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The Role of Muscle Fibers In Running

Posted by: on January, 20 2016

Found on Competitor.com and written by Jeff Gaudette

There are different types of muscle fibers, and it’s important for runners to work all of them.

Learn how to specifically target different types in your training.

Understanding the why of training and the science behind your workouts is important, even if you’re not writing your own training. First, with the multitude of training plans and books available, it allows you to distinguish between what’s good and what’s nonsense. Second, understanding the scientific purpose behind specific workouts helps you get the most out of each session while tailoring them to your needs.Muscle-fibers

In this article, we’re going to take an in-depth look at muscle fibers to better understand how they influence your training. Don’t worry, you don’t need a science degree to follow along, I’ll keep it simple. However, by developing a basic understanding the different types of muscle fibers and how they work, you can learn how to target them specifically to control your training.

Types Of Muscle Fibers

Skeletal muscle, the type that is responsible for moving our muscles when we run, is comprised of three different muscle fiber types, each with its own advantages, disadvantages and specialty.

Type I, better known as slow-twitch fibers, are the body’s primary method for less explosive, sustained movements. They do not contract forcefully and thus require less energy to fire, which makes them well suited to long distance running. More importantly, they house our main supply of oxygen-boosting power plants — mitochondria, myoglobin and capillaries.

Type IIx are best known as fast-twitch muscle fibers. These are the muscle fibers primarily responsible for fast, explosive movements like sprinting. However, they lack the endurance-boosting ability of slow-twitch fibers and can only be used for short periods of time.

Type IIa are what we call intermediate fibers. These are a blend between fast- and slow-twitch fibers. They have some aerobic capability, but not as much as the slow-twitch fibers, and they can fire more forcefully, but not quite as explosively as the fast-twitch fibers

Each individual has a genetic predisposition to certain muscle fiber types. The misconception that many runners have is that each fiber type is exclusive (i.e. you can only use one or another), we can’t train to improve how our fibers function, or the alter percentage we have of each. In reality, with the right training, we can manipulate and improve all three.

How Muscle Fibers Work

Before we can outline how to improve fiber function and conversion, we must understand under what circumstances we use each fiber type, when they are recruited, and when they change.

Recruitment Ladder

This process starts with what we call the recruitment ladder. The recruitment ladder is a way of envisioning how and when each fiber type is activated. At the bottom of the ladder, we have the slowest, least explosive fiber type, Type I slow-twitch, and at the top you have fast-twitch fibers.

You move up the ladder based on how much force you need to generate to sustain a given pace. If you were to head out to the streets right now and begin running easy, your body would start by using slow-twitch fibers. If you were to pick up the pace, your body would start recruiting some of the Type IIa intermediate fibers to supplement the need for more power from the muscles to generate more force during the stride. Finally, if you were to sprint across the road to beat traffic, your body would then engage the fast twitch muscle fibers to give you the explosive burst you need to sprint.


In addition to intensity, the other factor in muscle fiber recruitment is fatigue.

As you get further into a long run, the slow twitch fibers you’ve been using start to get tired and you can no longer fire them as efficiently. As a consequence, you start to recruit some intermediate fibers to help maintain pace. Of course, these intermediate fibers require more glycogen and are not as fatigue resistant as slow-twitch, so it won’t be long before you find yourself slowing dramatically as your muscles start to fail.

How We Can Improve Muscle Fiber Recruitment, Activation And Conversion

Now that we better understand the different muscle fiber types and under what circumstances we use them, we can employ this knowledge to better structure our training. In this section, we’ll outline some of the more common workouts, identify which muscle fiber they target, and what this means for your running-specific fitness.

Long Runs

The long run targets the slow-twitch fibers, making them more efficient by building their aerobic capabilities, and also making them more fatigue-resistant. Continuous long runs also help convert a greater percentage of your muscle fibers into slow-twitch fibers, which is one reason you continue to get better with years of mileage.

Tempo Runs

Tempo runs target slow-twitch and intermediate muscle fibers. Slow-twitch fibers reach maximum recruitment and contraction speed at tempo pace, which is one reason why tempo runs are so critical to endurance training. In addition, tempo runs help improve the recruitment patterns of intermediate fibers with slow twitch fibers. In essence, it improves the ability of both fiber types to work together for maximum effectiveness.

Short Repeats

Traditional interval workouts like 12 x 400 meters help recruit intermediate and fast twitch muscle fibers. By being used together, these two fiber types learn to interact more efficiently by reducing activation of unnecessary fibers. More importantly, it improves our neuromuscular coordination — the speed at which the brain can send signals to the muscles to fire, thus making you more efficient.

Speed Development And Sprint Work

Speed development work, like strides, hill sprints, and short, maximum effort sprints on the track, help recruit the maximum amount of fast-twitch muscle fibers. While this may not seem like a necessary benefit to someone running the marathon, this type of training makes each stride more explosive and enables you to generate more power without increasing effort. This increased power is what makes your stride more fluid and efficient.

Ancillary Training Like Strength Work, Stretching And Drills

Drills, strength training and dynamic stretching improve recruitment patterns (the ability of your muscle fiber types to work together concurrently), increase strength, and reduce inhibitions (discussed more in-depth in this article). By incorporating this type of work in your training, you develop more biomechanically sound running mechanics, become more efficient, and train your fiber types to work together.

Runners typically talk about training in terms of metabolic improvements — aerobic development, VO2 max, and lactate threshold — but we often forget the important role muscle fibers play in our fitness and ability to run faster. The next time you plan your training, don’t forget to factor in how you can better train your muscle fibers as well.