Final answer:
Lower repetition ranges with heavier weights recruit higher levels of motor units for strength and power, while higher repetition ranges focus more on endurance and engage slow-twitch muscle fibers.
Step-by-step explanation:
Training in the 4 to 6 rep range will recruit higher levels of motor units than training in the 12 to 15 rep range. This is because lifting heavier weights for fewer repetitions requires more force, which in turn recruits more motor units and their associated muscle fibers to perform the exercise. The higher intensity associated with lower rep training primarily activates fast-twitch muscle fibers, which are required for strength and power movements, while the higher rep range focuses more on endurance and utilizes more slow-twitch muscle fibers.