Why does one friend do barbell squats for ten reps in the gym and pack on muscle, while you barely grind out six reps with the same weight, yet you glide along the morning running track more easily than anyone else in your training group? The answer lies largely in your muscle types. Which muscle fibers dominate in your muscles determines how fast you run, how much weight you lift and how long you can keep going over a long distance without tiring.
Over the past twenty-odd years I have seen in the gym how two men on the same training program and nutrition get completely different results. One adds 8 kg of muscle mass in eight months, the other barely 2 kg. Genetics? Yes, but more specifically, the fiber composition of their muscles. And it is worth understanding this before you start reviewing your training plan.
Why muscle types determine your training results
How muscles respond to a repeated stimulus (that is, physical training) depends largely on their fiber composition. And that composition is highly genetically determined. The reason some athletes are natural sprinters and carry more muscle than others, while others can run tens of kilometers without tiring, is no mystery. It comes down to which fibers their muscles contain.
To specialize sensibly in any sport, or to put together a specific program, every coach and informed recreational lifter should understand the fine structure of muscle. Otherwise you are rowing in the dark: maybe training the wrong fiber type, maybe leaving your inherited potential untapped. Muscle types are not an academic detail. They are a practical framework for choosing your weight, reps and rest periods.
Three muscle fiber types: what sets them apart
Humans have three main muscle fiber types. Each is built for a specific job, and each has its own strengths and weaknesses. Let’s go through them one by one.
Type I fibers: slow oxidative
Slow-twitch oxidative muscle fibers contract slowly but are extremely resistant to fatigue. This means they can stay at work for a long time. The motor neurons (nerve cells) that innervate them are small, and the muscle fiber’s own diameter is also small. They contain many mitochondria, have a high capillary density and contain myoglobin, a protein structure that binds reserve oxygen.
In terms of energy, they depend practically not at all on creatine phosphate and use little glycogen. Their main energy source is triglycerides. They have few glycolytic enzymes but plenty of oxidative enzymes (Krebs cycle, electron transport chain). Slow fibers are involved mainly in aerobic, low-intensity and prolonged activities: slow running, walking, carrying things upstairs. Most everyday activities recruit precisely these fibers.
Type IIA fibers: fast but fatigue-resistant
Fast glycolytic fibers are in turn divided into two. Type IIA fibers are intermediate when it comes to fatigue: their properties fall between those of type I and type IIB fibers. Structurally, IIA fibers are innervated by a large motor neuron, and the fiber’s own diameter is also large. Mitochondrial density is high, the capillary network is moderately developed and myoglobin content is moderate.
Their creatine phosphate level is high, as is their glycogen content, while triglyceride content is moderate. These fibers contain roughly equal amounts of glycolytic and oxidative enzymes. Functionally, IIA fibers work in longer-duration anaerobic activities, such as a 400 m sprint, where relatively high power and strength are produced, but not maximal.
Type IIB fibers: explosive strength, rapid fatigue
Fast type IIB fibers are extremely susceptible to fatigue. They are used in short activities that demand high force and speed: sprinting, hurdles, jumps, shot put. These fibers are capable of producing significantly greater force than slow oxidative muscle fibers. IIB fibers, too, are innervated by a large motor neuron and have a large diameter. Their myoglobin content and their capillary and mitochondrial density, however, are low.
Creatine phosphate and glycogen reserves are large, while triglyceride content is low. They contain a lot of glycolytic enzymes and minimal oxidative ones. The simple truth: IIB fibers are built to deliver their maximum within 2–10 seconds and then need a long recovery.
The main difference between the two basic types comes down to contraction speed. That in turn depends on how fast calcium is released from the sarcoplasmic reticulum and on myosin ATPase activity. These values are high specifically in fast glycolytic muscle fibers (Fitts Widrick, 1996; Harigaya Schwartz, 1969).
At any movement speed, force production depends on the specific fiber type. During dynamic contraction, when a muscle either shortens (concentric) or lengthens (eccentric), fast fibers produce much greater force than slow ones (Widrick, 1996). In isometric contraction (the muscle neither shortens nor lengthens, as in a hold), both fast and slow fibers develop force of equal magnitude. The difference arises specifically during dynamic contractions.
Muscle fiber type distribution varies widely among athletes. Endurance athletes have a higher percentage of slow fibers, while sprinters, jumpers and throwers have more fast glycolytic fibers (Costill et al., 1976; Ricoy et al., 1998). The higher a sprinter’s share of fast fibers, the more strength and speed their muscles produce (Fitts Widrick, 1996).
Are the differences down to genetics or training? Studies on identical twins have so far shown that muscle fiber composition is largely inherited and genetically determined (Komi Karlsson, 1979). At the same time, it is clear that specific training can tilt a muscle’s structure and metabolic capacity in one direction or the other — but it cannot reverse it completely.
How muscle fiber types come into play in training
Muscle strength is produced by recruiting motor units — a motor unit is a group of muscle fibers innervated by a single motor neuron. During voluntary isometric and concentric contractions, recruitment follows the size principle (Henneman et al., 1974).
Small motor units (which contain slow, small fibers) have the lowest excitability threshold and are brought into action first. As the demand for more strength grows, progressively larger units are recruited. The largest motor units contain large type II B fibers — they have the highest excitability threshold and switch on only when the effort demands it.
A side note: this holds as long as we stay within our rep range — up to 8 reps — which in turn depends on how much resistance we use. If a set stretches beyond 8 reps, say 10–12 reps, the emphasis shifts toward type II A fibers. When a set goes beyond 15 (15–20 and more), the emphasis tilts toward slow type I fibers.
An important nuance: all of this assumes you choose a weight that brings you to your limit within the chosen rep range. If you plan a set of around 15 reps and stop halfway, it is quite certain that type II B fibers are never recruited — the emphasis stays mostly on type II A fibers. For muscle mass that is fine, because we get hypertrophy either way. But here is the catch: if you happen to have more type II B fibers than you think, much of their potential would simply go to waste, because you never stimulate them with a heavy enough load. That is exactly why it pays to try different rep ranges and analyze which one your muscles respond to best.
It makes no difference what resistance you train with — the small slow motor units are always recruited first. If the intensity is low (the weight is light), it is likely that only these are working. The excitability threshold of the large units is simply not reached.
When the intensity is high — you lift heavy weights or run maximum-speed intervals on the track — the small units are working first, then the fast II A units, and finally II B. The size principle holds from the top down.
There is evidence that under certain conditions the size principle can even be reversed. According to some studies, emphasized eccentric muscle work (the lengthening of the muscle, the lowering phase of most exercises) recruits fast motor units earlier than small slow ones (Denier van der Gon et al., 1985; Grimby Hannerz, 1977; Nardone et al., 1989; Smith et al., 1980; Ter Haar Romeny et al., 1982). The science is not 100% certain on this, but it is an interesting nuance worth knowing — especially if you are planning a training program that emphasizes eccentric tempo.
How to recognize your own muscle types
Essentially the only precise way to determine the fiber composition of muscles is a muscle biopsy. A small needle is inserted into the muscle, a microscopic piece of muscle tissue is snipped off and examined under a microscope. In Estonia this is not done routinely — unless you are taking part in a sports medicine or physiotherapy research project, biopsy stays in the lab.
Fortunately, there are also indirect tests that give data on a muscle’s properties and its tendency in terms of fiber composition. Such methods have produced results specifically in identifying fast fibers and strength indicators (Coyle et al., 1979; Froese Houston, 1985; Gerdle et al., 1988; Gregor et al., 1979; Suter et al., 1993).
Any recreational lifter can use an indirect method on their own in the gym, but keep in mind that it does not give a clear answer for individual muscles. Lifting any weight engages a whole range of muscle groups, so you only get a generalized picture. As a guideline, though, it works well:
- Find out your 1-rep max (1RM) in the given exercise.
- Take 80% of it as your resistance.
- Perform as many reps as you can with that weight — in a controlled manner, keeping your technique.
- If you managed fewer than 7 reps, the muscle group working is likely to contain more than 50% fast fibers.
- If you managed more than 12 reps, it is more than 50% slow fibers.
- If you managed 7–12 reps, the proportion of fibers is roughly equal (Pipes, 1994).
As you can see, the picture is blurry, but it does point in a direction. To get a more precise read on an individual muscle, you still need a biopsy.
Here is another indirect method that coaches use: get a young athlete involved in as many different sports and activities as possible. Before long it becomes clear which ones they excel at and which don’t suit them. This is often how young athletes’ sporting direction is determined. This method also gives an uncertain picture and takes time, but combined with the 1RM test it already gives you a pretty good first impression of your muscle types.
Training guidelines for different muscle types
An athlete’s fiber composition plays an enormous role in how much weight they can lift, how many repetitions they can do in a set, how fast they can run and how high they can jump. Fibers determine whether an athlete is better suited to speed-strength sports or to endurance events.
A bodybuilder dominated by fast fibers will never be able to do as many repetitions with a given weight as a bodybuilder with the same muscle mass who has more slow fibers. The first will never reach the same muscular endurance as the second. And vice versa: an athlete with a predominance of slow fibers cannot lift such heavy weights or run intervals as fast as an athlete with fast fibers. They will never become as strong or as fast.
It’s worth noting that there are big differences even within a group of sprinters, and likewise among distance runners. Some sprinters can do 10 × 200 m in training, while others are exhausted after the eighth repeat. Those who can only manage 8 repeats can probably put out even more power in a single repeat, which is also why they tire faster. The same goes for middle- and long-distance runners: some can do 8 × 800 m in training, others only five 800 m repeats.
That is why a coach should decide for each athlete individually who gets a longer recovery time between repeats. Those who put out greater strength tire faster and need a longer break. All in all, they do fewer repeats, but faster ones.
What training changes in muscle types
If you train a muscle dominated by fast fibers in endurance mode, the number of slow fibers will definitely not increase as a result. Likewise, if you train a muscle with mostly slow fibers in strength mode, the number of fast fibers does not increase.
That said, limited adaptations do take place. Type II B fibers can take on certain II A characteristics, and with consistent strength training II A fibers convert to some extent toward II B. But there is no total transformation. It is not possible for fast fibers to become slow or the other way around. What you were born with is, to a large extent, what you live with.
Even though fiber types don’t convert into one another, training can influence the cross-sectional area of one fiber type. In other words, depending on the nature of the training, selective hypertrophy of muscle fibers is possible.
Example: an athlete has a 50/50 mix of fast and slow fibers in a particular muscle. Looking at the muscle’s cross-sectional area, say 65% belongs to fast fibers and 35% to slow fibers. When following a strength training program, the numerical ratio of fibers stays 50/50, but the muscle’s cross-sectional area changes: the fast fibers’ share of the area grows, while the slow fibers’ area stays the same or even atrophies.
Depending on the intensity and the program, the area ratio can shift to as much as 75% fast-fiber area and 25% slow-fiber area. Such a change leads to an increase in strength numbers. At the same time, oxidative capacity and endurance decrease. Because fast fibers are naturally bulkier, the athlete also gains muscle mass during such a shift.
The opposite: if an athlete trains consistently in endurance mode, the fast glycolytic fibers atrophy (the stimulating training load is missing). Although slow fibers don’t hypertrophy much, the area ratio still shifts, because the fast fibers simply shrink. If under normal circumstances the ratio was 65% / 35% in favor of fast fibers, after endurance training it could be 50% / 50%. This doesn’t mean the fast fibers have disappeared; only their area changes. The athlete’s endurance rises, strength drops, and muscle volume and circumference decrease.
Weights, reps, goals
Coaches and athletes know that to increase muscle strength, you have to train with heavy weights and a low rep count. This logic holds because heavy weights recruit type II B fibers, which produce more strength than I and II A. Hypertrophy occurs in the fibers that are loaded, and significant hypertrophy occurs specifically in II B fibers, because they are bulky and able to respond to the training load (Morehouse Miller, 1976).
Low-intensity training, and in many cases even moderate-intensity training, is not enough to recruit the high-threshold II B fibers. That is why it’s important for the intensity to be genuinely high, not just moving dumbbells around in the gym.
How heavy should the weight be, and how many reps?
Muscle strength develops primarily when you perform an 8-rep set to the limit of your ability or below it. Every set of up to 8 repetitions builds strength.
- Strength without significant muscle mass gain: resistance of 90–100% of 1 RM, rep count 1–3.
- Maximal strength with hypertrophy: resistance of 80–90% of 1 RM, 5–8 repetitions, close to failure (Zatsiorsky, 1995). The training focus here is hypertrophy, but the goal is strength gain, not mass for its own sake.
- Muscle mass (hypertrophy) with moderate strength gains: rep range of 6–12 (Fleck Kraemer, 1996).
Keep in mind: to increase strength, your fast type IIB fibers have to be stimulated, and that means a sufficiently heavy weight.
For maximal results, train according to your genetic predisposition. If you have more slow fibers, endurance sports suit you better: more repetitions, lighter weight. If you have more fast fibers, you’ll get more out of sprint training and do better at building strength and muscle mass: use fewer repetitions and a heavier weight.
One more important observation: if you look at rep tables for strength and hypertrophy (for example, Jacob Wilson’s thorough three-part article Muscle Fibers — An In-Depth Analysis), once we go beyond 8 repetitions we lose part of the type IIB fibers’ potential for developing primary strength. According to the table, the growth potential of fast type IIB fibers is only exhausted after 12 repetitions. But the best combined effect of strength and mass for fast fibers comes from the range of 5–8 repetitions, though how many of these fast fibers you have in the first place is another matter.
Let’s not read the table as if your muscle fibers switch the moment the 8th repetition ends and the 9th begins. It doesn’t work mechanically like that. But in the big picture, the table gives a good framework.
Rest intervals by intensity
If you train with a resistance that lets you perform:
- 3–5 repetitions → rest 3–5 minutes
- 6–8 repetitions → rest 3 minutes
- 9–12 repetitions → rest 1–3 minutes
- 13–15 repetitions → rest 1 minute
- 15–25 or more → rest 30 seconds to 1 minute
These rest periods are a rule of thumb. If you train type IIB fibers on heavy repetitions in the 3–5 range, they need several full minutes for the ATP-CP system to recover. If you cut the rest to 30 seconds, you’re no longer doing a strength workout; you’re simply doing an endurance-type metabolic training load with a light weight.
Muscle types FAQ
Can I change my muscle types through training?
Not significantly. The basic muscle fiber type is genetically determined. You can influence the share of cross-sectional area the fibers occupy (some hypertrophy, others atrophy) and the shift between type IIA and IIB fibers, but slow fibers don’t turn into fast ones or vice versa.
How can I tell without a biopsy whether my muscles are more fast or slow?
Do a 1 RM test, then perform as many repetitions as you can with 80% of it. Fewer than 7 repetitions points to a predominance of fast fibers, more than 12 to slow fibers, and 7–12 to an even distribution (Pipes, 1994). As an additional indicator: look at which sports you’re naturally more successful in.
Why do some friends build mass faster than others?
One reason is the proportion of type IIB fibers: they are large in volume and respond to strength training with the strongest hypertrophy. If someone has more of them, they’ll add more muscle mass on the same program. Other factors (hormones, recovery, calories, sleep) matter too, of course, but muscle types are the foundation.
Author: Translated by Janar Rückenberg
Source: WHO – physical activity.
Come and train! ArtGym
