Muscles are made up of a mix of two main types of muscle fibers: slow-twitch and fast-twitch. These differ in how they generate energy, affecting the kind of movements they excel at:

Slow-Twitch Muscle Fibers (Type I):

Marathoners: Slow-twitch fibers are all about endurance. They rely on oxygen (aerobic respiration) to create energy, allowing for sustained activity. This makes them ideal for long-distance running, cycling, swimming, or anything that requires low-intensity effort for extended periods.
Fuel Efficient: They extract energy from oxygen and fat stores, making them more fuel-efficient and resistant to fatigue.
Slower Contractions: The trade-off for endurance is that slow-twitch fibers contract more slowly, resulting in less forceful movements.

Fast-Twitch Muscle Fibers (Type II):

Sprinter Champs: Fast-twitch fibers are built for speed and power. They generate energy without oxygen (anaerobic respiration), allowing for rapid but short bursts of activity. This makes them perfect for sprinting, jumping, weightlifting, or any exercise requiring quick, forceful movements.
Fast But Tire Quickly: The downside is they fatigue quickly because anaerobic respiration is less efficient. They rely on stored glycogen for energy, which gets depleted rapidly.
Subtypes: There are actually two subtypes of fast-twitch fibers:
- Type IIa: These are a hybrid between pure fast-twitch and slow-twitch fibers, offering a mix of speed and endurance.
- Type IIb: These are the true "classic" fast-twitch fibers, known for maximum power and speed but minimal fatigue resistance.

Muscle Fiber Distribution:

The good news is you're not stuck with one type of fiber. Most muscles contain a mix of both slow-twitch and fast-twitch fibers. Genetics play a role in the initial distribution, but training can also influence the ratio to some extent. Endurance training can increase the number of mitochondria in slow-twitch fibers, enhancing their endurance, while strength training can promote hypertrophy (muscle growth) in both fiber types.

Training slow-twitch fibers is primarily about endurance exercises like long runs, but what about fast twitch training???

There are many myths and misconceptions out there about fast-twitch muscle training, but this trainer breaks it down pretty well.

The tough part of this workout is that you need a lot of experience lifting weights to safely do many of these exercises. Not everyone is going to be able to do Olympic-type lifts, especially young athletes realatively new to weight lifting.

Here’s my recommendation for teenagers training to gain speed, strength, and leaping ability. Particularly good for basketball, football, and soccer players.

Superset #1

Romanian split squats (slow down, explode up) for 10-15 reps each leg
Jumping lunges x25

Superset #2

Back squats one-and-half reps (down slow, halfway up, back down, explode up) x 10-15
Broad jump to box jump (onto box), walk back to starting position for 5-10 reps

Superset #3

Box jump down to floor - explode up in a 2-foot jump as high as you can for 5-10 reps
Reverse lunge, into explosive one-leg jump, driving back knee up

Superset #4

Single dumbbell lateral lunge to L with DB in right hand x 5 then lunge to R with DB in left hand

Lateral (side-to-side) tuck jumps over the dumbbell x 15-20

Superset #5 (if you can make it)

Smith machine or barbell, toes elevated, 2-foot toe-ups x 50 (yes 50) reps
One-foot hops in a square pattern; 25 reps each leg

If you watched the video above, you’ll notice that my exercises get further and further away from the ones in the video because, while those are great lifts and plyometrics, they also require some gym accessories that you might not have as well as excellent form and experience with technical lifts like the hang-clean.

Fast twitch training