Final answer:
Summation in muscle cells is true and happens when stimulus frequency is high enough to cause action potentials to overlap, leading to increased muscle tension and a more powerful contraction.
Step-by-step explanation:
True, summation does occur in muscle cells when stimulus frequency increases, leading to a greater force of contraction. When a muscle fiber is stimulated by action potentials at a frequency that allows the development of force before the muscle can fully relax from the previous twitch, the tensions from each twitch combine, resulting in a more forceful contraction. This phenomenon is known as wave summation.
As the stimulus frequency further increases, the muscle can enter a state of incomplete tetanus, and eventually complete tetanus, where it exhibits a continuous contraction. Summation and tetanus significantly increase the amount of tension a muscle can produce.