Main body focus: abs.
Why do this?
Jack knives is an advanced version of the leg raises and help you to strengthen your (lower) abdominal muscles. It’s an effective exercise to help you gain a stronger core that is necessary for most sports varying from endurance to explosive motions.
How to do this?
- Lie on a flat bench or ground with your arms stretched out alongside your head.
- Your legs should be unsupported by the bench or ground from below your knees.
- With your feet together and your toes flexed upwards, raise your straight legs up a few cm off the bench; both of your legs should have no contact with the bench. This is your starting position.
- Keep your legs straight with a slight bend in the knees and raise your legs to just before 90 degrees.
- Simultaneously raise your upper body and touch your ankles with your hands while keeping your arms stretched at all times.
- Pause at the top and lower your legs in slow controlled manner back to the starting position.
How to vary?
Increase difficulty: Increase the speed with which you do the exercise. All force and momentum should be mostly regulated with your abs.
Decrease difficulty: Bend your knees to shorten the distance you need to cross to touch your ankles. This lowers the total amount of stress experienced in your abs.