What is meant by "summation" in muscle physiology?

Summation in muscle physiology refers to the increased force of contraction that occurs when a muscle fiber receives a second stimulus before the first contraction has fully relaxed. This phenomenon happens because the muscle cell is still in a state of some degree of depolarization and calcium ions remain available in the sarcoplasm, leading to a stronger contraction when the second stimulus arrives. Essentially, the muscle does not return to its resting state before the next action potential hits, resulting in additive contractions and producing a more forceful muscle contraction known as tetanus if the stimuli are frequent enough.

This concept is critical in understanding how muscles can increase their strength of contraction and respond in various ways to multiple stimuli, highlighting the efficiency and adaptability of the muscular system in response to activities that require varying degrees of force.