Final answer:
No contraction occurs with a low-intensity stimulus because it does not trigger an action potential, hence calcium is not released, actin and myosin can't interact, and there is no power stroke to shorten the muscle fibers.
Step-by-step explanation:
No contraction is measured in the muscle with a low-intensity stimulus because the stimulus is insufficient to reach the threshold that triggers an action potential in the muscle fiber's sarcolemma. At the molecular level, the interaction between actin and myosin filaments, essential for muscle contraction, requires the presence of calcium ions. Calcium is released from the sarcoplasmic reticulum (SR) when an action potential is generated.
If the stimulus is too low to cause an action potential, calcium is not released. Therefore, the troponin-tropomyosin complex does not change its configuration to expose the binding sites on actin. As a result, myosin heads cannot attach to actin, and thus, the power stroke that pulls actin filaments toward the center of the sarcomere (the M line) does not occur, preventing muscle contraction.
Muscle tone is a state of partial contraction in resting muscles, which maintains posture and readiness for action. However, it does not generate significant movement. Also, without adequate frequency of action potentials and recruitment of multiple motor units, the muscle will not generate a substantial contraction. Therefore, low-intensity stimuli that do not meet the threshold for these processes will not result in muscle contraction.