At approximately what percent of maximal demand does CNS stimulation begin recruiting Type IIa muscle fibers?

• A. 10 percent of maximal
• B. 20 percent of maximal
• C. 35 percent of maximal
• D. 50 percent of maximal

Answer: (B)

When the body needs more force, the nervous system recruits muscle fibers in order of increasing size and speed, so slow-twitch, fatigue-resistant units do the work first, and faster, more powerful units come in as demand grows. Type I fibers handle low levels of effort, but as the workload increases beyond what they can sustain, Type IIa fibers are recruited to provide more power without waiting until the absolute maximum is reached. This transition to include Type IIa typically happens at a relatively modest level of maximal demand—around twenty percent. From there, as demands continue to rise, further recruitment of faster fibers (and eventually Type IIx) occurs. Individual factors like training and fiber composition can shift this threshold, but twenty percent is a commonly cited point for the onset of Type IIa involvement.