Not all muscle fiber is the same. Different types of fiber within the musculo-skeletal system respond to different types of stress, and your body will increase or decrease the size of these fibers in response to the frequency and volume of this stress. This post aims to distinguish the difference, and give examples of the different types of exercises that stimulate the different types of muscular fiber.
"Skeletal muscles are composed of fibers that have markedly different morphological and physiological characteristics." There are several different recognized systems of classification for these different fiber types based on a variety of criteria. The popular system that I will use in this post employs the terms "slow-twitch/type I," and "fast-twitch/type II" to separate skeletal muscle fiber into two primary categories. “A fast-twitch/type II motor unit (a motor neuron, and all the muscle fibers it innervates) is one that develops force rapidly. It also relaxes rapidly and thus has a short duration twitch time. In contrast, slow-twitch/type I motor units develop force and relax slowly and have a long duration twitch time.” Aside from mechanical characteristics, the two fiber types also exhibit a distinct difference in their ability to store and utilize a respective supply of energy for muscular contraction, and thus withstand fatigue. “Type I fibers are generally fatigue resistant and have a high capacity for aerobic energy supply, but they have limited potential for rapid force development. Type II fibers are essentially the opposite, characterized by fatigability, low aerobic power, rapid force development, and high anaerobic power.”
Lets put the differences of the two fiber types into layman’s terms. One of my favorite ways to compare is the “sprinter v.s. distance runner” model. Type I/slow-twitch fibers are the “cardio” or distance runner fibers. These are the fibers that will propel him/her at a medium intensity pace over a long course/period of time. Type II/fast-twitch fibers are the “strength” or sprinter fibers. These fibers can produce a high amount of force/strength very rapidly, but cannot sustain this level of output for a long period of time.
In an untrained individual, a group of muscles such as the quadriceps will have a fairly even mixture of type I and type II fibers so that this individual can respond to low power/long duration as well as high power/short duration movements in the legs, e.g. jogging as well as sprinting. With the introduction of specific types of training, this 50/50 ratio will start to shift towards the style with the dominating amount of volume and intensity. The shift won’t occur in the number of fibers in the group, but rather the efficiency and/or the size of the respective half of the 50/50 split. Basically, the style of training you use most often and with the most intensity will determine the output level and local endurance of the type I and type II fibers within the quadricep. If you are a sprinter, your type II fibers will increase in size, consume and be able to process more sugar, and improve the high output amount over a short range of time. If you are a distance runner, your type 1 fibers will stay at a relatively similar size, increase in mitochondria content, consume and be able to process more fat (which breaks down to sugar eventually) utilizing oxygen, and improve the medium output/endurance over a long range of time.
Make sure you realize these differences when selecting your exercise styles. Selecting a high level of exercise that stimulates either type I or type II will stimulate more of the hormones and utilize more of the specific fuel source that support the respective fiber type. You will need to eat accordingly and expect to see related bodily changes based on your selection, e.g. sprinting might make your natural levels of testosterone and growth hormone increase, as well as increase the size of the quads, glutes, hamstrings, and calves, whereas distance running might make your estrogen levels and cortisol increase, as well as the oxidative abilities of the muscle.
Another way to look at it: don’t rely solely on jogging if you wish to gain strength and size in the legs, and don’t rely solely on sprinting if you are aiming to improve your long distance endurance abilities. This comparison can be used for all the major compound portions of the body including the chest, shoulders, back and core muscle groups.
Adaptations to resistance v.s. aerobic endurance training
- Variable / Resistance / Aerobic
- Size of the muscle fibers / increase / no change
- Number of muscle fibers / no change / no change
- Movement speed / increase / no change
- Strength / increase / no change
- Aerobic capacity / no change / increase
- Anaerobic capacity / increase / no change
Relative involvement of muscle fiber types in different sports events
- Event - Type I - Type II
- 800-m run - high - high
- 100-m sprint - low - high
- Marathon run - high - low
- Barbell squat - med - high
- Olympic lifting - low - high
- Soccer - high - high
- Field hockey - high - high
- Football wide receiver - low - high
- Football lineman - med - high
- Distance bicycling - high - low
- Mountain bicycling - high - high
- Basketball - med - high
www.formandfunctiontraining.com - E of S&C
Topic: Primary differences between slow-twitch & fast-twitch muscular fiber, J Anderson (Read 1053 times)