Final answer:
When a skeletal muscle cell is stimulated again before it has fully relaxed from a previous contraction, this leads to a condition called wave summation, potentially progressing to incomplete or complete tetanus depending on the frequency of the stimuli.
Step-by-step explanation:
If a laboratory skeletal muscle cell receives a single stimulus to contract, it produces a single muscle twitch. However, if we quickly fire another stimulus for the muscle to contract before the muscle has fully relaxed, we induce a condition known as wave summation. This occurs when successive stimuli are provided to a muscle still in its contraction phase from the previous stimulus, leading to an increase in tension due to the additional Ca++ ions released.
When the frequency of action potentials increases sufficiently, it may result in incomplete tetanus, where the muscle undergoes quick cycles of contraction with short relaxation phases. If stimuli are provided at an even higher frequency, eliminating the relaxation phase entirely, this can lead to a state called complete tetanus, where contractions become continuous.