The Jackknife exercise works your shoulders, back, abs, obliques and legs.

Lift your hands up to the sky. Lift your right leg. Keeping the ribs tucked in, fold yourself in half, reaching out through to your fingertips, lower yourself to your leg and reach yourself all the way back up to the starting point. You can tap your leg down in between. And we'll do another set on the other side. Roll yourself down, keep reaching for those fingers. Lift up, press through the hands, and lower the leg.