Skip to content
Crucible Fit
  • The Crucible
  • The Week
  • Programs
  • Train Your Way
  • Pricing
Get Crucible
  • The Crucible
  • The Week
  • Programs
  • Train Your Way
  • Apple Watch
  • Pricing
  • FAQ
  • Training Guides
Get Crucible
Crucible Fit

A daily functional fitness test for iPhone, Apple Watch and Android. Stations, reps and the Crux, sized to your readiness and your kit.

Download on the App StoreGet it on Google Play

Crucible Fit

  • The Crucible
  • The Week
  • Programs
  • Train Your Way
  • Apple Watch
  • Pricing

Explore

  • Training Guides
  • Exercise Library
  • FAQ
  • Press
  • Support

Legal

  • Terms and Conditions
  • Privacy Policy
  • Consumer Health Data Notice

CRUCIBLE FIT™ and Crux by Crucible Fit are trademarks of Crucible Fit, LLC.

© 2026 Crucible Fit, LLC. All rights reserved. Apple, the Apple logo, iPhone and Apple Watch are trademarks of Apple Inc., registered in the U.S. and other countries. App Store is a service mark of Apple Inc. Google Play and the Google Play logo are trademarks of Google LLC.

  1. Home
  2. /
  3. Exercises
  4. /
  5. Legs
  6. /
  7. Lying Alternating Legs Extension

Exercise guide

Lying Alternating Legs Extension

  • Beginner
  • Compound
  • Rep-based
  • Lower legs

This exercise builds core stability and lower abdominal strength while isolating the quadriceps through controlled knee extension. It is highly effective for teaching pelvic control and maintaining a neutral spine under load.

Reviewed by the Crucible Fit team · Updated June 2026

Watch the Lying Alternating Legs Extension demonstrationVideo and instructions for every movement are in the Crucible Fit app.

Muscles worked

Primary

  • Abs

Secondary

  • Glutes

Equipment

  • Body weight

Setup

  1. Lie flat on your back on a mat with your knees bent at a 90-degree angle (tabletop position) and shins parallel to the floor.
  2. Press your lower back firmly into the mat to eliminate any gap between your spine and the floor.
  3. Place your arms straight by your sides with palms down for stability.

How to do it

  1. Exhale as you slowly extend one leg forward until it is fully straight and hovering just a few inches off the ground.
  2. Contract your quadriceps forcefully at the point of full extension.
  3. Inhale as you slowly return the leg to the tabletop starting position with control.
  4. Repeat the movement with the opposite leg, alternating sides for the duration of the set using a 2-1-2 tempo (2 seconds out, 1 second hold, 2 seconds back).

Form checklist

  • Keep your lower back glued to the floor; if it arches, do not lower your leg as far.
  • Ensure the stationary leg remains perfectly still in the tabletop position while the other moves.
  • Fully lock out the knee of the extending leg to maximize quad activation.
  • Keep your neck and shoulders relaxed to avoid unnecessary tension.

Pro tips

  • Focus on pulling your navel toward your spine to engage the transverse abdominis throughout the entire range of motion.
  • Point your toes (plantarflexion) as you extend to create a long lever and increase the tension on the lower abs.

Make it harder

  • Lift your head and shoulder blades off the floor into a 'hollow body' crunch position to increase upper abdominal engagement.
  • Slow the eccentric (return) phase to 4 seconds to increase time under tension for the core.

Frequently asked

What muscles does the lying alternating legs extension work?
The lying alternating legs extension primarily targets the abs, and also works the glutes as secondary muscles.
What equipment do you need for the lying alternating legs extension?
The lying alternating legs extension requires no equipment — just your body weight.
Is the lying alternating legs extension good for beginners?
Yes. The lying alternating legs extension is a beginner-friendly movement and a strong foundation to build on.

Related exercises

  • 45 Degrees Arms PlankIntermediate · abs, deltoids, obliques, and pectorals
  • 90 To 90Beginner · abs, glutes, obliques, and quadriceps
  • 90 To 90 SwitchIntermediate · abs, glutes, obliques, and quadriceps
  • Air BikeIntermediate · abs and obliques

Train it with Crucible Fit.

Every movement in this library is in the app with video and instructions, ready for your own sessions on the days between Crucibles. Your first Crucible is on us.

Download on the App StoreGet it on Google Play

See how a Crucible works