Type I motor units are not able to generate as much force as Type IIA motor units because they are recruited less quickly.

When talking about locomotion, and also about the generation of force by muscles in order to carry loads, it is first important to know that muscles are composed by different types of fibers that are all connected through a motor plate and the innervation from a motor neuron, and whose job is to respond to stimulus in an organized pattern, depending on the specific reason as for which the fibers were stimulated.

Muscles are composed mostly by two big groups of fibers: slow-twitching fibers (Type I) and fast-twitching fibers (Type II A and B). These two groupings, have different excitatory levels and thresholds and each of them contract differently to allow a muscle to perform a specific task. On the grander scale of things, Type I fibers are much slower than Type II A and B, and the amount of force, or speed, they can generate, is not very high. However, they are the first ones to be recruited when movement initiates. Fast-twitching fibers, or Type A and B, however, are highly excitable, their time of response to stimulus is really fast and in general they are recruited both when speed is needed, or, when a load's weight increases, as they are able to generate more power and therefore can carry bigger and heavier loads.

So essentially, it is true that Type I motor units are not able to generate as much force as Type II A and B fibers basically because their excitatory threshold is not as high as that of slow-twitching fibers. However, it must also be established that Type I fibers are the first ones to always be recruited, not the Type II, unless, the work needed from the beginning, requires speed, or muscle power to carry a heavy load. If this is not the case, there are instances in which Type II fibers may not even be engaged in movement.