Final answer:
Varying stimulus strength affects twitch force as it correlates to the number and frequency of action potentials received by muscle fibers, controlling myofiber activation and twitch force through graded responses, summation, and tetanus.
Step-by-step explanation:
Varying the stimulus strength affects twitch force because the tension in muscle fibers is related to the number and frequency of action potentials received from motor neurons. Stimulus strength directly influences how many muscle fibers are activated and the rate at which they are stimulated. Graded muscle responses and mechanisms such as summation and tetanus enhance the force of muscle twitch.
Each muscle fiber fully contracts when stimulated; however, different tasks require varying force levels. For lifting light objects like a pencil, fewer motor neurons signal fewer myofibers, resulting in a weaker twitch. Conversely, activities like lifting heavy weights call for stronger twitches, achieved by signaling more myofibers and increasing action potential frequency. Wave summation occurs when a muscle fiber is stimulated before a previous twitch has fully relaxed, leading to a more potent contraction. Calcium ion release during each stimulus facilitates the increased tension by allowing more sarcomeres to activate.
Ultimately, the maximum tension is reached during a state of incomplete tetanus, characterized by rapid contraction cycles and a short relaxation phase. If stimulus frequency is sufficiently high, removing the relaxation phase entirely, the muscle enters a state of complete tetanus with continuous contraction.