Understand Slow and Fast Twitch Muscle Fibers for Training

Human muscles are made up of two basic fibers: fast twitch muscle fibers and slow twitch muscle fibers. While there is some variation, most muscles in the body have about 50% of each type of muscle fiber. Even some of the smaller muscles are made up of at least 100,000 fibers. Knowing the difference between a slow twitch muscle fiber and a fast twitch muscle fiber and how they affect performance can definitely help you during weight training.

Let’s begin with the slow twitch muscle fiber, also known as Type I. As their name suggests, slow twitch muscle fibers contract at a slower rate than their fast twitch counterparts. Unlike fast twitch muscle fibers, however, the Type I fibers can continue twitching at a steady rate for extended periods of time. What this means is that slow twitch muscle fibers are great for aerobic activities such as long distance running, swimming, and biking.

Fast twitch muscle fibers contract at a higher rate than Type I fibers meaning that they ultimately produce more power. The drawback is that they cannot maintain this rapid pace for long periods of time so fast twitch muscle fibers are best for anaerobic activities like sprinting or body building.

So how does any of this help you when weight training? Well, because slow twitch muscle fibers only kick in during endurance activities, performing a small number of reps will work your fast twitch muscle fibers more. This will help you build strength without changing your size. With higher repetitions, more of your slow twitch fibers will be worked and thus your endurance will increase more than strength. A good rule of thumb is that reps in the 1-5 range will build strength while those in the 6-12 range will build endurance.