Which motor units are recruited first in volitional contraction and in NMES?

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Multiple Choice

Which motor units are recruited first in volitional contraction and in NMES?

Explanation:
In voluntary contractions, the nervous system uses the size principle: motor units are recruited from smallest to largest. Small motor neurons control slow-twitch, fatigue-resistant fibers and have lower activation thresholds, so they are activated first. As more force is needed, larger motor neurons (and their fast-twitch fibers) are recruited, allowing graded increases in force with relatively good fatigue resistance at low intensities. Electrical stimulation behaves differently. The external current more readily depolarizes larger-diameter axons, which have lower thresholds to extracellular stimulation, so larger motor units (fast-twitch fibers) are activated first. This leads to quick, strong contractions but tends to fatigue more rapidly. So, volitional recruitment starts with small motor units, while NMES tends to recruit large motor units first.

In voluntary contractions, the nervous system uses the size principle: motor units are recruited from smallest to largest. Small motor neurons control slow-twitch, fatigue-resistant fibers and have lower activation thresholds, so they are activated first. As more force is needed, larger motor neurons (and their fast-twitch fibers) are recruited, allowing graded increases in force with relatively good fatigue resistance at low intensities.

Electrical stimulation behaves differently. The external current more readily depolarizes larger-diameter axons, which have lower thresholds to extracellular stimulation, so larger motor units (fast-twitch fibers) are activated first. This leads to quick, strong contractions but tends to fatigue more rapidly.

So, volitional recruitment starts with small motor units, while NMES tends to recruit large motor units first.

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