Final answer:
A change of temperature does not act as a direct stimulus for initiating a muscle contraction. Neurotransmitters, hormonal activity, and changes in pH can initiate contractions.
Step-by-step explanation:
Among the options given, the one that does not act as a stimulus to initiate a muscle contraction is a change of temperature. Muscle contractions are typically initiated by electrochemical signals. For instance, hormonal activity can influence muscle contractions by altering the metabolism and sensitivity of muscles to other stimuli. Neurotransmitters like acetylcholine (ACh) are crucial as they depolarize the muscle membrane at the neuromuscular junction, leading to an action potential that spreads across the muscle membrane. Changes in the pH of a muscle can affect enzyme activity and thus influence muscle contraction. However, a change in temperature, unless it affects the structure of proteins or alters the rates of metabolic reactions substantially, is not typically considered a direct stimulus for muscle contractions.