Fiber type Across a number of studies, the vastus lateralis is comprised of approximately 50% type I and 50% type II muscle fibers. To the extent that this makes it more slow twitch than other muscles, it will therefore experience less muscle damage.

So How Do we Hit Type IIa Fibers in Training?

1. Weighted plyometric exercises, such as jump squats.
2. Speed squats.
3. Speed benches.
4. Olympic lifts.
5. Drop and catch moves.
6. Drop jumps.
7. Sprints.

Case in point: When Outside contacted the Journal of Strength and Conditioning to get a copy of a recently published article discussing this very question, editors said sure, we could have it, as long as we “make sure the answer is right, and the answer is NO, one cannot change inherent fiber types I to II, only within …

Fast-twitch muscle fiber is great at generating power and speed. These muscles are easier to grow because they are used less and are lighter in color than slow-twitch fibers because of low levels of myoglobin. These are reliant on oxygen to keep the reactions going within the muscle.

There are two different types of muscles and, as a runner, you are probably familiar with both. Slow-twitch muscles are smaller and less powerful than fast-twitch muscles and have less potential for growth. In a specific muscle, the amount of slow-twitch fibers versus fast-twitch fibers is based on genetics.

Slow twitch muscles are great for endurance exercises such as:

• long-distance running (marathons or 5Ks)
• cycling.
• swimming.

If you’re fast-twitch dominant, you thrive in activities with quick bursts of power, speed, and strength. In your training, you respond well to explosive exercises.

Lifting a heavy weight with slow acceleration will develop maximum force, but you can also achieve maximum force by lifting a lighter weight with fast acceleration. Heavy Squats, Deadlifts and Bench Presses are good ways to increase Type II fibers. So is running sprints, agility drills and med ball training.

More fast twitch muscle fibers means you have a higher capacity and storage of ATP. You also have the ability to recruit more muscle fibers, to add to the intensity of the workload.

To target type I muscle fibers, you need to train at lower intensities, but perform higher repetitions. For each exercise, aim to complete 3 sets of 12 or more repetitions. The training intensity should be individualized based on fitness level, but always 55% to 65% of 1RM to stimulate type I muscle fibers.

Fast-twitch fibers favor speed and power activities like sprints and throwing events that take only tens of seconds at most.