Your Muscle Fibers Are Not Created Equal

A muscle is a muscle, right? Not so fast.

Inside every one of your muscles are different types of muscle fibers, and these different types of fibers have different characteristics that can be targeted to produce a specific effect.

No matter what type of exercise you do, a greater knowledge of muscle fiber types allows you to more intelligently design your program.

Defining “Fast-Twitch” and “Slow-Twitch”

Slow-twitch muscle fibers are characterized by their low levels of strength and their high levels of endurance. These are the fibers we use most of the day during low-level activity.

They contain more mitochondria and myoglobin, efficiently use oxygen, and are also sometimes called “Type I” or “red fibers” because of their ample blood supply.

Fast-twitch muscle fibers are characterized by their high levels of strength and their low levels of endurance. These are the fibers we use sparingly, only during intense, powerful muscle contractions.

They use glucose as fuel, and are more anaerobic with less blood supply, so they’re sometimes referred to as “white fibers” or “Type II.”

Skeletal muscles contain both types of fibers, but the ratios can differ depending on a variety of factors including muscle function, age and training. There are also many sub-groups of fibers with distinct characteristics, but in this article we’ll focus on the big picture.

One final note is that while there are some fibers that are fast or fast-twitch and will always be fast-twitch and other fibers are slow-twitch and will always be slow-twitch, the majority of muscle fibers are actually better understood as “intermediate fibers” that can change their characteristics in response to an environmental stimulus.

So the environment to which the fibers are exposed has huge effects on your muscle fiber expression.

Additional Read: How to Build Your Fast & Slow-Twitch Muscle Fibers

From Principle to Application

So what types of activities do the different types of muscle fibers support? I’m glad you asked!

Slow-twitch muscle fiber exercises include longer-distance endurance activities like marathon running, triathlons, long-distance swimming, and various cross-country events.

Fast-twitch muscle fiber exercises include quick, powerful activities like sprinting, weight lifting, jumping, throwing, and swinging various bats, clubs, rackets, or sticks.

It’s also important to note that fast-twitch fibers are far larger than slow-twitch fibers. So if you’re looking to build or maintain muscle mass, it is smart to focus most of your attention on the fast-twitch fibers.

Time to Train the Slow-Twitch Fibers

To effectively stimulate the slow-twitch fibers, a two-pronged approach is best.

First, it’s important to perform the specific task to which you need to condition the muscles. If you’re running marathons you don’t train on a bike, if you’re long-distance swimming you don’t train on a cross-country track, and so on.

Next, your training needs to be of a low enough intensity that it can be prolonged for greater and greater durations of time. Low levels of intensity stimulate your slow twitch muscle fibers, but are not sufficient to call the fast-twitch fibers into action. This is exactly what you want for the purpose of building slow-twitch fibers.

Aside from your sport-specific conditioning, it can help your slow-twitch muscle fibers to perform some resistance exercise that increases the strength of the slow-twitch fibers. They’ll still retain their relationship relative to the fast-twitch fibers (weaker but slower to fatigue), but will increase the amount of strength with which they can endure.

This helps increase the pace that feels “easy.” A professional marathoner, for example, maintains a pace that would quickly exhaust a normal person. The fact that this pace can be maintained for hours indicates that the slow-twitch fibers are being utilized. It’s just that those slow-twitch fibers are so strong and developed that they can produce that impressive speed without requiring the fast-twitch fibers.

Best Way To Train Fast Twitch

While the slow-twitch fibers are always active whenever there’s muscle contraction of any type, the fast-twitch fibers are only called into action when the required level of force reaches a threshold above which the slow-twitch fibers cannot reach.

In other words: all the fast-twitch fibers care about is force demand.

And the greater the force demand, the greater the stimulation of the fast-twitch fibers.

It’s obvious then what to do to effectively stimulate a muscle’s fast-twitch fibers: create the greatest-possible force demand on that target muscle.

Typically this is done with heavy weight lifting, sprinting, plyometrics, and other “explosive” movement training. And an athlete with a greater proportion of fast-twitch muscle fibers is a more powerful athlete.

Problem is, the risk of injury increases as the force demand increases when you’re using traditional methods and tools. Surely there’s a better way…

Welcome to The 21st Century

That better way is called ARX. Whether you’re an endurance athlete (or just want to develop the conditioning of your slow-twitch fibers) or a power athlete (or just want to develop the explosiveness and size of your fast-twitch fibers) ARX is an essential component of your training.

During a maximum-effort set of ARX, every available muscle fiber is activated right away, since the force demand is at your maximum 100% of the time.

The fastest of the fast-twitch fibers—usually only accessible using dangerous speeds or mechanical loads—are safely and instantly recruited by ARX’s properly-loaded eccentric resistance. Since these fibers fatigue the quickest, they drop out within the first few seconds.

Then the next-fastest-twitch fibers drop out after having been powerfully stimulated, typically within 10-20 seconds.

As your ARX set continues, you experience the “orderly drop-out of fibers” as fatigue sets in, finally culminating in the end of your set as even the slow-twitch fibers finally reach full fatigue.

No muscle fibers left untouched. No capacity left unused. Whatever your training goal is, you’ve just effectively, efficiently, and safely worked the specific type of muscle fibers you’ll need to perform at your best.

Welcome to the 21st century.

Additional Read: ARX For The 21st-Century Athlete

Fast and Slow-Twitch Fibers: Explained

And there you have it. When you’re training to improve your performance in an activity or sport, it is useful to understand which fibers your training is stimulating, and why.

If you have access to an ARX machine to upgrade your training, your muscle fibers will thank you!