What's The Difference Between Fast-Twitch And Slow-Twitch Muscle Fibers?

Picture this: One woman is doing bicep curls with 12-pound dumbbells. Three slow sets of 12 reps, resting for about a minute between each set. Another woman is doing sprint interval training, rotating between 30 seconds of walking and 30 seconds of running as fast as she can. These exercises not only work different muscles; they work them in different ways.

You already know the body has a lot of muscles in it, from the neck and shoulders to the back and arms, and down to the core, legs, and feet. In fact, Healthline claims there are more than 650 skeletal muscles in the human body. Within these muscles are two types of fibers, as the National Academy of Sports Medicine (NASM) explains. There are slow-twitch muscle fibers (type I) and fast-twitch muscle fibers (type II), each of which has different functions from the other.

Some exercises involve slow-twitch muscles while others use fast-twitch muscles, and according to Verywell Fit, most people have an equal amount of each. Strengthening both of these muscle fibers provides a range of benefits to the body, but what's the difference between the two?

Slow-twitch muscle fibers support cardio exercises

The aforementioned slow bicep weightlifting exercises are supported by slow-twitch muscle fibers. As the name implies, these fibers contract slowly — slower than fast-twitch muscle fibers. They use oxygen efficiently to create "fuel for continuous, extended muscle contractions over a long time," according to Verywell Fit.

Weightlifting can develop slow-twitch muscle fibers, but only if you can comfortably "complete 12 to 20 reps per set, and rest for about 60 to 90 seconds between reps," according to azcentral.com. NASM describes slow-twitch muscles as being resistant to fatigue and "aerobic in nature," so if you're lifting light weights, your muscles will take longer to get tired, compared to heavy lifting. This is how you know you're using slow-twitch muscles.

NASM's description above serves as a clue to another type of exercise that incorporates slow-twitch muscles: Aerobics. Merriam-Webster defines "aerobic" as "of or relating to the body's ability to consume oxygen during exercise." So all those "Sweatin' to the Oldies" workouts and Zumba classes you did back in the day helped to develop your slow-twitch muscles.

Other examples of aerobic exercises that involve slow-twitch/type I muscles include long-distance running, jogging, walking, rowing, cycling, and swimming, as MasterClass explains. Because all of these exercises increase your heart rate, they can support the health of your heart (via Healthline).

Fast-twitch muscle fibers support heavy anaerobic exercises

The sprint interval training mentioned earlier incorporates fast-twitch/type II muscle fibers. These fibers produce more strength and power compared to slow-twitch fibers, but only for a short amount of time. They fatigue faster and typically require longer recovery periods, as Men's Journal explains. Unlike slow-twitch muscles, fast-twitch muscles use what's called "anaerobic metabolism" to produce energy. They rely on glucose for fuel, as Physiopedia explains.

In addition to sprinting, other exercises that incorporate type II muscle fibers include powerlifting (heavy weights), jumping, and basically any other activity that involves powerful, explosive movements for short amounts of time (via Healthline). Verywell Fit claims Olympic sprinters can have up to 80% fast-twitch fibers. In comparison, marathon runners can possess that same percentage, but in slow-twitch fibers.

If you find that you struggle with explosive exercises like sprinting or jumping, it could mean that your fast-twitch muscles are weak. The good news is, they can be trained. MasterClass claims that "uphill sprints such as stair climbing are great for building this type of muscle, as are box jumps, kettlebell exercises, and jump squats."

To summarize, slow-twitch muscle fibers support aerobic activities like running, cycling, and light weightlifting, and rely on oxygen for fuel. Fast-twitch muscle fibers support aerobic exercises like sprinting, powerlifting, and jumping, and rely on glucose for fuel. Most people have an equal amount of each type of fiber, and you can train the body to strengthen both.