Stability ball jackknife. A compound leg and core exercise. Target muscles: Iliopsoas (not abs. Visit site to learn why). Synergistic muscles: Tensor Fasciae Latae, Sartorius, Pectineus, Adductor Longus, Adductor Brevis, Rectus Abdominis, Obliques, Adductor Magnus, and Gluteus Maximus. Dynamic stabilizers: Hamstrings and Rectus Femoris. Keep your neck neutral. Pull your knees all the way up to your chest.