Humans rely heavily on skeletal muscles in the body for posture and locomotion, without them life as we know it would undoubtedly be much different. Skeletal muscles are made up of protein fibers (Silverthorn, Silverthorn, Johnson, Ober & Garrison, 2007). These fibers were originally categorized in the 19th century into two groups based on their contraction speed and rate of fatigue. Slow-twitch muscle tissue containing more myoglobin, an oxygen-transport protein, were observed to look redder and contract slower, while fast-twitch muscle cells were observed to contract more quickly and appear white (Zierath & Hawley, 2004).
Additional study of the fast-twitch fibers’ anaerobic and aerobic properties revealed two subgroups; fast-twitch oxidative (Type IIA) and fast-twitch glyolytic (Type IIB) (Brooke & Kasier, 1970). Today, muscle fiber groups are generally categorized by these three subgroups: slow-twitch muscle fibers (Type I), fast-twitch oxidative (Type IIA) and fast-twitch glyolytic (Type IIB), which combine in various ratios to form human muscles. In addition to the differences listed above, these muscle fiber groups also differ in oxygen use and energy generation, and play specific roles in several key physiological processes which we will explore in detail (Silverthorn et. al., 2007).
Slow-twitch muscle fibers have the slowest contraction rate, usually two to three times slower than fast-twitch fibers, and the lowest rate of fatigue of the three muscle fibers. For example, the contraction rate of slow-twitch muscle fibers have been observed to contract about 10-30 times per second, whereas fast-twitch muscles fibers, near peak performance, typically contract 30-70 times per second (Hamilton, 2017). This makes slow-twitch muscle fibers useful for steady state activities such as maintaining posture and walking (Karp, 2001). They are found in high proportions in secondary muscles, such as the soleus muscle of the calve. Slow-twitch are typically red in color, due to a high amount of myoglobin, a oxygen-transport protein, and contain three to five times the amount of blood flow compared to fast-twitch fibers (Reis & Wooten, 2008).
Slow-twitch muscle fibers fatigue much slower due a variety of factors, including an aerobic method of ATP production, a higher amount of mitochondria, and a greater number of blood vessels supplying oxygen relative to the fast-twitch group. (Karp, 2001). Another contributing factor to slow-twitch muscle fibers slow rate of contraction is having less myosin ATPase in the fibers’ filament. In addition, the duration of contractions are up to ten times longer when compared to fast-twitch subgroups. This is primarily due to the decreased rate at which CA²ᐩ is removed from the cytosol, a key factor of contraction duration. (Silverthorn et. al., 2007).
Conversely, fast-twitch muscle fibers are designed to have a fast contraction rate, up to two to three times faster when compared to slow twitch, but fatigue rapidly (Silverthorn et. al., 2007). Fast-twitch muscle fibers are found in muscles where the primary function is power output, such as the gastronecnemius in the calf. Both ATP splitting and CA²ᐩ reuptake rates are faster in the Type II fibers than compared to Type I (Karp, 2001). As stated above, fast-twitch muscles are categorized into two subcategories: fast-twitch oxidative (Type IIA) fibers and fast-twitch glyocolytic fibers (Type IIB) (Silverthorn et. al., 2007).
Type IIA fibers are used in moderate intensity activities such as running or weight lifting and have a medium endurance. This fiber is composed of medium amounts of myoglobin, mitochondria, and blood capillaries, which are used to generate ATP through both aerobic and anaerobic processes. While still capable of delivering a moderately high power output, Type IIA fibers fatigue less quickly than the Type IIB fibers by primarily using aerobic ATP generation. (Karp, 2001).
Type IIB fibers are used in high intensity and/or maximum effort activities such as sprinting and have the highest power output and speed of contractions of all fiber types. Composed of a low amount of myoglobin, few mitochondria, and very limited blood supply, Type IIB fibers generate ATP using only anaerobic processes, resulting in the most rapid fatigue of the three fiber types (Silverthorn et. al., 2007). Type IIB fibers can be found in areas of the body where maximum output is required, such as the thighs of a sprinter or the arms of a weightlifter (Karp, 2001).
Type I and II fibers are not distributed equally in all muscle types. For example, the larger gastronecnemius muscle in the calf, which is responsible for plantar flexion and knee bending, typically has a higher proportion of Type II fibers compared to the smaller soleus muscle in the calf. This is due to the large power output required by the gastronecnemius for activities like jumping and sprinting, compared to the secondary plantar flexion functions of the soleus, which aids activities including balance or walking. (Hamilton, 2017). A primary component of Type II fiber distribution and size is determined by the sex group of an individual. In general, men retain a higher amount of Type II fibers than women due to the effects of testosterone. Testosterone is a steroid hormone, produced primarily by males, that promotes synthesis of myosin and actin, two critical proteins for muscle function and development (Sherwood, 2013).
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