Final answer:
The muscle's active force output decreases as it lengthens due to reduced overlap of myofilaments, meaning fewer cross-bridges can form, resulting in less tension. Thus, the statement is true and the given cause is false.
Step-by-step explanation:
The statement active force output of the muscle decreases as the muscle is lengthened is true. This is because when a sarcomere is lengthened, the overlap between the actin and myosin filaments—the myofilaments—is reduced. Less overlap means fewer cross-bridges can form between actin and myosin, which in turn results in less tension being generated during muscle contraction.
The reason 'There is more overlap of the myofilaments' as being the cause for the decrease in force is actually false. In fact, the opposite is true; the active force output decreases because there is less overlap of the myofilaments when the muscle is over-lengthened.
To summarize, the optimal length for muscle contraction is where there is the greatest overlap of thick and thin filaments, allowing for maximal cross-bridge formation and tension. Stretching the muscle beyond this optimal length results in decreasing force output due to reduced opportunities for cross-bridge formation.