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  3. Exercises
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  5. Core & Abs
  6. /
  7. Side To Side Knee Tuck

Exercise guide

Side To Side Knee Tuck

  • Intermediate
  • Compound
  • Rep-based
  • Upper legs
  • Waist

The Side To Side Knee Tuck is a dynamic core exercise that targets the rectus abdominis and obliques by combining a vertical crunch with lateral rotation. It builds rotational stability and hip flexor strength while keeping the core under constant tension.

Reviewed by the Crucible Fit team · Updated June 2026

Watch the Side To Side Knee Tuck demonstrationVideo and instructions for every movement are in the Crucible Fit app.

Muscles worked

Primary

  • Abs
  • Obliques

Secondary

  • Hip flexors

Equipment

  • Body weight

Setup

  1. Sit on the floor with your knees bent and feet flat.
  2. Lean your torso back at a 45-degree angle and place your hands on the floor behind your hips for support.
  3. Lift your feet off the floor, bringing your knees toward your chest to find your balance on your sit bones.

How to do it

  1. Inhale as you extend your legs straight out at a 45-degree angle to the right side, keeping your feet hovering just above the floor.
  2. Exhale and use your core to pull your knees back in toward the center of your chest.
  3. Inhale again as you extend your legs straight out to the left side.
  4. Continue alternating sides in a fluid, controlled motion, maintaining a steady tempo.

Form checklist

  • Keep your chest lifted and avoid rounding your shoulders or lower back.
  • Ensure your feet stay glued together throughout the entire movement.
  • Maintain a slight bend in your elbows to avoid joint strain.
  • Focus on pulling the knees in using your abs rather than just swinging your legs.

Pro tips

  • To maximize oblique engagement, imagine your knees are drawing a 'U' shape in the air.
  • Focus on the peak contraction at the top of the tuck, squeezing your abs hard before extending again.

Make it harder

  • Remove your hands from the floor and hold them out to your sides or behind your head to challenge your balance.
  • Slow down the extension phase to a 3-second count to increase time under tension.

Frequently asked

What muscles does the side to side knee tuck work?
The side to side knee tuck primarily targets the abs and obliques, and also works the hip flexors as secondary muscles.
What equipment do you need for the side to side knee tuck?
The side to side knee tuck requires no equipment — just your body weight.
Is the side to side knee tuck good for beginners?
The side to side knee tuck is rated intermediate. Build a base with simpler variations first, then progress to it with light load and strict form.

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.

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