Under Knee Ab Crunch
The under knee ab crunch is a bodyweight exercise that combines a standard crunch with a knee tuck. Placing hands beneath the knees provides light feedback and assists in coordinating the simultaneous torso and knee movement, targeting the full abdominal wall.
How to do it
- Lie on your back with your knees bent and feet flat on the floor.
- Place your hands beneath your thighs or knees for light support.
- Curl your upper body off the ground, bringing your chest toward your knees.
- At the same time, draw your knees toward your chest to meet your rising torso.
- Slowly lower back to the starting position and repeat for the desired number of repetitions.
Form cues
- Keep your lower back gently pressed into the floor.
- Exhale as you lift your chest and knees toward each other.
- Use your hands lightly under your thighs for coordination, not pulling.
- Move slowly and control the descent back to the floor.
Breathing: Exhale as you crunch your torso and knees together; inhale as you slowly return to the starting position.
Common mistakes
- Pulling on the head or neck with hands, which strains the cervical spine instead of targeting the abs.
- Using momentum to swing the legs up, which reduces muscle engagement in the core.
- Holding your breath, which raises blood pressure and makes the movement harder to control.
Muscles worked
Primary: abs. Secondary: hip flexors, obliques.
Suggested sets & reps
Get fitter
3 sets × 10–15 reps, rest 60 s
Lose weight
3 sets × 12–15 reps, rest 45–60 s
Build muscle
3 sets × 10–12 reps, rest 60 s
Easier, harder & alternatives
Take care if…
- If you feel lower back discomfort, reduce your range of motion or pause and rest.
General educational guidance, not medical advice. Stop on sharp pain and check with a qualified professional if unsure.
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Start free →Frequently asked questions
What if I feel this in my neck?
Make sure your hands are resting gently beneath your knees rather than pulling on your head, and focus on lifting your chest with your upper abs.
How many times a week should I do this?
2 to 3 times per week is a great starting point to allow your core muscles adequate recovery time.
Can I do this if I am a complete beginner?
Yes, this exercise is beginner-friendly and uses light hand placement to help coordinate your movement.