What accounts for the decrease in the length of an entire muscle?

Combined decrease in sarcomere length.

Explanation:

Sarcomere is the functional unit of the striated muscle tissue. The units get repeated between the two Z lines. The striation under the microscope is shown by the sarcomere.

Actin and myosin protein plays an important role in the muscle contraction. The individual length of the thick (myosin) and thin (actin) filament do not changes and slide over one other. But the sarcomere overall length decreases that result in muscle contraction.

Thus, the answer is combined decrease in sarcomere length.