Basic Muscle Fiber Type Info

[iron addict]

All of us are composed of two basic muscle fiber types called slow twitch muscle fiber and fast twitch muscle fiber. Slow twitch (Type 1A) muscle fiber is characterized by smaller size, less force capacity and more endurance capacity. Their production is utilized after about 90 seconds of any given activity. Slow twitch muscles are predominately used in such athletic activities as long distance running and cycling.

Fast twitch muscle fibers (Type 2) are characterized by larger size, more force capacity, and less endurance capacity. Fast twitch muscle fibers actually have two subsets called type 2A and type 2B. Type 2A production is utilized between 25 and 89 seconds. Type 2B production is utilized from 0 to 24 seconds. (Please note that range times are approximate and dependent upon intensity.) Fast twitch muscles are predominately used in such power events as sprinting, jumping, throwing and squash.

Intensity (force) plays a factor in recruitment of muscle fiber. Depending on the intensity of an activity different muscle fiber types are recruited. Type 1A muscle fiber is recruited with intensity from 58% to 69%. Type 2A muscle fiber is recruited with intensity from 70% to 84%. Type 2B muscle fiber is recruited with intensity from 85% to 100%. The more we train our Type 2A muscle fiber the better it will be able to sustain intensity for a longer period of time.

Fiber Type Intensity (force) Time Utilization Weight Training
Slow Twitch (Type 1A) 58%-69% > 90 secs Light
Fast Twitch (Type 2A) 70%-84% 26-89 secs Moderate
Fast Twitch (Type 2B) 85%-100% 0-25 secs Heavy

As we do any given activity whether lifting a weight, swinging a racquet, or running a 200-meter sprint, the body goes through a process of utilizing these muscle fiber types. In the case of weight lifting, by lifting heavier weight our bodies are required to output more force production. To do this we enlist predominately type 2 muscle fiber. These fast twitch muscle fibers contract at a faster rate than slow twitch muscle fibers resulting in greater speed. We appear to be moving the weight slowly because it is so heavy. This type of training will not slow our game down. Training with heavier weight will result in faster movement on court.

* as you can plainly see you will not become slower by lifting weights but faster.


12-20 or above RM mainly train type 1 fibers. 6-12 RM can activate and train most type 2 fibers. Thus, 6-12 RM is most effective to build muscle since type 2 fibers have large hypertrophy potential.
Details will be discussed in the chapter of Resistance Training Program Design.
On average, most muscles consist of 50% of type 1, 25% of type 2a and 25% of type 2b fiber.
However, different muscles have different muscle composition, Those muscles which is always used for contraction have higher % of type 1 fibers such as lower back muscle. Also, those muscles which seldom contract have less % of type 1 and larger % of type 2 fibers such as shoulder muscles (deltoid).
Due to the genetic determination, the composition of muscle fibers vary in different people.
People with larger % of type 2 fibers will develop strength and muscle mass more easily.
People with larger % of type 1 fibers need to use slightly lighter resistance (10-12 RM) to develop muscle mass and strength. Similarly, people with larger % of type 2 fibers should use slightly heavier resistance (6-8 RM) to increase muscle mass and strength.