Final answer:
When a muscle is stimulated, whether it changes length depends on the load on the muscle. Muscle contraction mechanics involve calcium ion-regulated cross-bridge formation and ATP-powered power strokes, but adequate tension must overcome the external load for muscle shortening to occur.
Step-by-step explanation:
What determines whether a muscle will change length when stimulated is primarily the load on the muscle. If the load is higher than the tension the muscle fibers can generate, the muscle will not shorten and the muscle length will remain the same or even lengthen during contraction. Muscle contraction occurs as described in the sliding filament model, where the binding of calcium ions to troponin exposes active sites on actin, allowing the myosin heads to bind and form cross-bridges. The muscle fibers shorten as cross-bridges 'pull' on actin filaments during the power stroke, which is powered by the hydrolysis of ATP. However, if the load exceeds the generated tension, no shortening occurs. Therefore, the amount of load determines the possibility of muscle length change upon stimulation.