Do you love sprinting but the thought of a marathon makes you cringe? And does it seem like your friend could easily run 100 miles in a day without blinking an eye? It could all be in your genetics, thanks to the number of fast- and slow-twitch muscle fibers you were born with.

Despite the name, these fibers have nothing to do with a physical muscle twitch; the term “twitch” just refers to how quickly the nerve is sending the message to your muscle to contract. Both fast- and slow-twitch fibers are important for working out and functional activities like lifting your child, so even if you’re not an athlete, you’ll need to keep them in tip-top shape. We talked to physical therapist Kristin Hietbrink at OrthoCarolina all about the difference between these muscle fibers, and how to train each.

Slow-twitch fibers

  • Used in endurance activities like long-distance running or swimming
  • Found in posture and stabilization muscles like your core
  • Most important muscle fiber type because they provide the foundation for smooth, controlled, coordinated movement
  • If you naturally excel at marathons, swimming other endurance-type sports, you may have been born with more slow-twitch fibers

HOW TO TRAIN: Kristin told us slow-twitch muscle fibers are vitally important for athletes of every skill level (and even non-athletes) because they make your movements more smooth and coordinated. They also help stabilize your joints and can prevent injury to your fast-twitch fibers. That’s why training slow-twitch fibers should be an athlete’s first priority.
“If you don’t have that underlying strength of endurance muscles, it can make you prone to injury of bigger muscles because they become overused due to the lack of a strong base,” Kristin said. “The core is important and many people don’t work on it correctly. You need deep core control before you start box-jumping and swinging kettle bells. That’s how people injure their back and shoulders.”

The key for training slow-twitch muscles is high repetitions and low resistance. Kristin says you need to excel at doing high repetitions at whatever slow-twitch muscle group you’re working (the gluteus medius muscle and rotator cuff are other good examples) before you start the power training. Here are some moves she recommends:

Rotator cuff exercise

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Using a light resistance band, rotate your forearm away from your body, keeping the elbow bent to 90 degrees. A towel roll between the body and upper arm helps to maintain good form.
Start with three sets of 15 and work up to three sets of 30 with one minute of rest between sets. It should not be painful. When working on endurance, the fatigue should be toward the end of the repetitions — if you fatigue early in the repetitions, you probably are using too much resistance.

Side-lying clam (works gluteus medius — a hip stabilizer)

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Lying on your side with knees bent to 90 degrees, keep the feet together and raise the top knee. Do not rotate your body backwards.
Start with three sets of 15 and work toward three sets of 30 with one-minute rest breaks between sets. Fatigue should be at the end of the repetitions; if it is starting early in the repetitions, start with three sets of 10 and slowly build up from there.

Side bridge (strengthens core)

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Lying on your side with knees bent and hips straight, tighten in the abdomen and raise your hips up off the floor. Start with three sets of 10 working up to three sets of 30. There should be no pain with this. As always, fatigue should set in toward the end of the repetitions. If it is setting in early in the repetitions, start with three sets of 10 and gradually build up to three sets of 30.



Fast-twitch fibers

  • Used in “power-activities” like sprinting, weight-lifting and explosive sports like football
  • Found in big and dominant muscles (think hamstrings and quads)
  • More prone to injury
  • If you naturally excel sprinting, swinging kettle bells, playing soccer, etc., you may have been born with more fast-twitch fibers

HOW TO TRAIN: Kristin says it’s important to make sure your slow-twitch fibers are adequately trained before you start working the fast-twitch fibers. She says the key is a progression: start doing squats and once you can do those well, add weights or start doing single-leg squats. After that, make them more dynamic by doing jump squats.



Even if you’re not a gym rat, you still need your fast-twitch fibers intact. For example, if you’re routinely picking up your toddler, that’s a power activity.

Have more questions about slow- and fast-twitch fibers? Contact OrthoCarolina.






Meet our expert

Kristin Heitbrink has a physical therapy degree from University at Buffalo. She’s been practicing orthopedic physical therapy for 13 years. She works at OrthoCarolina’s Eastover office specializing in spine therapy. In her free time she enjoys yoga, running and outdoor activities with her family and friends.