Slow Twitch vs. Fast Twitch: Muscle Fibers Explained

Slow Twitch vs. Fast Twitch: Muscle Fibers Explained

When you jump on the treadmill, flex in front of the mirror, or do a pull-up, you might feel the tension and the burn in your muscles. And while that burn may feel similar, there are actually two very different types of muscle fibers involved in each activity. If you want to take your workouts to the next level and increase your performance as an athlete, bodybuilder, or weekend warrior, you need to understand the key differences between slow-twitch muscle fibers and fast-twitch muscle fibers (and what exercises target each type).

Biomechanics 101: Slow-Twitch vs. Fast-Twitch

Before we get into how to increase the strength of your slow-twitch muscle fibers and fast-twitch muscle fibers, we first need to explain exactly what this means from a biomechanical and physiological level. Let's trade the gym tank top for a lab suit and get science-y for a second.

Slow-Twitch Muscle Fibers Explained

Slow-twitch muscle fibers contract slowly and are thus "weaker" and incapable of creating explosive power. However, what these fibers lack in peer strength is made up by their high aerobic capacity. Your body recruits its slow-twitch muscle fibers whenever you're doing any form of cardio exercise, and these muscles are especially critical during endurance events (e.g., marathons or long-distance swimming).

You'll also activate your slow-twitch muscle fibers when you:
  • Yoga
  • Bike
  • Jog
  • Walk

Fast-Twitch Muscle Fibers Explained

Fast-twitch muscle fibers are polar opposites to slow-twitch. As their name implies, these muscles contract quickly and are able to move significant weight. 

While slow-twitch fibers are recruited during aerobic exercise, anaerobic movements like weightlifting, as well as anything requiring quick, high-intensity power, will activate your fast-twitch muscle fibers. 

Other movements that require fast-twitch muscle fibers include:

  • Box Jumps
  • Sprinting
  • Olympic Weightlifting

Slow Twitch vs. Fast Twitch: Can Each Type Change?

Genetically, you may have been born with more than one type of muscle fiber versus the other. However, your body is very good at adapting. Limited biomechanical research suggests that you may convert fast-twitch muscles to slow-twitch muscles (or vice versa) if you train properly consistently over time.

For example, if you are training as a competitive sprinter, you're putting an intense load on your body's need for fast-twitch muscle fibers. If your body needs more fast-twitch muscle fibers to meet this demand, it may slowly convert some of your slow-twitch muscle fibers to fast-twitch fibers. 

3 Exercises to Target Your Fast-Twitch Muscle Fibers

Essentially, any movement designed to train your muscle explosiveness will also help activate, strengthen, and increase your fast-twitch muscle fibers. The following three exercises are a powerful (pun intended) start.

  • 1. Dumbbell Clean & Split Jerk
  • 2. Box Jumps
  • 3. Weighted Step Ups

        3 Ways to Target Your Slow-Twitch Muscle Fibers

        Recruiting and strengthening your slow-twitch muscle fibers is less about specific exercises and more about how you breathe and move through those movements. 

        1. Lift Weights

        Slow, steady weightlifting with a rep range of 10-12 per set is one of the best ways to build your slow-twitch muscle fibers. Just remember that the more you increase your speed and velocity, the more you activate your fast-twitch muscle instead of your slow-twitch fibers.

        2. Reduce Your Rest Time

        If you're one of the many people who like to scroll through their phone for extended periods of time between sets, you may want to rethink that. The longer your rest period, the more time your fast-twitch muscle fibers have to recover, and the more load they'll carry in your workout. 

        Instead, cutting your rest time to 30 seconds or less keeps your fast-twitch muscle fibers fatigued, thus forcing your body to rely on your slow-twitch muscles.

        3. Increase Your Time Under Tension

        As you lower the weight, try and reduce the speed at which you do so. For example, during a bicep curl, lift it and lower it down 30% slower than you normally do. The slower you lower the weight, the longer your time under tension, and the more you fatigue your muscles and tap into the endurance-focused slow-twitch muscle fibers. 

        Your Takeaway: Keep Mixing Up Your Workouts

        Remember, your body needs both slow-twitch and fast-twitch muscle fibers, regardless of your favored fitness outcomes. One is not more important than the other, although you may wish to emphasize a specific type of muscle fiber based on your sport or athletic goals. Keep your workouts diverse, change the tempo and speed and resistance, and make sure you're doing both endurance movements and explosive movements.

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