Muscle fiber types can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers. Type II fibers can subsequently be broken down into two types: type IIA, which is referred to as "fast twitch oxidative glycolytic", and type IIX, which is referred to as "fast twitch glycolytic". Slow twitch fibers are referred to as "slow twitch oxidative". Type I fibers are characterized by low force/power/speed production and high endurance, Type IIX fibers are characterized by high force/power/speed production and low endurance, while Type IIA fall in between the two.
- Type I (slow twitch oxidative), red in color
- Type IIA (fast twitch oxidative glycolytic), red in color
- Type IIX (fast twitch glycolytic), white in color
|Characteristic||Type I (SO||Type IIA (FOG)||Type IIX (FG)|
|Contraction time||Slow||Fast||Very fast|
|Size of motor neuron||Small||Large||Very large|
|Resistance to fatigue||High||Intermediate||Low|
|Activity used for||Aerobic||Long term anaerobic||Short term anaerobic|
|Force production||Low||High||Very high|
|Major storage fuel||Triglycerides||CP, Glycogen||CP, Glycogen|
- muscle fiber is also known as "slow twitch oxidative" fibers. Muscle fiber types can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers. Type I fibers are used in lower-intensity exercises such as very light resistance work aimed at muscular endurance and long-duration aerobic activities such as 5K and 10K runs. Type I fibers are identified by slow contraction times and a high resistance to fatigue. Structurally, they have a small motor neuron and fiber diameter, a high mitochondrial and capillary density, and a high myoglobin content. ST fibers also have a low supply of creatine phosphate, low glycogen content, and a high store of triglycerides (the stored form of fat). ST fibers contain few of the enzymes involved in glycolysis, but contain many of the enzymes involved in the oxidative pathways (Krebs cycle, electron transport chain). Type I
ST fibers are predominantly used for aerobic activities requiring low-level force production, such as walking and maintaining posture, but are also the primary fiber type found in endurance athletes. Most activities of daily living use ST fibers.
Type II muscle fiber is also known as fast twitch muscle fibers. These fast twitch fibers can be further categorized into Type IIa and Type IIx fibers, which are also known as "fast twitch oxidative" and "fast twitch glycolytic," respectively. Type I fibers are characterized by low force/power/speed production and high endurance, Type IIx fibers are characterized by high force/power/speed production and low endurance, while Type IIA fall in between the two.
It is possible that a fiber might be transformed from Type IIx to Type IIA with exercise training. Furthermore, researchers at Boston University School of MedSTDicine have found that increasing the mass or size of type II muscle fibers will lead to a significant decrease in fat mass or the amount of fat in the body. A new study in the February issue of Cell Metabolism suggests that in regards to weight loss, .
Type IIA fibers (Fast Twitch 1), or fast oxidative fibers, are used more during sustained power activities such as sprinting 400 meters or doing repeated lifts with a weight below maximum (but not with very light weights). They contain very large amounts of myoglobin, very many mitochondria and very many blood capillaries. Type IIA fibers are red, unlike Type IIx fibers, which are white. Type IIA fibers have a very high capacity for generating ATP by oxidative metabolic processes, and split ATP at a very rapid rate. They have a fast contraction velocity and are resistant to fatigue. Such fibres are infrequently found in humans.
Type IIx FibersEdit
Type IIx fast-twitch fibers (Fast twitch 2), or fast glycolytic fibres, are recruited for very short-duration high-intensity bursts of power such as maximal and near-maximal lifts and short sprints. Type IIx fibres contain a low content of myoglobin, relatively few mitochondria, relatively few blood capillaries and large amounts glycogen. Type IIx fibres are white, while Type I (slow twitch) fibers are red. Type IIx fibers are geared to generate ATP by anaerobic metabolic processes, however, they are not able to supply skeletal muscle fibres continuously with sufficient ATP, and fatigue easily. Type IIx fibers split ATP at a fast rate and have a fast contraction velocity. Such fibres are found in large numbers in the muscles of the arms.
Determining Type of FiberEdit
The only way to directly determine the fiber-type composition in an athlete is to perform an invasive muscle biopsy test. Since this is not always feasible, an indirect method that can be used to determine the fiber composition of a muscle group is to initially establish the athlete's 1RM. Then the athlete performs as many repetitions at 80% of 1RM as possible. Fewer than seven repetitions and the muscle group is likely composed of more than 50% FT fibers. Greater than 12 and the muscle group has more than 50% ST fibers. If the athlete can do between 7 and 12 repetitions, then the muscle group probably has an equal proportion of fibers. This method does not work for individual muscles, just muscle groups.
- ↑ 1.0 1.1 1.2 Elizabeth Quinn (October 30, 2007). Fast and Slow Twitch Muscle Fibers. About.com. Retrieved on 2008-05-13.
- ↑ 2.0 2.1 2.2 Baggett, Kelly (n.d.). Understanding Muscle Fiber Types. Bodybuilding.com. Retrieved on 2008-10-17.
- ↑ 3.0 3.1 3.2 Leyland, Tony (July 2008). "Human Power Output and Crossfit Metcon Workouts". CrossFit Journal (71).
- ↑ 4.0 4.1 4.2 Karp, Jason R.. Muscle Fiber Types and Training. Coachr.org. Retrieved on 2008-10-17.
- ↑ University of Oregon (n.d.). Muscle Physiology. UOregon.edu. Retrieved on 2008-05-13.
- ↑ Nick H. (n.d.). Type I Vs Type II Muscle Fibers and Weight Lifting Programs. EzineArticles.com. Retrieved on 2008-05-13.
- ↑ Dr. Sanjukta Acharya (review) (5 February 2008). Type 2 muscle important in body metabolism and obesity. Rxpgnews.com. Retrieved on 2008-05-13.
- ↑ 8.0 8.1 Brian Mac (n.d.). Muscle Types. BrianMac.co.uk. Retrieved on 2008-05-13.