Muscle Fiber Types
Two types of skeletal muscles are the slow twitch fibers or Type 1 fibers and the fast twitch fibers or Type II fibers.
The fast twitch, which is also called white muscles fibers because of lack of blood, focuses on anaerobic exercise, primarily used in bodybuilding or sprints. Bodybuilders have a greater number of fast twitch muscle fibers or Type II.
The slow twitch muscle fibers are often referred to as red muscle fibers because of the flow of blood and is focused on endurance activities, such as running, rowing, etc. Long distance runners, marathoners, tri athletes have a great muscle density of Type I muscle fibers or the slow twitch.
Skeletal Muscle Anatomy
Muscle cells are composed of myofibrils, actin, myosin, and sarcomeres. The myofibrils are protein strands that run the length of each muscle fiber. Several proteins make up the myofibril, and the main muscle proteins are actin and mysosin. The acting and myosin proteins are contractile proteins. Sarcomeres repeat along the muscle cell length.
Muscle Contraction
According to the sliding filament theory, muscles contract when myosin projections attach to the actin, forming an actinmyosin cross bridge. ATP energy causes myosin heads to swivel to the center of the sarcomere, pulling the attached actin filament to actin slides inward toward center of sarcomere, which shortens the sarcomeres, shortening the entire muscle fiber.
A concentric contraction or a positive contraction shortens muscle fibers, eccentric contraction or a negative contraction lengthens muscle fibers. An isometric contraction occurs when no change in the length of whole muscle. The force of contraction is a function of muscle fiber size an the number of motor units, the activity of the number of motor units.
