Final answer:
Wave summation is when a muscle receives a second stimulus before complete relaxation causing greater tension. This leads to incomplete tetanus with quick contractions and short relaxation or complete tetanus with continuous contraction. It occurs due to extra Ca++ ion release and sarcomere activation.
Step-by-step explanation:
When a muscle receives a second stimulus before it has time to completely relax from the first one, the produced tension is greater. This phenomenon is known as wave summation. If action potentials continue to be sent to the muscle before previous twitches can fully relax, tension accumulates, eventually leading to a state called incomplete tetanus. During incomplete tetanus, the muscle alternates between quick contractions and shortened relaxation phases. If the frequency of stimuli becomes so rapid that relaxation phases are eliminated altogether, the muscle experiences complete tetanus, where contraction is sustained until the muscle fatigues.
At the molecular level, wave summation occurs because the second stimulus allows for the release of additional Ca++ ions, which activate more sarcomeres while the muscle is still contracting from the first stimulus. This results in a more powerful overall contraction of the muscle. In contrast, treppe is a different muscle contraction phenomenon where tension increases in a step-like fashion, resembling stairs, as the muscle is repeatedly stimulated after a period of rest.