Jackknife
You should do this move only for 3 seconds. Lie flat on your back. You can place a rolled up towel under your lower back for support. Stretch out your arms, above your head and tighten your abdominal muscles. Then lift up your legs and arms together and try to touch them above your body. Lower them and repeat. To make this exercise a little easier, you can place your legs straight above your body and touch them with your hands. This will increase the flexibility in the core area.
Push-up and Side Plank
Do a regular pushup with hands placed wider than the shoulders and your legs straightened behind you. Pull in your abdominal muscles and keep your body in a straight line as you continue the pushups. As you complete each push up, bring your body to the side and do a side plank.