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Muscle fiber types can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers.[1] 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.[2]

  • 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
Characteristics of the Three Muscle Fiber Types
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
Mitochondrial density High High Low
Capillary density High Intermediate Low
Oxidative capacity High High Low
Glycolytic capacity Low High High
Major storage fuel Triglycerides CP, Glycogen CP, Glycogen

Type I[]

Main article: Type I Muscle Fiber
Type I muscle fiber is also known as "slow twitch oxidative" fibers.[2] Muscle fiber types can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers.[1] 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.[3] 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).[4]

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 IIx Fibers[]

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.[3] Type IIx fibres contain a low content of myoglobin, relatively few mitochondria, relatively few blood capillaries and large amounts glyrs are red

The only way to directly determine the fiber-type composition in an athlete is to perform an invasive muscle biopsy test.[4] 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.[4]


  1. 1.0 1.1 Elizabeth Quinn (October 30, 2007). Fast and Slow Twitch Muscle Fibers. Retrieved on 2008-05-13.
  2. 2.0 2.1 Baggett, Kelly (n.d.). Understanding Muscle Fiber Types. Retrieved on 2008-10-17.
  3. 3.0 3.1 Leyland, Tony (July 2008). "Human Power Output and Crossfit Metcon Workouts". CrossFit Journal (71).
  4. 4.0 4.1 4.2 Karp, Jason R.. Muscle Fiber Types and Training. Retrieved on 2008-10-17.