Final answer:
Motor units that contain type I and type II fibers are organized based on a size factor, which determines the number of muscle fibers in the motor unit.
Step-by-step explanation:
Motor units that contain type I and type II fibers are organized based on a size factor. The size of a motor unit varies depending on the muscle. Some motor units may contain up to 1000 muscle fibers, while others may have only 10 fibers. The number of muscle fibers in a motor unit corresponds to the precision of control of that muscle.