There are three types of muscle fibers.
Type 1 are slow twitch, oxidative fibers or red fibers. They respond to electrical stimuli slowly. They can resist fatigue with repeated stimulation. They possess many mitochondria and have a higher lipid content. Endurance training enhances metabolism of these fibers.
Type 2a are fast twitch, oxidative-glycolytic fibers and have inttermediate properties between type 1 and type 2b.
Type 2b are fast twitch, glycolytic fibers. they are also called white fibers. They respond rapidly and with greater force of contraction but also fatigue rapidly. These fibers contain more glycogen and have higher myophosphorylase and myoadenylate deaminase activity.
Strength training like weight lifting, sprinters, jumpers lead to hypertrophy of these fibers.
Muscle contraction
Muscle contraction occurs by shortening of myofilaments within muscle fibers. Stimulation causes an action potential to be transmitted along the sarcolemma, then through the T-tubule system to the sarcoplasmic reticulum. This causes release of calcium into the sarcoplasm.
As the calcium concentration increases, actin is released from a state of inhibition, allowing actin-myosin cross-linkage and shortening of the myofilaments. The muscle fiber shortens until calcium is actively pumped back into the sarcoplasmic reticulum, which breaks the cross-links to normal fiber contraction and relaxation.
