Review Question - QID 213070

Type I muscle fibers are slow oxidative and are the first to be recruited during muscle activation.

Variability in the isoforms of the contractile and structural proteins result in the different myofibril types. Most important are the types of myosin ATP, which produce fast and slow forms of myosin, resulting in three general muscle fiber types: slow oxidative (type I), fast oxidative-glycolytic (type IIA), and fast glycolytic (type IIB). When myofibrils are activated, the smaller type I units are activated first, whereas the larger type IIA and IIB units are activated when more forceful contractions are necessary. Type I fibers are smaller and contract more slowly and less forcefully but are fatigue-resistant. Type IIA units are intermediate, and type IIB units are the fastest and most powerful but also the most fatigue-prone.

Staron published a review on human muscle fiber types. They state that children (2-5 years old) have a higher percentage of type I fibers than newborns and adults. Aging causes loss of function from sarcopenia (loss of muscle mass, loss of motor units, particularly type II) and reduced maximum oxygen consumption beginning at 25 years old. Regarding sex differences, females have muscles 40% smaller than men because of smaller fibers and fewer total numbers of fibers diameter cross-sectional area.

Scott et al. published a review on human skeletal muscle fiber type classifications. They reported that muscle fiber types can be described using histochemical, biochemical, morphological, or physiologic characteristics. Histological analysis, in particular, showed that there is a correlation between myosin ATPase activity and the speed of muscle shortening, and led to the original division of muscle fibers into type I (slow) and type II (fast).

Incorrect Answers:
Answers 1, 2, 3, and 5: Type I muscle fibers are slow oxidative and are the first to be recruited during muscle activation.