Waist Exercise · Body Weight

Expand

Flexion Leg Sit Up (bent Knee)

Category

Waist

Equipment

Body Weight

Target

Abs

Share Embed & Cite

Biomechanical cues verified by the PlanWorkouts Science Review Board under CSCS editorial oversight; see our editorial standards.

Bending the knees during a sit-up shortens the hip flexors' leverage, which shifts a bit more of the work onto the abs compared to a straight-leg sit-up. It's still primarily a hip-flexor-dominant movement despite the ab target listed — that's the long-running criticism of sit-ups generally, and it's part of why crunches have become more popular for isolating the abs specifically.

How to Do the Flexion Leg Sit Up (bent Knee)

1. Lie on your back with your knees bent and feet flat on the floor.
2. Place your hands behind your head, elbows pointed out to the sides.
3. Using your abs, curl your upper body up off the floor until your torso reaches about a 45-degree angle.
4. At the same time, lift your feet off the floor and draw your bent knees toward your chest.
5. Pause momentarily at the top, then lower your upper body and legs back down together.
6. Continue for the set reps, taking care to lower with control and reset your posture between each rep.

Biomechanical & Joint Load Cues

Trunk flexion, anti-extension, or anti-rotation targeting rectus abdominis and obliques. Requires posterior pelvic tilt to isolate abdominal fibers.

Common Mechanical Mistakes & Form Errors to Avoid

• Pulling on the Neck / Cervical Spine: Yanking the head forward with hands during crunches places excessive strain on cervical vertebrae without engaging the abs.
• Arching Lower Back Under Load: Allowing the lumbar spine to hyperextend during leg raises shifts work to the psoas hip flexors and pinches facet joints.
• Rushing Through Reps: Using fast bouncing tempo bypasses deep transverse abdominis engagement; slow reps with a full exhale build real core strength.

Proper Breathing Patterns & Core Engagement

Exhale completely during spinal flexion or maximal tension to maximize abdominal contraction; inhale on controlled return.

Injury Modification & Joint Safety

Individuals with lumbar disc herniations should prioritize isometric anti-extension (planks, dead bugs) over repetitive loaded spinal flexion. If joint irritation persists, immediately stop and substitute this movement with another exercise from the Waist exercises category.

Muscles Worked

The Flexion Leg Sit Up (bent Knee) primarily targets the Abs, with Hip Flexors as the primary synergist muscle group.

More Waist Exercises

3/4 Sit-Up Body Weight · Abs 45° Side Bend Body Weight · Abs Air Bike Body Weight · Abs Alternate Heel Touchers Body Weight · Abs Arm Slingers Hanging Bent Knee Legs Body Weight · Abs Arm Slingers Hanging Straight Legs Body Weight · Abs

More Abs Exercises

Arms Overhead Full Sit-Up (male) Body Weight · Abs Assisted Hanging Knee Raise Assisted · Abs Assisted Hanging Knee Raise With Throw Down Assisted · Abs Assisted Lying Leg Raise With Lateral Throw Down Assisted · Abs Assisted Lying Leg Raise With Throw Down Assisted · Abs Assisted Motion Russian Twist Medicine Ball · Abs

Equipment Alternatives

Don't have body weight? These target the same abs with different equipment.

Assisted Hanging Knee Raise Assisted · Abs Assisted Motion Russian Twist Medicine Ball · Abs Band Alternating V-Up Band · Abs Barbell Press Sit-Up Barbell · Abs

FAQ

What muscles does the Flexion Leg Sit Up (bent Knee) work?

The Flexion Leg Sit Up (bent Knee) primarily targets the abs as the primary agonist. The hip flexors acts as the primary supporting muscle group during force transmission. Maintaining strict biomechanical alignment ensures maximal tension remains focused on the intended muscle fibers rather than shifting to surrounding stabilizer joints.

What equipment do I need for the Flexion Leg Sit Up (bent Knee)?

The Flexion Leg Sit Up (bent Knee) is performed using no equipment — just body weight. Ensure all equipment is securely positioned with collars or safety pins locked before loading working sets. If you train in a home gym or minimal setup, explore our equipment-free and resistance band alternatives below.

What is the most common form mistake on the Flexion Leg Sit Up (bent Knee)?

The most frequent error is pulling on the neck / cervical spine: Yanking the head forward with hands during crunches places excessive strain on cervical vertebrae without engaging the abs.. Controlling the 2 to 3-second eccentric lowering phase ensures maximal hypertrophic stimulus while shielding connective tissue.

How many sets and reps should I perform on the Flexion Leg Sit Up (bent Knee)?

For muscle growth and density, execute 3 to 4 working sets of 10 to 15 controlled repetitions with 60 seconds rest, emphasizing a deep stretch and deliberate peak contraction on every rep.

What's a good alternative to the Flexion Leg Sit Up (bent Knee)?

If you don't have access to the space or setup it needs, the Assisted Hanging Knee Raise targets the same abs using assisted instead. This alternative replicates the same joint angle and movement plane while allowing you to train effectively with different available gear. You can also substitute it seamlessly in our free Workout Plan Builder.