Why does the rectus femoris create passive insufficiency during knee flexion?

The rectus femoris, as a part of the quadriceps muscle group, crosses both the hip and knee joints. During knee flexion, if the hip is also flexed, the length of the rectus femoris becomes insufficient to generate effective force because it is already elongated across both joints. This results in passive insufficiency, where the muscle cannot maintain optimal tension and ability to contract effectively, leading to a reduced capacity for knee flexion.