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Muscle Fiber Type Evolution:
Almost all multicellular animals depend on muscle to move [1]. For many species, the ability to move is essential for catching prey or fleeing from predators contributing to a species' evolutionary fitness. Generally, muscular systems of multicellular animals are similar due to the fact that different muscle groups will contain proportions of both slow-twitch and fast-twitch muscle fibers. The proportions vary for unique purposes of the muscle which often are responses to the environment [2].
Invertebrates:
In a fiber polymorphism study of the American lobster, Homarus americanus, 3 fibers type were analyzed. The 3 distinct fiber types include: fast twitch fibers, slow-twitch and slow-tonic fibers [3]. Slow-tonic is a slow twitch-fiber that can sustain longer contractions (tonic) [4, 5]. Fast twitch fibers are primarily found in the cutter claw closer and in the deep abdominal flexor muscles. The slow-twitch fibers make up the majority of the fiber type for the crusher claw closer and are present in the leg opener and claw muscles, in superficial flexors and extensors of the tail and the ventral muscles of the cutter claw closer. The slow tonic fibers presence were mainly found within the distal fibers of the cutter claw closer, leg openers and claw [3].
Zebrafish embryo:
In early development of vertebrate embryos, growth and formation of skeletal muscle happens in successive waves or phases of myogenesis. The myosin heavy chain isotype is a major determinant of the specific fiber type. In zebrafish embryos, the first muscle fibers to form are the slow twitch fibers. These cells will undergo a radial migration from their original medial location to form a superficial monolayer of slow twitch muscle fibers. These muscle fibers undergo further slow twitch fiber differentiation as the embryo further matures [6].
Turtles:
In turtles, Trachemys scripta elegans, a muscle fiber type study indicated the percentage of different fiber types within different muscle groups. In larger animals, there is a trend of the need for different muscle groups for different purposes. This study looked at different neck and hindlimb muscles. What was interesting was that complementary muscles within the neck showed a potential inverse trend of fiber type percentages since transversalis cervicis (TrC) showed to primarily be composed of fast twitch fibers (the largest percentage of the 5 muscles studied) while it’s counterpart test-cervicis (TeC4) had the highest percentage of slow twitch muscle fibers. This trend is not seen within the hindlimbs of turtles as the complementary muscles: flexor digitorum (FDL) and external gastrocnemius (EG) have similar fiber type percentages leaning in favor of more fast twitch muscle fibers [4].
Mammals:
In a study of chimpanzee strength compared to humans, chimpanzee muscles are known to be composed of 67% fast-twitch fibers and have a maximum dynamic force and power output 1.35 times higher than human muscle of similar size. This is interesting when considering chimpanzees are one of our closest relatives. Among mammals, there is a predominance of type II fibers utilizing glycolytic metabolism. Because of the discrepancy in fast twitch fibers compared to humans, chimpanzees will outperform us in power related tests. Humans, however, will do better at exercise in aerobic range requiring large metabolic costs such as walking (bipedalism) [8].
Genetic Conservation versus Functional Conservation:
A potential mechanism behind changes in fiber type expression is the expression of the gene Prdm1. Within the zebrafish embryo, this gene is expressed and represses the formation of new slow twitch fibers through direct and indirect mechanisms. This gene is conserved genetically within mice, but is not functionally conserved. In zebrafish, Sox6, an indirect mechanism of repressing slow muscle genes, is controlled by Prdm1, but does not control slow muscle genes in mice. As a result, slow as well fast twitch fibers are expressed. This is an indicator of the importance of the necessity of both fiber types [7].
Plasticity and Environment:
In fish, different fiber types are expressed at different water temperatures [6]. Cold temperatures require more efficient metabolism within muscle and fatigue resistance is important. While in more tropical environments, fast powerful movements (provided from higher fast-twitch proportions) may prove more beneficial in the long run [9]
In a genetic determinism of fiber type proportions in human skeletal muscle study, genetics and environment play an equal role in fiber type determinism. The specific attributes of environmental influence investigated included diet, exercise and lifestyle types. Humans are able to consciously eat better food that will help build or train the muscle in desired outcomes. Aerobic exercise will shift the proportions towards slow twitch fibers, while explosive powerlifting and sprinting will transition fibers towards fast twitch [10]. In animals, "exercise training" will look more like the need for long durations of movement or short explosive movements to escape predators or catch prey [11].
The biggest evolutionary advantage among organisms with skeletal muscle is the plasticity of the muscle fiber types. In rodents such as rabbits and rats, the transitory nature of their muscle is highly prevalent. They have high percentage of hybrid muscle fibers and have up to 60% in fast-to-slow transforming muscle [12]. As previously mentioned, throughout history animals have had the ability to shift their phenotypic muscle fiber type proportions through training and/or responses to the environment [10]. This has served organisms well because an organism with higher proportions of slow twitch fibers that didn’t have fibers that could remodel into a faster twitch placed into an environment that would favor being able to move quickly would not survive. This change can be proven in bodybuilding, changes in muscle mass and force production can be made relatively quickly within a matter of months. [13].