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Slow and fast twitch muscles are the two groups of muscles that are responsible for all your gains in the gym, whether you’re running on the treadmill or bodybuilding. Considering how they impact your performance will allow you to tweak your workout routine for maximum returns.

How Do We Get Stronger?

Your muscles are made up of bundles of individual muscle fibers called myocytes. Myocytes are mainly made of strands of protein called actin and myosin. The actin and the myosin work together to create a muscle contraction. When you exercise, muscle fibers undergo trauma — they’re damaged by the force you put on them. But that’s a good thing, because it activates the cells around the muscles to bind together and to the muscle fibers themselves, increasing their mass. That’s how muscles grow — it’s a constant cycle of damage during exercise and repair during recovery.

However, muscle growth isn’t the only way strength increases. You also get stronger by activating neuromuscular adaptation — the ways your entire muscular system, controlled by signals from the brain, adapts to increasing stressors without necessarily damaging muscles enough to stimulate growth. This is important to understand if you’re looking to get stronger without getting bigger — often a concern of athletes in sports like gymnastics, dance, and rock climbing. And the different types of muscle fibers play a big part in this process.

The Different Kinds of Muscle Fibers 

Muscles are divided between slow and fast twitch muscle fibers. Slow twitch muscle fibers are called type I fibers, while fast twitch muscle fibers consist of type IIa and type IIb.

Each fiber type contracts in its own way. You have a unique mix of fast and slow fiber types, which is largely genetically determined.

Fast twitch muscle groups can be further broken down into type IIa and type IIb fibers. Type IIa fibers tend to showcase properties that bridge fast twitch and slow twitch muscles. They are activated before type IIb fibers, as the muscles are gradually called on to exert more power.

Type IIb fast switch muscles are activated for the highest-intensity movements. This muscle fiber group has more rapid contractions than all other muscle groups. However, the trade-off is that they fatigue at a faster rate and can’t last as long before needing rest.

Which Should I Focus on to Improve Performance?

It depends on your goals. If you just want to get stronger, it helps to focus more on the fast-twitch muscle group by doing strength exercises with a lower number of reps at heavier weights, closer to your one-rep maximum (the heaviest weight you can handle for one rep of a given movement). The higher intensity needed to lift those heavier weights activates fast-twitch muscle fibers immediately, and over time you’ll train your body to activate more of those muscle groups at the same time, increasing overall power.

If you keep the reps low (one to five), you won’t do enough damage to your muscles to add significant muscle mass — though there will be some growth.

If you also want to build muscle (which will always add strength as well), add some lower-weight exercises to the mix, but do more reps. This way, you’re also activating your slow-twitch muscle groups, and both types are incurring the damage required to build more muscle during recovery.

And of course, if your goal is to increase endurance, keep doing what you’re doing: running, cycling, or swimming farther than you did last time to build those slow-twitch muscles. But keep in mind that adding in exercises to activate your fast-twitch muscle fibers will make you stronger overall, and make you look more toned, since fast-twitch muscles are larger.

Focusing on only one type of muscle group means you’re not using your body to its fullest potential — and it can also increase your chances of injury. Plus, doing a wide variety of exercises makes workouts more fun! Just make sure to allow enough recovery time between workouts, and get plenty of water and proper nutrition. Your muscles — both slow and fast — will thank you!

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