Question

18) How would fiber composition affect the speed and tension of different muscles, for example, the ocular vs. intercostal vs soleus muscles?

Answer 1

FIBER COMPOSITION OF MUSCLES

Different types of muscle fibers possess different contractile properties. Therefore, their distribution within a muscle influences the contractile performance of a whole muscle. However, because human muscles are composed of a mix of fiber types, fiber type has less influence on the force-producing capacity of a muscle than do the factors discussed to this point. There are a variety of ways to categorize voluntary muscle fibers based on such characteristics as their metabolic processes, their histological composition, and their phenotype. A classification based on metabolic properties classifies most human muscle fibers as type I, type IIa, or type IIb fibers. In general, the contractile force of a type IIb fiber is greater than that of a type I fiber.Thus muscles composed of more type IIb fibers are likely to generate larger contractile forces than a comparable muscle consisting of mostly type I fibers . Type I fibers are innervated by small-diameter axons of the motor nerve. They are recruited first in a muscle contraction. Type IIb fibers are innervated by large axons and are recruited only after type I and type IIa fibers. Type IIb fibers are recruited as the resistance increases.

Fiber composition of intercostal muscles

Intercostal muscles include slow and fast fibres which reflects their functional tasks. While quiet breathing uses mainly slow fibres, fast muscle fibres are recruited specifically when the breathing rate increases—a shift in recruitment from slow to fast muscle fibres during tachypnea. Based on their pattern of activity the respiratory muscles can be classified into two main groups. During inspiration electromyographic (EMG) activity can be recorded in the diaphragm, scalenus, sternomastoid, parasternal, internal intercostal, and external intercostal muscles while during expiration the internal intercostal and abdominal muscles are active. During quiet breathing most of the respiratory work is carried out by the diaphragm and other respiratory muscles (accessory respiratory muscles) become important only when required by increased ventilation depth. The diaphragm is therefore engaged in a continuous rhythmic activity which does not permit any pause to rest, so the diaphragm fibres must be very resistant to fatigue.

Fiber composition of ocular muscles

The myosin heavy chain pattern of adult Extra Ocular Muscles is different from that in adult skeletal muscle. In adult ocular muscle single muscle fibers express developmental myosin heavy chain (MHC) isoformsin combination with adult MHC isoforms. Furthermore, in adult ocular muscles, variations in MHC characteristics are observed along single muscle fibersand an extraocular specific MHC isoform is detected.Normal adult extrafusal skeletal muscle fibers contain only an adult MHC pattern.

Fiber composition of soleus muscle

The soleus muscle contained predominantly slow twitch (Type I) fibers with the mean for all subjects, while the ocular muscles are expressed as MHC isoforms. Slow twitch muscle fibers are predominantly used during aerobic exercise, such as long-distance running. These fibers contract slowly and have a very high aerobic capacity. As a result, they produce ATP through aerobic glycolysis.