How to Perform a Bent-Leg Jackknife.

Starting Position: Lie flat on your back on an exercise mat. Extend your arms completely straight behind your head and let your legs hover slightly above the floor.The Setup: Press your lower back firmly into the floor by drawing your belly button toward your spine. Maintaining this flat-back posture is crucial for keeping your core engaged.The Crunch: Inhale, then exhale as you simultaneously bend your knees and draw them toward your chest. At the same time, sweep your arms forward over your torso, lifting your head and shoulder blades off the mat to reach toward your feet or shins.