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  3. Exercises
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  5. Legs
  6. /
  7. Knee - Flexion

Exercise guide

Knee - Flexion

  • Beginner
  • Compound
  • Timed hold
  • Hips
  • Lower legs

This isolation exercise targets the hamstrings and calves through active knee flexion, improving mind-muscle connection and joint stability without the need for equipment.

Reviewed by the Crucible Fit team · Updated June 2026

Watch the Knee - Flexion demonstrationVideo and instructions for every movement are in the Crucible Fit app.

Muscles worked

Primary

  • Calves
  • Hamstrings

Secondary

  • Adductors
  • Glutes

Equipment

  • Body weight

Setup

  1. Stand tall with your feet hip-width apart and your core engaged.
  2. Shift your weight onto your left leg, keeping a slight micro-bend in the knee for stability.
  3. Place your hands on your hips or hold onto a wall or sturdy chair for balance.
  4. Position the working (right) leg slightly behind the standing leg with the toes lightly touching the floor.

How to do it

  1. Exhale as you slowly bend your right knee, pulling your heel toward your glutes as far as comfortably possible.
  2. Squeeze the hamstrings and calves at the top of the movement for a one-second pause.
  3. Inhale as you slowly lower the foot back toward the floor in a controlled manner.
  4. Maintain a steady tempo of 2 seconds up and 2 seconds down, completing all reps on one side before switching.

Form checklist

  • Keep your thighs parallel to each other; do not let the working knee drift forward.
  • Maintain an upright torso and avoid leaning forward or arching your lower back.
  • Keep the foot of the working leg flexed (toes toward the shin) to maximize calf engagement.
  • Ensure the movement is driven solely by the knee joint, keeping the hip stationary.

Pro tips

  • Focus on the 'mind-muscle connection' by imagining you are pulling against a heavy weight to increase muscle fiber recruitment.
  • Keep the standing leg's glute squeezed to help maintain a neutral pelvis and better balance.

Make it harder

  • Add a 3-5 second isometric hold at the peak of the contraction (heel closest to glute).
  • Perform the movement with a slow 4-second eccentric (lowering) phase to increase time under tension.

Frequently asked

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

Related exercises

  • Back Shuffle Side KickoutIntermediate · calves, glutes, hamstrings, and quadriceps
  • Barbell Band Assisted DeadliftIntermediate · calves, glutes, hamstrings, quadriceps, and trapezius
  • Barbell Bench Lateral Step UpIntermediate · calves, glutes, hamstrings, and quadriceps
  • Barbell Paused Sumo DeadliftAdvanced · calves, glutes, hamstrings, quadriceps, and trapezius

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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