Final answer:
Type IIB/X fibers are indeed recruited last during muscle contraction which is true, but the claim that the alpha motor neuron is larger and has fewer inputs is false since larger motor neurons actually innervate more muscle fibers for greater force production.
Step-by-step explanation:
The statement that Type IIB/X fibers are recruited last during contraction is TRUE. These muscle fibers are fast-twitch glycolytic fibers, known for their high force production and rapid fatigue rate. They are generally recruited for short bursts of high-power activities.
However, the claim that an alpha motor neuron is larger and has fewer inputs is FALSE. Larger alpha motor neurons innervate more muscle fibers, not fewer, and are typically associated with higher-threshold motor units that generate greater force, as seen with Type IIB/X fibers. These neurons are recruited when more power is needed, using a process known as recruitment.
Thus, muscle fibers' recruitment order is determined by the size principle, where small motor units with smaller, more excitable motor neurons and muscle fibers are activated first, and as more force is needed, larger units with larger motor neurons are subsequently activated.