Final answer:
A type I single muscle fiber would have similar contraction velocity to a type I motor unit, but the force produced would be less for the single fiber compared to the motor unit because a motor unit contains multiple muscle fibers that work collectively to generate a larger force.
Step-by-step explanation:
Regarding the differences between a type I single muscle fiber and a type I motor unit on a force-velocity curve, a single fiber contracted alone would produce less force than an entire motor unit, since a motor unit consists of multiple muscle fibers. Therefore, we can assume that the correct option is 1) the single fiber and motor unit would have similar velocity intercepts but different force intercepts (motor unit would be larger). Each motor neuron can innervate multiple muscle fibers, which allows motor units to generate greater force as a collective than an individual muscle fiber could alone. The velocities at which they contract would be similar because the type of muscle fiber (type I, which is slow-twitch) dictates this characteristic, and it remains consistent whether the fiber is contracting alone or as part of a motor unit.
The force produced by a motor unit is larger due to the cumulative force of all the muscle fibers it contains, whereas the velocity represents the contraction speed, which is inherent to the muscle fiber type, thus remains comparable between the single fiber and the motor unit.