ABSTRACT
Some red fibers are slow-twitch and some are fast-twitch, whereas all white fibers are generally fast-twitch (Table 1). Whether a given fiber contracts quickly or slowly is dependent upon the rate of crossbridge cycling (see Topic I3). The faster the ATPase portion of the myosin head (myosin ATPase) can split ATP and then interact with actin (a process repeated many times during contraction), the faster the rate of shortening of the sarcomere. Thus, fibers that have fasttwitch characteristics express forms of myosin ATPase that have fast ATPase kinetics and those that have slow-twitch characteristics express forms of myosin ATPase that have slower kinetics. Histochemical staining with antibodies against the different forms of myosin can be used to distinguish fast-and slowtwitch fibers. Fast-twitch muscle fibers must also relax quickly, and since this is dependent on the sequestration of calcium ions into the sarcoplasmic reticulum by calcium-ATPases (see Topic I3), this enzymatic activity is high in fast-twitch muscle fibers.
