Top leg hamstrings are lengthening. Tightness there can contribute to spinal flexion. The bottom leg’s hamstrings are active and help counter spinal flexion with extensor action.

With the bottom leg extended, there is more adduction on the top leg and possibly more internal rotation which leads to increased length in the iliotibial band; gluteus minimus, medius, and maximus; piriformis; gemelli; and obturator internus.