Muscle tissue is categorized into three separate types based on function, control, and ability to produce force based on its size and shape. These categories are skeletal muscle, smooth muscle, and cardiac muscle. For our intended purpose we will be focusing on the skeletal muscle category.
Skeletal muscle attaches to the skeleton via tendons and through contraction, exerts force on the bones causing them to move, is considered voluntary muscle because it is normally under the conscious control of the individual.
Skeletal muscle can be divided into two general groups based on how quickly it contracts: slow-twitch fibers and fast-twitch fibers.
Slow-twitch fibers (or Type I) contain relatively large amounts of mitochondria and are surrounded by more capillaries that fast-twitch fibers. Additionally, slow-twitch fibers contain higher concentration of myoglobin* than fast-twitch fibers. The high concentration of myoglobin, the large number of capillaries, and the high mitochondrial content make slow-twitch fibers resistant to fatigue and capable of sustaining aerobic activity. furthermore, as the name implies slow-twitch fibers create lower force outputs but are more efficient than fast-twitch fibers.
Fast-twitch fibers ( or type II) are split into two subtypes: Type IIx and Type IIa.
Type IIx: fibers are the largest and fastest and are capable of producing the most force of all the skeletal muscle fibers, but contain a relatively small number of mitochondria, have a limited capacity for aerobic activity and are notably less efficient than type I muscle fibers causing faster fatigue. However, they also have a high number of Glycolytic enzymes** which provide them with a considerable anaerobic capacity.
Type IIa: muscle fibers possess speed, fatigue resistance, and force-production capabilities somewhere between type I and type IIx muscle fibers. They can sustain an effort for longer than type IIx fibers, up to three minutes in highly trained athletes. They are unique in that they are highly adaptable. That is, with proper training, can increase their oxidative capacity (muscle’s maximal capacity to use oxygen) to levels similar to Type-1 fibers.
A muscles fiber-type composition is usually and equal mixture of both fast- and slow- twitch fibers, though some muscle groups are known to be made up of primarily fast- or slow- twitch muscle fibers. percentages can be influenced by things like genetics, hormones, and activity level/habits.
Fiber composition is thought to play an important role in sport and exercise performance, it is commonly believed that successful power athletes possess a relatively large percentage of fast-twitch fibers, whereas endurance athletes possess a high level of slow-twitch fiber. However, it should be noted muscle-fiber composition is only one small factor in overall physical performance.
* A red protein that carries and stores oxygen in the muscle cells.
** Glycotic enzymes burn glucose and help produce ATP (energy-carrying molecule).