types of skeletal muscle fibers
motor units
composed of 1 fiber type
shoulders & arms
high in fast fibers
legs (postural & active)
high in slow & intermediate fibers
postural muscles (back & neck)
high in slow fibers
intermediate fibers
intermediate diameter; pink: little myoglobin but more capillaries; mitochondria & glycogen: ATP from aerobic & anaerobic processes; fast-twitch: fast ATPase; intermediate properties: faster & more fatigue resistant, but less strength & endurance
fast fibers
largest diameter: most powerful; white: little myoglobin & few capillaries w/ little oxygen; few mitochondria, lots of glycogen: ATP from glycolysis; fast-twitch: fast ATPase; fast & strong but fatigue rapidly: strength & speed activities
distribution of fibers
most muscles have all 3 types
regeneration of skeletal muscle
muscle fibers don't divide; satellite cells regenerate fibers to a small degree; muscle growth is due to hypertrophy (increased cell size due to more thick & thin filaments) significant muscle damage results in fibrosis
skeletal muscle fibers
vary in content of myoglobin, capillaries, ,mitochondria, & glycogen; variable speed of conttraction cycle due to differences in ATPase, variable sources of ATP
recruitment order
slow, intermediate, fast; ratio of fast & slow twitch fibers is genetic
slow fibers
smallest diameter: least powerful; dark red: lots of myoglobin & capillaries w/ lots of oxygen; many mitochondria: ATP mainly from aerobic respiration; slow twitch: slow ATPase (slow contraction cycle); slow but very resistant to fatigue: posture & endurance activities