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