What are Type I and Type II Muscle Fibers... and Why Do They Matter?
So you've started working out and training several times a week. You have goals, right? But how do you know if you're doing the right exercises for your goals? That depends largely on what your goals are, and if you're working the right muscle fibers.
Every muscle is made of up a bundle of nerves and fibers, and the fibers can be broken down into three different types: type I, type IIa, and type IIx.
Type I is commonly called “slow twitch,” and are smaller than the type IIs. Slow twitch muscles create less force, are slower to reach max tension, and are smaller than the Type IIs. They are optimal for low intensity, longer duration workouts such as long distance running. In fact, most endurance athletes have a higher percentage of slow twitch muscle fibers. People that have a higher percentage of slow twitch muscle fibers typically have smaller, leaner body types (again, think of the typical marathoner).
Type IIa fibers are larger than Type I and are commonly called “fast twitch.” They create more force, are quicker to reach max tension, and more suited to shorter, high intensity workouts, like heavy lifting or sprinting. Most power athletes have a higher percentage of type II a fibers, which give a fuller, more muscular appearance.
Type II x fibers can produce the most force, but are the least efficient of the muscle fibers. They're commonly called the “couch potato” fibers, because unathletic people have a higher percentage of them than athletic people. They're the body's way of protecting itself in the event of an emergency when running quickly, or lifting heavy is required. With exercise you can convert the inefficient Type IIx fibers into more useful Type IIa fibers.
Most non-athletes have a pretty even mix of 50/50 Type I and Type II fibers, but some research suggests you might be able to increase your percentage of one fiber type over the other based on the type of training you do. Definitely picking a goal and working with that in mind it will you the best result regardless of whether or not you're able to change the makeup of your muscle fibers.
If your goal is to be a great endurance athlete, doing 3-4 endurance workouts week would best help you attain your goals. Training with lighter weights, higher reps, and shorter breaks when lifting will engage the Type 1 fibers; and of course include several longer cardio days. If your goal is to gain strength and muscle mass, lifting heavier weights for shorter reps, with longer rests in between sets will give you the gains you're after (provided that your nutrition is on point!) Keeping your cardio short and sweet and keeping the intensity up will work those Type IIa muscle fibers. Remember, however, that variety in any training plan is key, so even if you're trying to target one muscle fiber type, make sure to include one or two of the opposite workouts a week.