Final answer:
Smaller motor units with lower-threshold motor neurons are recruited first during a muscle contraction of increasing intensity. As more strength is required, larger motor units with higher-threshold motor neurons are enlisted.
Step-by-step explanation:
The type of motor units that are recruited first during a muscle contraction of increasing intensity are the smaller motor units with smaller, lower-threshold motor neurons. These motor units are more excitable and fire first to activate the smallest muscle fibers. As more strength is needed, larger motor units with bigger, higher-threshold motor neurons are recruited to activate larger muscle fibers, resulting in a progressively stronger muscle contraction.