Stability Ball Knee Tuck
Also known as: stability ball jackknife
- Primary muscleAbs
- EquipmentStability Ball
- DifficultyIntermediate
- TypeCore · Core flexion
- Secondary muscles
- Hip Flexors
- Body part
- Core
- Position
- Lying
- Laterality
- Bilateral (both sides together)
- Category
- Strength
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From a plank with your shins on the ball, pulling the knees in rolls the ball toward your hands while the shoulders hold everything level, so the abs flex the trunk and brace it at the same time. The ball drifting sideways tells you which obliques are slacking - feedback a bench tuck never gives.
How to do the Stability Ball Knee Tuck
- Start by sitting on the stability ball with your feet flat on the ground and your knees bent at a 90-degree angle.
- Place your hands on the sides of the stability ball for support.
- Engage your abs and slowly roll your hips forward, bringing your knees towards your chest.
- Pause for a moment at the top of the movement, then slowly extend your legs back to the starting position.
- Repeat for the desired number of repetitions.
Tip: Breathe out as you crunch and let the last bit of air leave the lungs at the top of each rep.
Muscles worked
Primary
Abs · Core
The rectus abdominis shortens to bend the spine forward, and each rep should end with the upper abs fully contracted and the ribs pulled down.
Secondary
- Hip Flexors — The hip flexors take over if your feet are anchored and you come up too high; keep the range short to leave them out.
Common mistakes
- Pulling on the neck — hands behind the head should support, not yank; keep the elbows wide and the chin about a fist away from the chest.
- Sitting all the way up — a crunch is a curl of the ribs toward the pelvis, not a sit-up; the lower back should stay on the floor the whole time.
- Cranking out fast reps — momentum does the work if you bounce; lift to a squeeze, hold for a count and lower slowly.
- Never adding load — once bodyweight reps get easy, hold a plate on the chest or use a cable rather than doing sets of 50.
Sets, reps and where it fits
| Goal | Sets × reps | Rest |
|---|---|---|
| Strength | 3 × 8–12 | 60 s |
| Endurance | 2–3 × 15–25 | 45 s |
Crunches close out a session after the heavy work, or they can fill the rest periods between sets of a lift that does not need much bracing. Pair a cable crunch with a cable side bend on the same stack.
Variations and alternatives
Similar exercises
Without stability ball
Frequently asked questions
What muscles does the Stability Ball Knee Tuck work?
The Stability Ball Knee Tuck primarily works the abs. It also involves the hip flexors, which assist the movement and stabilise the joints.
Is the Stability Ball Knee Tuck good for beginners?
It is rated intermediate, so it suits people who already train regularly. Beginners can still do the Stability Ball Knee Tuck with light load and a spotter or coach watching the first few sessions; if it feels awkward, pick an easier variation from the alternatives below first.
What can I do instead of the Stability Ball Knee Tuck?
If you cannot do the Stability Ball Knee Tuck, try the Arms Overhead Crunch, the Decline Crunch or the Incline Bench Leg Hip Raise. Each of them trains the abs through a similar movement, so you can swap them in without changing the rest of the workout.
How many sets and reps should I do?
It depends on your goal. For the Stability Ball Knee Tuck, a sensible starting point is strength: 3 × 8–12 with 60 s rest; endurance: 2–3 × 15–25 with 45 s rest. Train it at the end of a session or pair it with a lower-body lift as active rest.
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Exercise data and animations are licensed ExerciseDB content used in SetGrip. This page is for general information and is not medical or coaching advice.