posed of50% fast-twitch motor units and 50% slow twitch motorunits. The A muscle

Question

posed of50% fast-twitch motor units and 50% slow twitch motorunits. The A muscle is com muscle is tetanized by stimulating the motor nerve at a very high frequency and relatively high voltage. The contractile strength declines over time (fatigue) as shown below. With two additional lines (marked as shown in the legend), show the changes in force generated by the fast-twitch (FT) motor units and the slow-twitch (ST) motor units over the period of time indicated. Explain your answer. 100 %max force whole muscle FT motor units_ ST motor units . . 1 2 34 5 67 89 10 20 30 minutes

Explanation / Answer

Type I (black)

Also called "slow twitch"

Take 110 ms to reach full tension when stimulated

~50% of fibers in an average muscle

Slow contractile speed:
Slow myosin ATPase
Low glycolytic capacity
Less developed SR

Low peak force:
Small motor neuron (innervates <300 muscle fibers)

High resistance to fatigue:
High in oxidative enzymes (used during the Krebs cycle)

Most useful for long duration steady pace

Peak force is low, but repeat contractions can occur for a long duration

Most activities of daily living use ST Fibers

Postural muscles have higher ^ of ST Fibers- small contractions are needed for extended periods of time.

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