Everyone is born with a different combination of three types of muscle fibers and that, in part, determines what sort of athletic pursuits you can excel in. Although you can make a little shift in how much of each your muscle has, but for the most part, you're stuck with what you're born with. People who excel in explosive sports such as sprinting and weightlifting typically have more of a certain type of muscle called "fast twitch."
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If you cut a rope in half, you would see that it's made up of a lot of little strings twisted together. While your muscles aren't twisted, they're also made up of a bunch of smaller muscle fibers bundled together.
These bundles of fibers get grouped together in little bunches called motor units. There are a lot of little motor units that make up each muscle in your body. Each motor unit has a little nerve that attaches to it. This nerve makes the motor unit contract by using an electric signal that comes from your brain and spine.
Read More: Combining Slow Twitch & Fast Twitch Workouts
Fast and Slow Twitch Fibers
Each motor unit is made up of two general types of muscle fibers. The smaller units, which are made of slow twitch muscle fibers, are called so because they don't contract very quickly. They're designed to save energy and are used in endurance activities such as walking or jogging.
Fast twitch muscle fibers are in bigger groups and are more powerful. They contract much faster but burn out of energy more quickly than slow twitch. You use fast twitch fibers when you lift heavy weights or sprint.
Different Fast Twitch Fibers
There are two different types of fast twitch fibers, called IIa and IIx. The IIa fibers are a middle ground between slow twitch fibers and fast twitch type IIx fibers. They can act like endurance fibers but are much more powerful than the average fast-twitch fiber.
When you train, your muscle fibers can change slightly. However, they don't usually change over between slow twitch and fast twitch fibers. What usually happens is a shift between the two fast twitch fiber types.
The more you train, whether you do endurance or strength training, the more you'll turn type IIx fibers into type IIa fibers. When you stop training, some of them go back to type IIx fibers, which are the most powerful but get tired very quickly.
Building Fast Twitch Fibers
You use fast twitch muscle fibers during a heavy weightlifting movement. If you bench press as much weight as you possibly can for one rep, you're going to recruit almost all of your fast twitch muscle fibers. They kick on when you really need them to do the heaviest activities.
Read More: How to Work Your Fast Twitch Muscle Fibers
Fast twitch fibers also kick in when you're really fatigued. If you were to do 20 squats with a heavy weight your slow twitch fibers would get tired and gradually more and more fast twitch fibers would join in to help out.
Best Workouts for Fast Twitch Fibers
The jury is still out on whether or not you can turn one type of fiber into another through training, but if you could, the answer would be in heavy weightlifting. For your legs, that means barbell squats and deadlifts with weights that you can only lift for 5 reps or less.
Your best bet is to train for muscle growth. While there may not be a way to convert muscle fibers, you can still grow the fast twitch fibers that you already have. Use traditional weightlifting exercises like the squat, lunge, deadlift, and step-up to begin building up your leg muscles.
Start with one leg workout a week where you do three sets of 10 reps for each exercise, then add another workout during the week once you feel comfortable.