Final answer:
The decrease in the length of an entire muscle is due to the combined decrease in sarcomere length, as the actin and myosin filaments slide past each other, not because these filaments themselves shorten.
Step-by-step explanation:
The decrease in the length of an entire muscle is accounted for by the combined decrease in sarcomere length. This occurs during muscle contraction as explained by the sliding filament model of muscle contraction. When a muscle fiber contracts, it is the result of sarcomeres, which are the functional units of a muscle, shortening as myosin heads pull on the actin filaments.
It's important to understand that thick and thin filaments do not shorten themselves. Instead, they slide past one another, causing the sarcomere to shorten while the filaments remain at a constant length. This sliding process is what makes the muscle itself shorter and is powered by the hydrolysis of ATP. Thus, when all the sarcomeres in a muscle fiber shorten, the muscle fiber as a whole contracts.