Final answer:
When multiple motor units are stimulated in a muscle, the contraction becomes stronger due to motor unit recruitment. This process allows muscles to generate varying levels of force, from small to maximal strength, and is enhanced by summation and tetanus during rapid stimuli.
Step-by-step explanation:
If more than one motor unit in a single muscle is stimulated, the resulting contraction will grow progressively stronger. This is because of a process called recruitment, where sequentially larger and more motor units are activated to produce an increased contractile strength in the muscle. Initially, smaller motor units with more excitable motor neurons and smaller muscle fibers fire first, causing small contractions. As the demand for strength increases, larger motor units with bigger, higher-threshold motor neurons join in, and the muscle contraction becomes more forceful. This can go from picking up a feather using minimal force to lifting a heavy weight by engaging the largest motor units.
Summation and tetanus are additional factors that influence muscle contraction. Summation occurs when the fibers are stimulated while a previous twitch is still in effect, leading to a stronger contraction. If the stimuli are rapid enough, it can lead to tetanus, which is a continuous contraction without relaxation phases.