To achieve maximal force, the CNS recruits which of the following?

• A. Only Type I fibers
• B. All motor units serving the muscle across Type I, IIa, IIb
• C. Only Type IIa and IIb
• D. Some Type I and IIa but not all

Answer: (B)

Maximal force comes from recruiting every motor unit available in the muscle, across all fiber types. The nervous system follows the size principle: it starts with small motor neurons that control Type I fibers and progressively recruits larger motor neurons that control faster, more forceful fibers (Type IIa and Type IIb) as the demand increases. When force demand is at its peak, all motor units are activated, including those that drive the fast, high-torque fibers. That’s why recruiting across Type I, IIa, and IIb yields the greatest possible force.