Final answer:
Muscle force is greater during repetitive stimulation compared to a single twitch due to the summation of muscle twitches, where the release of additional calcium ions leads to increased sarcomere activation, potentially resulting in incomplete or complete tetanus.
Step-by-step explanation:
The force produced by repetitive stimulation of a muscle fiber is greater than the force produced by a single twitch due to a phenomenon called summation of muscle twitches or wave summation. When muscle fibers are stimulated in rapid succession, the contractions can build upon each other because the muscle fiber does not fully relax in between the stimuli. This results from additional release of Ca++ ions, which are necessary for sarcomere activation that leads to muscle contraction. When these consecutive stimuli are close enough together, the muscle tension increases and eventually might reach incomplete tetanus, where the contractions occur so rapidly that only partial relaxation occurs between them. If the stimulation frequency is high enough, it can lead to complete tetanus, where the muscle is in a state of continuous contraction, producing maximal tension.