When does the demand for type 1 switch to type 2?

• A. Once power demand reaches +/- 20% of max
• B. Immediately at start of exercise
• C. After 60 seconds
• D. Only during max effort

Answer: (A)

Muscles recruit fibers in order of their speed and fatigue resistance, matching the demand for force. Slow-twitch Type I fibers handle lower-intensity work because they’re efficient and fatigue-resistant. When the power demand rises above what Type I fibers can sustain, larger, faster Type II fibers are recruited to generate more force. In many training contexts this shift starts around a practical threshold of about 20% of maximum power. So as you push beyond that level of demand, Type II fibers begin to contribute and take on more of the work as intensity increases. It’s not only at the start or only at max effort, and it’s not something that happens strictly after a fixed time like 60 seconds.