Module 9.14 Fast, slow, and intermediate skeletal muscle fibers differ in size, internal structure, metabolism, and resistance to fatigue.

slow fibers also contain a red pigment myoglobin,

a globular protein that is structurally related to hemoglobin, the red auction carrying pigment in blood

because slow fibers have both an excessive capillary supply and a high concentration of myoglobin, skeletal muscles dominated by slow fibers appear

darker in color

fast fibers have large diameters and contained

densely packed myofibrils, large glycogen reserves, and relatively few mitochondria

Intermediate fibers have more extensive capillary networks around them and have more resistance to fatigue than

fast fibers

most of the skeletal muscle fibers in the body are called

fast fibers because they can reach a peak twitch tension in 0.01 seconds or less after stimulation

Intermediate fibers are more closely related to

fast fibers because they contain more myoglobin and look relatively pale

the human body has three major types of skeletal muscles

fast fibers, slow fibers, an intermediate fibers

The percentage of fast versus slow fibers in each muscle is determined

genetically

There are no slow muscle fibers in the muscles of the

hands or eyes

the ratio of intermediate fibers too fast fibers can

increase as a result of athletic training. For example, if a muscle is used repeatedly for endurance events, some of the fast fibers will develop the appearance in functional capabilities as intermediate fibers

both myoglobin and hemoglobin respectively bind to

oxygen molecules

slow muscle fibers are surrounded by a more extensive network of capillaries then fast muscle tissue, so they have a dramatically higher

oxygen supply to support mitochondrial activity

slow fibers are specialized to continue contracting for extended

periods of time , long after a fast fiber word become fatigue

the tension produced by a muscle fiber is directly proportional to the number of myofibrils, so muscles dominated by fast fibers produced

powerful contractions

skeletal muscles demonstrated by fast fibers appear pale and are often called

white muscle

Fast muscle fibers and slow muscle fibers have different

sizes, densities, number of mitochondria, and amount of capillary supply.

Many back and calf muscles are dominated by

slow fibers because these muscles are continuously contracting in order to maintain upright posture

myoglobin is most abundant and slow fibers

so resting slow fibers contain substantial oxygen reserves that can be mobilized during a contraction

however, fast fibers fatigue rapidly because

their contractions use ATP in massive amounts, and they have relatively few mitochondria to generate ATP . as a result, prolonged activity is supported primarily by anaerobic metabolism

slow fibers have only about half the diameter of fast fibers and take

three times as long to reach peak tension after stimulation