The Muscle Fiber Test

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Have you heard of fast-twitch and slow-twitch muscle fibers? These are the two types of muscle that are responsible for all your gains in the gym. And depending on how you train — and even your own genetics — you could be either slow or fast-twitch dominant. But how can you tell? Take our test to find out which muscle-fiber type you have more of. Plus, learn the best way to train for your type for better and faster muscle gains. 

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Slow-Twitch vs. Fast-Twitch Fibers

Slow-twitch muscle fibers (also known as “Type I”) generate less power and strength than fast-twitch fibers, but they have can sustain activity for longer. The cells within these muscles excel at clearing waste and using oxygen as fuel and, within the muscles, there’s a high-density of capillaries, which helps bring blood to the muscles. During a marathon, you primarily use slow-twitch fibers. 

Fast-twitch muscle fibers (also known as “Type II”) generate far more power and strength, but they fatigue much faster and require more time for recovery. There are also two types of fast-twitch fibers: Type IIa and Type IIb. Type IIa yields more endurance but produces slightly less strength; Type IIb creates the most strength, but yields less endurance.

When playing football, you primarily use fast-twitch fibers (Type IIa and IIb). Olympic weightlifting, however, will target Type IIb muscle fibers specifically. 

During exercise, you fire your Type I fibers before your Type II (unless you’re an Olympic-level athlete — those guys can skip right to their Type II fibers). And while you can change the composition of your muscle fibers to some extent, the rest of it is determined by your genetics.

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The Muscle-Fiber Test

On a big strength exercise like the bench press or back squat, load the bar to 80% of your one-rep maximum (1RM). For example, if your one-rep max on the bench press is 200lbs, then use 160lbs. Then, do as many reps until you fail or your technique gets really bad. 

•    Less than 7 reps: fast-twitch dominant
•    7 to 9 reps: balanced between slow- and fast-twitch fibers 
•    More than 9 reps: slow-twitch dominant

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Slow-Twitch Training

If you’re slow-twitch dominant, you have a great advantage in longer-duration activities. With your resistance training, focus on increasing your time under tension. 

Use higher reps — eight or more — and focus on a slower tempo (i.e. two seconds up and two seconds down) with your strength exercises. Also, use shorter rest periods (like 30 seconds) and, every few weeks, train to failure, count how many reps you could do, and try to beat that next time. 

If you think, however, slow-twitch training will make you look like a skinny marathon runner, think again. Those guys run for hours per day—you don’t. 

Finally, occasionally target your fast-twitch muscles with low-rep training to boost your maximal strength. Also, add a few power exercises and perform some high-intensity intervals with less than 15 seconds of work. 

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Fast-Twitch Training

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. 

When creating your workout, add fast movements like box jumps, medicine ball slams, and kettlebell swings. Then, add heavier power exercises like power cleans, push presses, and jump squats; with your strength training, lift very heavy (90% or more of your 1RM) and lift as fast as you can. 

Finally, periodically target your slow-twitch muscles with high-rep sets or slow eccentric movements. Also, use long jogs, hikes, or bike rides for cardio to improve your aerobic system. 

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