Which muscle fiber type has the highest threshold of excitation?

• A. Type I
• B. Type IIa
• C. Type IIx
• D. Type IIb

Answer: (D)

When we look at how motor units are recruited, the nervous system tends to activate lower-threshold units first and reserve higher-threshold units for stronger demands. The highest excitation threshold belongs to the fast, glycolytic fibers, which are capable of producing rapid, powerful contractions but fatigue quickly. They have the largest motor neurons and require a much stronger neural drive to be recruited compared to more fatigue-resistant fibers.

So among the options, the fastest, most glycolytic fiber type is the one described as the highest-threshold. It’s recruited last and only when the force demand is substantial, which is why it’s the best answer here.