Your body is made up of two kinds of muscle fibers: slow twitch (type 1) muscles, and "fast twitch" (type 2) muscles. "Slow-twitch muscles are the ones in charge of long endurance challenge—so they activate first and they last for longer, so you won’t fatigue quite as quickly," explains Pilates Instructor and health coach Helen Phelan. "There are a few types of fast-twitch muscles, but the general difference is that you use them for power. They activate in seconds, but they also burn out much quicker."
To break it down a bit more simply: "Slow twitch muscles activate first and for longer," Phelan says. "I think of fast twitch muscles as the backup generator that clicks on when you're at exhaustion, but they don't have the ability to sustain that power and force, so it's a 'short-term solution."
Rondel King, MS, an exercise physiologist at the NYU Langone Sports Performance Center, explains that type 1 muscles tend to be more anaerobic (which helps with endurance activities, like distance running), and type 2 fibers are more aerobic (which helps with more "explosive" activities, like sprinting), and the more you have of one type of muscle can dictate how good you are at a certain type of activity. So for example, if you can run a 10K and barely break a sweat, but struggle to make it through a 5-minute Tabata sequence, you likely have more type 1 muscle fibers than type 2.
You're born with what King calls a "blank slate" (though some people may have a genetic predisposition to more of one type of muscle), and the activities you do as you grow up and go through puberty tend to dictate which types of muscle fibers take hold. "If you're doing a bunch of endurance events, you set the stage for developing more type 1 muscle fibers. On the other hand, if you're doing more sprinting exercises at that point in your life, you have the propensity to develop more Type 2 fibers," he explains.
That said, even if you do have more of one type of muscle than the other, you can use your workouts to develop both. "For the general population it helps to train in both," says King, noting that you want to do aerobic activities to build type 1 muscles and keep your blood flowing, and also do explosive activities to get the strength of type 2 fibers. "So developing a program where you tap in and stimulate both fibers is the best way to go."
In order to do this properly, Phelan explains that you should know which types of muscles you're actually focusing on to get the most out of your workout. "Increasing mass requires working to absolute burnout to engage the fast twitch fibers. Increasing endurance means your body needs to learn to burn energy over an extended period of time, which means you need to build the slow twitch fibers," she says. "So, ideally a well-rounded practice incorporates both types of training so that you are not just challenging your body and keeping it mentally stimulating but you'll then be in a continued state of building strength and also be able to sustain that strength over a period of time."
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