Detailed measurements of sarcomere length and tension during isometric contraction in striated muscle provided crucial early support for the sliding-filament model of muscle contraction. Based on your understanding of the sliding-filament model and the structure of a sarcomere, propose a molecular explanation for the relationship of tension to sarcomere length in the portions of Figure $Q 16-1$ marked $I,$ II, III, and IV. (In this muscle, the length of the myosin filament is $1.6 \mu \mathrm{m}$, and the lengths of the actin thin filaments that project from the Z discs are 1.0 $\mu \mathrm{m} .$