Fast-twitch muscle fibers are the largest and most powerful muscle movers in your body.

The human body is equipped with a variety of muscle-fiber types. These range on a spectrum from the smaller, endurance-based, slow-twitch fibers to the larger fast-twitch fibers designed for strength and power activities. But you're right that these larger fibers have physique implications as well. Aside from their sheer size, fast-twitch fibers also store a great deal of carbohydrates. For every gram of carbohydrate you store, you also draw about 3 grams of water into the muscle. Thus, bodybuilders who optimize fast-twitch fiber development will obtain a fuller and denser look onstage.

The balance of fast-twitch and slow-twitch fibers in your body is determined by genetics, but there's still plenty you can do in your training to maximize growth and strength in the muscles you have. Specifically, consider two variables when trying to activate fast-twitch muscle fibers: the amount of weight you lift and how you manage fatigue during sets.

GO HEAVY!
Your body recruits muscle fibers based on the force demands placed upon it. If the force demands from an exercise are less, you'll use more slow-twitch fibers. The greater the intensity—meaning percentage of your one-rep max, not just how strenuous an exercise feels—the more you'll tap into fast-twitch fibers.

In 2004, researchers found that powerlifters and Olympic weightlifters had much greater fast-twitch muscle-fiber development than bodybuilders.1 It's at least partially a question of programming. The traditional bodybuilding program focuses on the 8-12 repetition range with moderately heavy loads and 60-90 seconds of rest between sets. Compare this with powerlifters, who train largely in a 1-5 repetition range with very heavy weights and 3-5 minutes of rest. Studies show that individuals who neglect training in a heavier range will not program their nervous system to effectively recruit their largest fast-twitch fibers.

But it's also true that in the original study, bodybuilders still had larger overall muscle size. This is because, unlike powerlifters, they had also drastically increased the size of their slow-twitch muscle fibers. Based on this research, it is clear that bodybuilders looking to maximize gains should incorporate heavy low-rep training sessions in addition to the traditional 8-12 repetition range for improved fast-twitch muscle fiber development, and thus greater growth.

FATIGUE IS YOUR FRIEND
Along with intensity, fatigue is the second surefire way to increase fast-twitch muscle fiber recruitment. Your body's first impulse is to recruit slow-twitch fibers, but once you fatigue those fibers, it has to recruit fast-twitch fibers to do what is being asked of it.2

Making the most of this phenomenon is more a matter of how you train rather than what your program looks like. For example, most lifters take brief rest periods between repetitions when a set becomes painful. When you do this intentionally, it is known as cluster set training, but what I'm talking about is largely subconscious, such as when a subject in my lab pauses between reps during a difficult set of squats.


After seeing it countless times, my colleagues Jordan Joy, Ryan Lowery, and I decided to study what happens on squats when you mimic this natural intra-set rest technique.3 We had athletes perform 4 sets of 8 maximal repetitions either with or without brief rests in the middle of the set. We found that intra-set rest decreased fatigue, but it also prevented the body from recruiting the fast-twitch muscle fibers.

The take home message here is to analyze how you perform a given set. If you find yourself pausing when a movement gets difficult, you may be short-changing your gains!

Make this principle work for you throughout a workout by keeping your workout density high. Here's what I mean: An athlete who takes 2 hours to perform 15 sets has low density, while an athlete who performs 15 sets in 30 minutes has extremely high density. By shortening your rest periods on traditional bodybuilding days, your slow-twitch muscle fibers will fatigue much sooner and fast-twitch muscle fiber recruitment will skyrocket.

MAKE FAST-TWITCH FIBERS A PRIORITY
The existing evidence strongly supports the conclusion that heavy lifting and muscle fatigue largely dictate the recruitment of fast-twitch muscle fibers. To put these principles into action, perform a heavy training day every 2-3 workouts. A good training split for a given body part could look like this:

Workout 1: Heavy, 1-5 repetitions, 3-5 minutes rest, compound movements
Workout 2: 8-12 repetitions, 60-90 seconds rest, mainly compound movements
Workout 3: 12+ repetitions, 30-60 seconds rest, supersets, compound and isolation movements

On heavy days, prioritize movements that recruit the most muscle, such as squats, deadlifts, bench presses, shoulder presses, dips, and pull-ups. On your 8-12 repetition days, keep workout density high and hammer out repetitions one after the other. Regardless of the pain you may experience, don't pause in the middle of a set.

If it helps, remember one of my favorite quotes from Lance Armstrong: "Pain is temporary. It may last a minute, or an hour, or a day, or a year, but eventually it will subside and something else will take its place. If I quit, however, it lasts forever." Keep fighting and pushing, and your body will adapt accordingly.

Now get out there and lift some heavy weight!