Final answer:
Muscle tension rises in a muscle twitch due to wave summation, where overlapping stimuli lead to more calcium ion release and increase cross-bridges between actin and myosin, facilitating stronger muscle contractions.
Step-by-step explanation:
Tension rises in the curve of a muscle twitch because of the phenomenon known as wave summation. As the stimuli to the muscle fibers overlap, more calcium ions (Ca++) are released, which means additional sarcomeres can become activated. This results in the formation of more cross-bridges between actin and myosin within the muscle fiber, generating greater muscle tension and causing the muscle to contract more forcefully.
When the frequency of motor neuron signaling increases, this can lead to a state of incomplete tetanus. In this state, the muscle undergoes rapid cycles of contraction with only brief periods of relaxation, vastly increasing tension. If the stimulation is high enough, it can lead to complete tetanus, where the contractions fuse and tension remains continuous due to a sustained high concentration of Ca++ ions in the sarcoplasm.