Jack Knife
CoreMuscles worked
corehip flexors
- Equipment
- No equipment
- Level
- Intermediate
- How much to do
- 8–12 reps per set.
How to do it
- Lie on your back with your arms extended overhead and your legs straight.
- Press your lower back gently into the floor and brace your abs.
- Lift your arms and legs together, folding at the hips like a closing jackknife.
- Reach your hands toward your shins at the top.
- Lower slowly with control until you are nearly flat again.
How to breathe
Breathe in as you lower, out as you push. Never hold your breath at the hardest point.
Common mistakes
Swinging into it. If momentum is doing the work, slow down until the muscle has to.
Letting your lower back peel off the floor. Press it down, and end the set the moment you cannot.
Rushing. Speed hides the range; slow down until the last rep looks like the first.
Frequently asked questions
- Which muscles does Jack Knife work?
- Jack Knife mainly works your core. It also engages your hip flexors.
- What equipment do you need for Jack Knife?
- None — Jack Knife is a bodyweight move you can do anywhere.
- Is Jack Knife good for beginners?
- Jack Knife suits intermediate exercisers. Build a base first, then add reps.
- How many reps of Jack Knife should you do?
- 8–12 reps per set.