Fitness Test: Are You Fast or Slow Twitch? Find Out.

Fitness Test: Are You Fast or Slow Twitch? Find Out.

Are you a natural-born sprinter or more primed to run a marathon? It’s true that genetics have a lot to do with which way your fitness abilities lean, but how you train is possibly just as important. And it all has to do with muscle fibers. Here's what you should know to cultivate better total-body muscle response and train your weaknesses. 

A Quick Primer on Muscle Fibers

Fast-Twitch Muscle Fibers

Fast-twitch fibers include your IIA (or hybrid) fibers and IIB (or IIX) fibers. As their names imply, you use these fibers to move fast, like to sprint all-out for 10 seconds, or to generate a high amount of power for a short amount of time. Think: explosively executing one rep of a heavy lift. 

The weakness of these fibers is their ability to endure, says Chris Hinshaw, a top endurance coach who has worked with more than 30 CrossFit Games champions. The energy systems that primarily fuel the fibers — the phosphagen system and the glycolytic energy system — only last fully charged for 10 to 12 seconds. The problem? The byproduct of using glycolytic (or anaerobic) energy is fatigue-causing lactate. “So if you continue to use these fast twitch fibers you essentially are a ticking time bomb,” Hinshaw says. You essentially create a greater amount of lactate than the body can clear and you’re going to blow-up or “hit a wall.” 

muscle-fiber types

Slow-Twitch Muscle Fibers

That’s where, ideally, your slow twitch fibers come into play. “One way that lactate clears the body is through your slow-twitch fibers,” Hinshaw explains. They not only love oxygen as a fuel, they also love lactate. And by consuming it, they remove the acidity, or fatigue, from the body. “So that's why when you talk about aerobically-fit athletes, what you're really saying is that those athletes recover, or clear fatigue, fast.” 

The Bottom Line

Ideally, you want a good muscle balance between slow twitch and fast twitch to not only move fast, but also clear lactate and endure. “What we need to do, especially as Spartan athletes, is to develop the entire spectrum of muscle fibers, everything from the fastest of the fast to those walking fibers, because every single speed within that spectrum recruits different fibers,” Hinshaw explains. 

In order to improve, you need to know your baseline capabilities. That’s where this muscle-fiber test comes into play. 

Take the Muscle-Fiber Test

Think of yourself as a volleyball player making a three-step approach to the net. As soon as you plant both feet to jump, do your hips drop or do you immediately fire up? [Consider having a friend video you so you can analyze it afterward.] 

If your hips drop: You’re slow twitch

If you immediately fire up: You’re fast twitch

“The three steps [pre jump] tune the muscles for what they're about to do,” Hinshaw explains. You're getting the brain, the neurological system, ready for what's about to happen. “A slow-twitch athlete doesn't get tuned in three steps, which is why they bend down to make the jump,” he says. They should target an improvement in the neurological pathway, the percentage of fibers that are recruited, and the speed at which they are recruited.

A fast-twitch explosive athlete, on the other hand, is going to take a fractional drop in the hips to make the jump occur because they’re literally fired up from the three-step approach. “This athlete should find a way to shut the fast twitch fibers down when necessary, and to settle into a more efficient slow-twitch aerobic speed.”

How to Fine-Tune Your Fibers

muscle-fiber types

If you’re slow twitch, here are three ways to start developing your fast-twitch fibers: 

  1. End your long run with five to 10 minutes at a fast pace, or with several 10-second sprints. 
  2. Include short “anaerobic” surges within your long runs (think: sprint for 20 to 30 steps every two minutes during a six-mile run).
  3. Incorporate short hill sprints with full recoveries (think: six 12-second sprints with up to six minutes of recovery between each) into your training routine to build strength and power. 

If you’re fast twitch, here are two ways to start developing your slow-twitch fibers:

  1. Do long runs at a slower pace to improve fuel economy by restricting fast-twitch recruitment. Pro tip: Do them based on time (vs distance) so you don’t feel compelled to push the intensity and finish faster.
  2. Break long runs into two workouts (think: run five miles in the AM, rest three to five hours, run three miles in the PM). 
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