Exercise guide
Single Leg Extension Crunch
- Intermediate
- Compound
- Rep-based
- Waist
This exercise combines a standard crunch with a unilateral leg extension to intensely target the rectus abdominis and obliques while improving core stability. It challenges the lower abs through the leg lever and the upper abs through the spinal flexion.
Reviewed by the Crucible Fit team · Updated June 2026
Muscles worked
Setup
- Lie flat on your back on a mat with your knees bent and feet flat on the floor, hip-width apart.
- Place your fingertips lightly behind your ears or head, keeping your elbows flared wide.
- Engage your core by pressing your lower back firmly into the floor to eliminate any arch.
How to do it
- Exhale as you lift your shoulder blades off the floor while simultaneously extending one leg straight out at a 45-degree angle.
- Inhale as you lower your shoulders back to the floor and return the extended leg to the starting bent-knee position.
- Repeat the movement on the opposite side, alternating legs with each repetition.
- Maintain a controlled 2-1-2 tempo: 2 seconds to crunch/extend, 1 second pause at the top, and 2 seconds to return.
Form checklist
- Keep your lower back glued to the mat throughout the entire movement.
- Avoid pulling on your neck; use your abdominal strength to lift your chest toward the ceiling.
- Keep your chin tucked slightly, as if holding an orange between your chin and chest.
- Ensure the non-extending foot remains stable and flat on the floor.
Pro tips
- Focus on pulling your navel toward your spine during the exhale to engage the deep transverse abdominis.
- For better oblique activation, add a subtle rotation of the opposite elbow toward the knee that remains bent.
Make it harder
- Perform the exercise with the non-working leg in a 'tabletop' position (90-degree hip/knee bend) rather than flat on the floor.
- Lower the extended leg closer to the ground (about 6 inches high) to increase the lever length and tension on the lower abdominals.
Frequently asked
- What muscles does the single leg extension crunch work?
- The single leg extension crunch primarily targets the abs and obliques, and also works the hip flexors as secondary muscles.
- What equipment do you need for the single leg extension crunch?
- The single leg extension crunch requires no equipment — just your body weight.
- Is the single leg extension crunch good for beginners?
- The single leg extension crunch is rated intermediate. Build a base with simpler variations first, then progress to it with light load and strict form.