1). Whether you have more of type I or type II depends on your activity level and age. Activity LevelNonathletic 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. Power athletes have a higher ratio of fast-twitch fibers (e.g., sprinters 70-75% type II), whereas for endurance athletes have 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. AgeAge is also a factor for our muscle fibers. Aging causes 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 and are metabolically efficient fibers. Thus, 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 combat this decline. Training Both Type I and Type II Muscle FibersYou can modify fiber types through exercise. Type I muscle fibers can be developed through endurance training, such as low resistance, high repetition, or long duration, low intensity. (As seen in OPT ™ Phases 1 and 2.) Type II muscle fibers can be developed through strength training. Resistance training increases the size of both type I and type II muscle fibers. Greater growth (i.e., hypertrophy) occurs in type II fibers and increases actin and myosin filaments. This results in an increased ability to generate force (2). Fast-twitch fibers can also recruit slow-twitch fibers: endurance training at high-intensity intervals can be effective in improving aerobic power (2,6). Tapering during training programs (reducing volume and intensity), can also improve the strength and power of type IIA fibers without decreasing type I performance (9). One study investigated muscle fiber changes in recreational runners training for a marathon. After 13 weeks of increasing mileage and a three week tapering cycle, not only did the functions of type I and type IIa fibers improve, but type IIa continued to improve significantly during the tapering cycle (9). If you are interested in training athletes, becoming a strength and conditioning coach, or getting trained as a performance enhancement specialist, get in touch! 3 Sports PErformance Blogs to Check outReferences
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