Ever wondered why you’re better at long-endurance exercise versus sprints? Or maybe you’re explosive out of the gate but burn out fast. Whether you tend toward a turtle or a hare depends on what muscle fiber type is predominant in your body. If you were to take a microscopic look at a slice of your bicep muscle, you’d see a mix of two general types of muscle fibers: slow-twitch (type I) and fast-twitch (type II). Here, we break down both types and share how to train each type with targeted workouts.
Slow-twitch vs. Fast-twitch
Slow-twitch muscle fibers contain little powerhouses of energy called mitochondria that use oxygen to fuel muscles. Because oxygen is the fuel, it’s considered aerobic, and this type of oxidative fueling can power a moderate level of force for a sustained time. In general, slow-twitch fibers have a slow contraction speed, low force, and low glycolytic capacity (using stored glycogen for fuel). They’re high in endurance, capillary density (lots of blood flow to the fibers), and oxidation capacity. These muscle fibers are the first ones activated during a contraction, but if the power needed is greater than what can be generated with slow-twitch fibers, the fast-twitch fibers are recruited.
Fast-twitch muscle fibers are recruited when the force needed exceeds what type I fibers can provide. They’re the ones that provide short, high-intensity force, but they don’t have staying power because their fuel source depends predominantly on stored muscle glycogen. When it runs out, it takes time to build back stores. Fast-twitch fibers have a higher activation threshold but, when activated, reach maximum force more quickly than slow-twitch fibers. These attributes enable type II fibers to provide the fast, peak forces needed with weightlifting, sprinting, and other high-intensity, short-duration activities.
We can further break down fast-twitch muscle fibers into type IIa and IIb. Type IIb can produce a moderate level of force and endurance, have a high contraction speed and glycolytic capacity, and have a medium oxidative capacity and capillary density. This type is the intermediate between type I and type IIb because it uses a mix of oxidative (aerobic) and glycolytic (anaerobic) energy production. Because type IIa fibers are a mix of aerobic and anaerobic, they can provide more force more quickly than type I muscle fibers.
Type IIb fibers are the racehorses of muscle fibers. They have the fastest contraction speed and highest force—all powered by a high glycolytic capacity. The capillary, mitochondrial, and endurance levels are low in this type of fiber. They’re good for a fast, intense effort but burn out their fuel very quickly. When trained, fast-twitch fibers, especially type IIb, increase the size and definition of a muscle.
Endurance vs. Speed
Your genetics dictate whether you have a higher percentage of type I or type II. A quick assessment of what type of exercise you prefer can give you a clue. If you dread long, moderate-level exercise bouts but love HIIT workouts, you probably favor fast-twitch. In a study in the Journal of Applied Physiology that observed a world-class sprinter’s thigh muscle biopsy composition, the researcher found it was composed of 71 percent fast-twitch fibers. That gave that sprinter the ability to achieve impressive power and speed.
On the other hand, if you’re always the last person across the finish line in a sprint but can out-hike most of your friends, slow-twitch is your dominant type. There are also differences between men and women in muscle composition. A 2015 study in Physiology that looked at gender differences in muscle fiber composition reported that women have a higher percentage of type I and type IIa muscle fibers than men. Age factors in as well, with age-related muscle mass loss consisting of more type II fibers lost than type I, leading to a reduction in strength and muscle size.
How to Train Both Muscle Types
Although your body may tend toward one muscle type, you can improve either your endurance or power by doing workouts that target the fibers recruited for the activity. Below are some tips for training both types of muscle fiber.
Type I Muscle Fiber Training
- Sustained isometric contractions challenge the type I fibers to improve their aerobic capacity. Think exercises such as planks, standing balance from leg to leg, and a press-and-hold against a wall. Work toward increasing the duration of the contraction to challenge slow-twitch fibers.
- Do resistance training with more reps (aim for 15 or more) at a lighter weight. Don’t speed through your reps—use a count of three to lift and return, keeping your movements as smooth as possible.
- Circuit training using low resistance and continued movement without a rest break between each exercise is an excellent way to improve slow-twitch fiber capacity.
- Incrementally increase your distance with running, biking, or walking while maintaining a moderate intensity to focus on building endurance. It’s all about boosting your body’s ability to use oxygen to fuel the workout, and it takes time to get into the aerobic zone.
- Incorporate long-interval training with lower-peak intensity. Mix it up with different activities, but keep the intervals at a level that allows you to exercise for at least 20 minutes and build duration as you become more aerobically fit.
Type II Muscle Fiber Training
- If you’d like to improve your fast-twitch muscle capability, focus on near-peak intensity, short-duration exercise. With weight training, opt for heavier weights that you lift as quickly as you can.
- The key is to challenge your muscles with the heavy weights and low reps (no more than six) with an effort that is fast and forceful.
- Build in longer rest periods between exercises to allow time to replenish energy stores—you need at least a minute of rest between maximal lifts.
- The trick to activating fast-twitch muscles and training them to become stronger is by pushing the muscles with fast, intense workloads. Think heavy squats, deadlifts, bench presses, vertical and broad jumps, sprints, or sled drags. Mix it up to work different muscle groups with a circuit of these exercises.
- HIIT training recruits fast-twitch fibers, and the shorter and more intense the burst, the more you’ll target those fibers. Try a series of 15-second all-out sprints with a two-minute recovery between each. Or try indoor cycling at the highest workload you can for 15-20 seconds with a two-minute recovery.