His knees do not flex as readily, limiting his power output.

He stopped at her side; his knees flexed.

However, the body should be relaxed and the knees nicely flexed.

He turned, braced, knees flexed, and expected to be struck from behind.

The extra force is needed because, during running, the knees must stay flexed to enable the leg to act as a spring.

It is most easily palpated (felt) with the knee partially flexed.

His knees flexed but didn't buckle as he'd thought they might, especially the left one.

His knees flexed, and he leaped to one side.

His knees flexed until he was almost squatting on the ground.

Jennifer stood still, knees flexed for flight, and watched them come.