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  3. Exercises
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  5. Arms
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  7. Finger Flexion While Extending Thumb

Exercise guide

Finger Flexion While Extending Thumb

  • Beginner
  • Isolation
  • Rep-based
  • Lower arms
  • Upper arms

This isolation exercise targets the deep flexors of the fingers while stabilizing the thumb, improving grip strength and manual dexterity. It specifically engages the flexor digitorum muscles while maintaining isometric tension in the thumb extensors.

Reviewed by the Crucible Fit team · Updated June 2026

Watch the Finger Flexion While Extending Thumb demonstrationVideo and instructions for every movement are in the Crucible Fit app.

Muscles worked

Primary

  • Forearms

Secondary

  • Biceps
  • Triceps

Equipment

  • Body weight

Setup

  1. Sit on a bench or chair with your forearm resting flat on a bench, palm facing up.
  2. Position your wrist at the very edge of the bench so your hand is fully supported but the fingers are free to move.
  3. Extend your thumb away from your palm, keeping it parallel to the bench surface.

How to do it

  1. Inhale and slowly curl your four fingers toward your palm while keeping the thumb strictly extended and stationary.
  2. Squeeze your fingertips into the top of your palm, creating a 'claw' or partial fist without involving the thumb.
  3. Exhale as you slowly uncurl your fingers back to the flat, starting position.
  4. Maintain a controlled tempo of 2 seconds for flexion and 2 seconds for extension.

Form checklist

  • Keep the forearm and wrist glued to the bench to prevent 'cheating' with wrist flexion.
  • Ensure the thumb does not move toward the fingers during the curling phase.
  • Move only at the knuckle and finger joints, not the wrist.
  • Maintain a steady, rhythmic breathing pattern throughout the set.

Pro tips

  • Focus on pressing the tips of your fingers firmly into the pads at the base of your fingers for maximum muscle recruitment.
  • To increase mind-muscle connection, visualize the tendons in your forearm pulling the fingers closed while the thumb remains anchored.

Make it harder

  • Perform the movement while holding a small, rolled-up towel or a stress ball to add resistance to the flexion.
  • Slow the eccentric (opening) phase to 4-5 seconds to increase time under tension for the forearm flexors.

Frequently asked

What muscles does the finger flexion while extending thumb work?
The finger flexion while extending thumb primarily targets the forearms, and also works the biceps and triceps as secondary muscles.
What equipment do you need for the finger flexion while extending thumb?
The finger flexion while extending thumb requires no equipment — just your body weight.
Is the finger flexion while extending thumb good for beginners?
Yes. The finger flexion while extending thumb is a beginner-friendly movement and a strong foundation to build on.

Related exercises

  • Band Wrist CurlBeginner · forearms
  • Barbell Behind Back Finger CurlBeginner · forearms
  • Barbell Palms Down Wrist Curl Over A BenchIntermediate · forearms
  • Barbell Palms Up Wrist Curl Over A BenchBeginner · forearms

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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