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.
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 altered 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.
Fatigue
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 the 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. 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.
Muscle fibres come in two forms: type I slow twitch (ST) fibres and type II fast twitch (FT) fibres. Type II fibres can be further broken down into two categories: type IIa and type IIb (also referred to as IIx in some texts). A “twitch” refers to the amount of time required to stimulate contraction of a fibre. A “fast twitch” fibre, therefore, contracts quickly and is beneficial to sprinting and explosive-type movements. A “slow twitch” fibre is optimal for endurance sports, as they are highly resistant to fatigue and because endurance events occur at a submaximal workload.
Source: Plowman & Smith, Exercise physiology: for health, fitness, and performance, 2nd Ed - Tab 19-2
Slow twitch fibres: As evident in the above table, structurally ST fibres contain a high proportion of mitochondrial and capillary density, and myoglobin content compared to FT fibres. Cast your minds back to our article discussing improving VO₂ max and you will remember that these three components play massive roles in improving your aerobic capacity.
Functionally, ST fibres produce low amounts of force and have a slow contraction velocity. This is fine, however, as explosive movements are a rarity in endurance events. Importantly, ST fibres are highly resistant to fatigue, so they can contract for long periods of time without reducing force output. What do you think of when you picture an elite marathon runner? Probably a very thin and light athlete without an ounce of fat on them. ST fibres have a small fibre diameter, which is why elite marathoners appear to have very little muscle. They actually have a similar overall number of muscle fibres to everyone else, ST fibres just don’t experience much hypertrophy (size gain).
Metabolically, ST fibres have high triglyceride (fat) stores and oxidative enzyme capacity, assisting them to specialise in aerobic energy production. They are not required to have high phosphocreatine stores or a huge glycolytic enzyme activity, because their contraction velocity and energy demand is submaximal.
Fast twitch IIb muscle fibres are polar opposites to ST fibres. They have characteristics which favour high intensity, explosive, sprint-type work, and a very low oxidative capacity. They contract quickly and forcefully, but fatigue in a short period of time. FT fibres are prone to hypertrophy, which is why you see elite sprinters having a heap of muscle mass.
FT type IIa fibres fall in between ST type I and FT type IIb fibres, and elicit moderate characteristics of both. As you can see in the table, most of their characteristics are “intermediate”.
Can We Manipulate The Type of Muscle Fibre We Have Through Training?
Yes. Around 50% is genetic, and 50% is influenced through the training environment. Every individual will be born with a certain percentage of each muscle fibre type. Elite sprinters have been shown to express up to 70-75% of FT type II fibres in their muscles, and endurance athletes 70-80% ST fibre. Therefore, there is a genetic limit to what you are born with.
Source: Powers & Howley, 8th Ed - Tab 8.2
Source: Simoneau JA & Bouchard C. (1995). Genetic determinism of fiber type proportion in human skeletal muscle. FASEB J 9, 1091-1095
You can, however, modify fibre type to an extent through training. FT type IIa fibres (the ‘intermediate’ fibres) are highly influenced by the training environment. If you complete a lot of endurance training, they will shift towards eliciting ST fibre characteristics. If you complete high intensity sprinting, they shift towards becoming explosive FT IIb fibres.
The take home message? Your body adapts to the stressors you place on it. If you complete a heap of endurance work, you will both strengthen your current ST fibres, as well as manipulate your FT IIa (intermediate) fibres into expressing and/or transforming into ST fibre characteristics. Ever wonder why those Masters endurance athletes can hold a steady pace literally all day long? Their muscle fibres have adapted to many, many years of endurance training.
That wraps up the physiological determinants of successful endurance performance. The last three articles will begin to focus on the nutritional and behavioural determinants of performance enhancement.