Exercise guide
Flexion Leg Sit-Up Straight Arm
- Intermediate
- Isolation
- Rep-based
- Waist
This intermediate core exercise combines a sit-up with a single-leg raise to target the rectus abdominis and obliques while simultaneously engaging the hip flexors and quadriceps. The straight-arm reach increases the lever length, creating higher tension in the upper and lower abdominals.
Reviewed by the Crucible Fit team · Updated June 2026
Muscles worked
Setup
- Lie flat on your back on a mat with your legs fully extended and feet hip-width apart.
- Extend your arms straight overhead, resting them on the floor behind you.
- Press your lower back into the mat to engage your core and eliminate any arch in your spine.
How to do it
- Exhale as you simultaneously lift your torso and one leg off the floor, keeping both arms and the lifted leg perfectly straight.
- Reach your hands toward your toes, sitting up until your torso is nearly vertical and your leg is at a 45-to-90-degree angle.
- Inhale as you slowly lower your torso and leg back to the starting position with a controlled, 2-second tempo.
- Repeat the movement on the opposite side, alternating legs for each repetition.
Form checklist
- Keep the lifted leg locked straight by squeezing the quadriceps.
- Avoid swinging your arms for momentum; use your abdominals to 'peel' your spine off the floor.
- Maintain a slight tuck in your chin to prevent neck strain.
- Ensure the non-working leg stays pinned to the floor throughout the movement.
Pro tips
- Focus on the mind-muscle connection by imagining you are folding your body like a jackknife at the hip.
- At the top of the movement, pause for a half-second and reach as far past your foot as possible to maximize oblique engagement.
Make it harder
- Keep both heels hovering 2 inches off the floor throughout the entire set to maintain constant tension on the lower abs.
- Hold a light medicine ball or dumbbell between your hands to increase the resistance on the upper abdominals.
Frequently asked
- What muscles does the flexion leg sit-up straight arm work?
- The flexion leg sit-up straight arm primarily targets the abs and obliques, and also works the erector spinae as secondary muscles.
- What equipment do you need for the flexion leg sit-up straight arm?
- The flexion leg sit-up straight arm requires no equipment — just your body weight.
- Is the flexion leg sit-up straight arm good for beginners?
- The flexion leg sit-up straight arm is rated intermediate. Build a base with simpler variations first, then progress to it with light load and strict form.