Final answer:
The muscles in a single motor unit are defined by their shared fiber type, contraction speed, and fatigue resistance, as they are all innervated by the same motor neuron.
Step-by-step explanation:
The two shared characteristics that define the muscles innervated by a single motor neuron, or the fibers belonging to a single motor unit, are fiber type and contraction speed along with fatigue resistance. A motor unit comprises a motor neuron and the group of muscle fibers it innervates. These fibers typically share the same characteristics, as they respond to signals from that singular motor neuron. For example, all fibers within a motor unit can either be fast-twitch or slow-twitch, which in turn affects their contraction speed and fatigue resistance.