Final answer:
Wave summation occurs because muscle fibers are already partially contracted due to previous stimuli, leading to stronger subsequent contractions as further calcium ions are released, intensifying muscle contraction. It progresses from partial to complete tetanus as the frequency of stimulation increases, preventing muscle relaxation between stimuli.
Step-by-step explanation:
Wave summation occurs when successive stimuli are provided to a muscle before it has had a chance to fully relax after a previous twitch. This results in a stronger contraction because the muscle fibers are already partially contracted when the next stimulus arrives.
At the molecular level, the second stimulus causes the release of more calcium ions, making them available to activate additional sarcomeres, which are the fundamental units of muscle contraction. Consequently, the overall effect of successive stimuli is summed, intensifying the muscle contraction, referred to as wave summation.
This process can escalate to incomplete tetanus, where the muscle goes through quick cycles of contraction with very short relaxation phases, or if stimuli are frequent enough, complete tetanus, where the muscle no longer relaxes between stimuli, leading to a continuous contraction. The amount of tension produced by a muscle is determined by the number of cross-bridges formed between the actin and myosin within the muscle fibers.