Final answer:
Type S (slow oxidative) motor units can contract for many hours due to their slow contractions and utilization of aerobic respiration to generate ATP. Option A is correct.
Step-by-step explanation:
The type of motor unit that can contract for many hours is type S, which stands for slow oxidative (SO) fibers. These muscle fibers are able to sustain long contractions because they contract relatively slowly and use aerobic respiration to produce ATP (adenosine triphosphate), providing them with a high endurance capacity.
In contrast, fast glycolytic (FG) fibers contract rapidly and use anaerobic glycolysis, making them fatigue quickly. Likewise, fast oxidative (FO) fibers contract faster than SO fibers and primarily use aerobic respiration but may switch to glycolysis under certain conditions, thus, they can also fatigue more quickly than SO fibers.
The type of motor unit that can contract for many hours is Type S. Type S motor units are slow-twitch muscle fibers that have a high resistance to fatigue and are capable of sustaining contractions over long periods of time. They are primarily used for endurance activities such as long-distance running or cycling.