Flexion Leg Sit Up (bent Knee)
intermediate strength exercise · body weight · targets abs
- Body part
- waist
- Primary target
- abs
- Equipment
- body weight
- Difficulty
- intermediate
The flexion leg sit-up with bent knee is a sit-up variation performed with knees bent and hands across the chest, focusing on a controlled curl of the upper body up from the floor. The bent-knee position reduces hip flexor involvement compared to straight-leg sit-ups, biasing the work toward the rectus abdominis (front abs) more directly. Most sit-ups engage hip flexors significantly — to the point that the hip flexors do most of the work in straight-leg or anchored sit-ups. The bent-knee version reduces this hip-flexor dominance, producing more efficient ab work per rep. For trainees focused on building front-ab strength, this variation is more productive than traditional sit-ups. Where this earns its place is as foundational front-ab work for trainees pursuing visible abs or general core strength. Combined with planks (anti-extension) and rotational work, sit-ups complete the ab training spectrum. The trade-off is the lower back stress some trainees experience with sit-ups in any form — for those individuals, crunches or planks are safer alternatives.
Why train the Flexion Leg Sit Up (bent Knee)?
- Reduces hip flexor dominance compared to straight-leg sit-ups.
- Targets the rectus abdominis more directly.
- Builds visible front-ab muscle.
- Accessible to most fitness levels.
- Useful in foundational core programs.
- Pairs naturally with planks for compound core training.
How to do the Flexion Leg Sit Up (bent Knee): step by step
- 1Lie flat on your back with your knees bent and feet flat on the ground.
- 2Place your hands behind your head with your elbows pointing outwards.
- 3Engaging your abs, slowly lift your upper body off the ground, curling forward until your torso is at a 45-degree angle.
- 4At the same time, lift your legs off the ground, bending your knees and bringing them towards your chest.
- 5Pause for a moment at the top, then slowly lower your upper body and legs back down to the starting position.
- 6Repeat for the desired number of repetitions.
Muscles worked
Primary
abs
Secondary
hip flexors
Common mistakes to avoid
Anchoring the feet
Anchored feet recruit hip flexors. Keep feet free for ab focus.
Pulling on the neck
Hands across chest is the safer position. Avoid hands behind head with neck pulling.
Going too fast
Slow controlled motion produces ab engagement.
Hyperextending lower back at top
Sit up only until torso is at about 60-70 degrees. Going further engages hip flexors.
Bouncing off the floor at bottom
Control descent. Bouncing reduces ab stimulus.
Easier and harder variations
Easier
Perform smaller crunches (lifting only shoulders off floor). Or use 3/4 sit-up range.
Harder
Add weight (plate against chest). Or slow tempo (3 seconds up, 3 seconds down).
Alternative exercises
Crunch
Smaller-range version. Less hip flexor involvement.
Plank
Anti-extension core work. Different stimulus.
Hollow body hold
Isometric core work.
How to program the Flexion Leg Sit Up (bent Knee) into your training
Sets and reps: 3 sets of 12-20 reps. Frequency: 2-3 times per week. In core sessions: 3 sets of 15 sit-ups, 3 sets of 30-second planks.
Recovery and frequency
Recovery within 24-48 hours. Watch for lower-back discomfort.
Frequently asked questions
How many sets and reps?
3 sets of 12-20 reps.
How often?
2-3 times per week.
Sit-up vs crunch?
Sit-up has full range; crunch has shorter range. Crunch is safer for back; sit-up engages more muscle.
Will this build a six-pack?
Builds the muscle. Visibility comes from low body fat.
Why bent knee?
Reduces hip flexor involvement, biasing work toward abs.
Are sit-ups bad for back?
Done correctly with proper form, no. With anchored feet or bad form, can stress lumbar spine.
Useful tools for this exercise
Build a workout with the Flexion Leg Sit Up (bent Knee)
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