Force is made when the heads on the thick filament grip the thin filament and pull.
So the only part that can make force is the part where the two filaments overlap. A head with nothing in front of it has no partner to grip and does nothing.
The figure moves the length back and forth. When the overlap is wide there are many dark heads, and when it is narrow more of them turn pale. The dark ones are the heads that are working.
Held at 3.4 µm. The I band is wide and so is the H zone.
The thin filaments have withdrawn, so the heads near the centre cannot reach them. A head that cannot reach adds nothing, and the tension falls by that much.
Stretch past 3.6 µm and the overlap is gone altogether. No head can find a partner, and the tension is zero.
Now held at 1.7 µm. The H zone has closed and the thin filaments are crossing at the centre.
The thin filaments coming in from each side run into each other, and end up in front of the heads on the wrong side, which cannot use them.
Squeeze further and the ends of the thick filament run into the Z lines. From there the thick filament itself is being compressed, and the tension drops sharply.
All of that gathered into one curve is the length-tension relation. Sarcomere length runs along the bottom, and the tension it can make runs up the side.
There is a flat stretch between about 2.0 and 2.25 µm. Here every head reaches a thin filament and the tension is at its maximum. This is the optimal length.
To the right of it the curve falls in a straight line as the overlap shrinks. To the left it falls steeply, for the two reasons we have just seen.
In the body, the range a joint moves through keeps the muscle near that flat stretch. Where the tendons attach to bone is arranged so the muscle works where it is strongest.