Slow & Fast Twitch Muscle Fibers.

In this article you will learn that the higher the reps, the more slow twitch fibers you work and the lower the amount of reps, the more fast twitch fibers you work and much more...
It is generally accepted throughout the world that there are two different types of muscle fibers. Slow twitch (Type I) muscle and fast twitch (Type II) muscle fiber. From there, you can further categorize fast twitch muscle fiber into Type IIa and Type IIb.

Type I Muscle Fibers

Type I muscle fibers have the slowest-contractile speed, the smallest cross-sectional area, the highest oxidative (aerobic) capacity, and the lowest glycolytic (anaerobic) capacity. They contract slowly and are able to hold a steady paced twitch for long durations without fatigue. Type I muscle fibers are predominately used in endurance activities. Long distance runners, swimmers, and cyclists mostly use Type I fibers.

Type II Muscle Fibers

Type IIb muscle fibers have the fastest-contractile speed, the largest cross-sectional area, the lowest oxidative capacity, and the highest glycolytic capacity. They are ideally suited for short fast bursts of power. These muscle fibers are used in such activities as sprinting, powerlifting, and bodybuilding. Type IIa muscle fibers are intermediate and their properties lie between type I and type IIb.

How Type I & Type II Muscle Fibers Are Different

Type I fibers are different than type IIb fibers for many reasons. You can think of them as opposites. Type I is for long endurance activities while type IIb is for short fast bursts. Type I fibers are highly oxidative and are not likely to hypertrophy as much. Type IIb fibers are highly gycolytic and tend to hypertrophy more than type I fibers. Type I fibers are also known as red fibers due to their abundant supply of blood. Type IIb fibers have little blood causing them to be white in appearance.

How Your Body Recruits Muscle Fibers

Even the small muscle groups in your body have over 100,000 muscle fibers. A motor neuron is what stimulates our muscles to contract. It carries impulses (messages) from our brain and spinal cord to our muscles. One motor neuron controls anywhere from 2-2,000 muscle fibers. A single motor neuron and the fibers it stimulates are called a motor unit. Each motor unit mainly contains muscles of its kind. Also, the motor unit fires with a frequency that is conducive to the fibers it stimulates. Simply put, a slow twitch motor neuron will cause the muscles in it to contract slowly while a fast twitch unit will fire quickly.

The quicker it fires the more power it produces. If the activity is light, it will mainly stimulate type I muscle fibers. When it becomes too intense it will call upon type IIa muscle fibers. And finally, for the highest intensity movements, it will recruit the type IIb fibers. This is why type I fibers are called low threshold, and fast type IIb fibers are called high threshold. Low threshold because they are the first muscle fibers to be recruited and high threshold because they are only recruited under the most intense circumstances. Your body always activates its muscle fibers in this fashion.

An Example Of How Your Muscle Fibers Are Recruited

Say you were to help someone lift a heavy couch up a flight of ten stairs. You would use your hands as grips and let your legs do all the work. On the first step your legs will start to recruit type IIa fibers. By the 2nd or 3rd step your nervous system does not recruit more motor units. This being the case the first set of fibers rest and more type IIa fibers are recruited. Along with these, a number of type IIb fibers are called into play (to maintain fluent motion up the stairs).

As your journey continues more type IIa and type IIb fibers are recruited until by the last step they have all come into play. Your muscle fibers weren't twitching at maximum speed until the end of the stairs when they neared failure. The faster a muscle fiber twitches the greater the force is. At the beginning, the fibers weren't forced to twitch at maximum frequency to overcome the weight, but at the end they had to produce as much force as possible to overcome the weight. This is how recruitment is designed to maintain a certain amount of force.

Recruitment In Low Rep Sets

Low repetition work (in the 1-5 rep range) provides an extremely unique adaptation. To overcome the weight, your body must recruit as many motor units as humanly possible. This will cause your nervous system to become more efficient at this process. Over time, you will learn to lift the heavier weight with all (or close to as possible) of your motor units in one rep. Powerlifters are brutally strong for this reason. They can basically make all the their motor units fire at once.

Strength Gains Without Muscular Hypertrophy?

Strength gains in the 1-5 rep range can take place without muscular hypertrophy. This doesn't mean that growth cannot occur at these junctions. It just means that growth is not the optimal method of adaptation in this zone. This is for two reasons. First, although more motor units are recruited at once, low repetition sets cannot recruit as many muscle fibers as in a higher repetition set.

This is due to signaling problems occurring in the nervous system. These problems occur because the nervous system is asked to act extremely fast and furious and is taxed to its limit. Second, contractile proteins in a cell are responsible for muscular growth. These must be exposed to enough stress (which they aren't in low repetition sets) or they will not be damaged enough to overcompensate and increase in size.

How Does A Certain Rep Range Affect Your Muscle Fibers & Strength Gains?

Overview Growth In Muscle Fibers Below
Repetition Range Type I Type IIA Type IIA Strength Gains
1-2 repetitions Very Low Low Low Excellent
3-5 repetitions Very Low Low Decent to Good Excellent
6-8 repetitions Very Low Good Excellent Good
9-12 repetitions Low Excellent Very Good Good Within Rep R.
13-15 repetitions Decent Very Good Decent to Good Endurance
16-25 repetitions Very Good Diminishing Low Endurance
25-50 repetitions Excellent Low Very Low Endurance

How To Apply This Knowledge To Bodybuilding

From this article, you learned that in general the higher the amount of reps, the more slow twitch fibers you work and the lower the amount of reps, the more fast twitch fibers you work. You also learned that sets in the 1-5 rep range don't recruit as many fast twitch fibers as sets in the 6-12 rep range although they do recruit a higher percentage of fast twitch fibers compared to slow twitch. Low rep sets can be used to your advantage to bust through a plateau or gain strength while maintaining the same size.


Sports Medicine with Elizabeth Quinn
Skeletal Muscle Function and Metabolism
SJ Valberg DVM, PhD, and JM MacLeay, DVM, Department of Clinical and Population Sciences, College of Veterinary Medicine, University of Minnesota, St Paul, Minnesota
Muscle Fibers - An In Depth Analysis Part 2 by Jacob Wilson

Good Luck,