Final answer:
The increased tension during tetanus is due to an increased concentration of calcium ions, leading to continuous muscle contractions without relaxation phases, a phenomenon which lasts until muscle fatigue ensues.
Step-by-step explanation:
The increased tension seen during tetanus is explained by an increase in the concentration of calcium ions around the myofibrils. This calcium influx enables the ongoing formation of cross-bridges between actin and myosin filaments within the muscle cells, facilitating a continuous contraction. This state can endure until the muscle eventually fatigues and can no longer produce tension.
Tension increases in a graded manner, akin to stair steps, in a phenomenon known as treppe or the "staircase effect," where muscle contractions become more efficient. The continuous, uninterrupted muscle contraction that defines tetanus is opposite to a single twitch or graded contractions that include a relaxation phase.
Increase in motor unit recruitment would lead to more tension, as more muscle fibers would be activated, but this is not what causes tetanus. Instead, the persistent contraction during tetanus is due to the high frequency of motor neuron signals, which surpasses the need for relaxation phases typically seen in incomplete tetanus.