Dumbbell Fly (knees at 90 degrees)
- Synergist muscles
- Biceps Brachii, Deltoid Anterior
- Equipment
- Dumbbell
- Body part
- Chest
- Type
- Strength
The dumbbell fly with knees at 90 degrees is a chest isolation exercise that targets the pectoralis major (both the upper clavicular head and the lower sternal head), with help from the front deltoids and biceps. Holding your knees bent at 90 degrees with your feet up flattens your lower back against the bench and keeps your core engaged, so the work stays on the chest instead of leaking into a big back arch.
How to do the Dumbbell Fly (knees at 90 degrees)
- 1Lie back on a flat bench (or the floor) holding a dumbbell in each hand, resting them on your thighs.
- 2Lift your feet off the floor and bend your knees to 90 degrees, hips flexed so your shins are roughly parallel to the floor; press your lower back flat into the bench.
- 3Press the dumbbells up over your chest with your palms facing each other and your arms nearly straight.
- 4Set a soft bend in your elbows and lock that angle in place — you will keep it constant for the whole rep.
- 5Open your arms out to the sides in a wide arc, lowering the dumbbells under control until you feel a stretch across your chest, with the weights roughly level with your shoulders.
- 6Keeping the same elbow bend, squeeze your chest to bring the dumbbells back up along the same arc until they nearly touch over your chest.
- 7Pause and squeeze the chest at the top, then begin the next rep.
- 8After your last rep, bring the dumbbells to your chest, lower your feet, and sit up safely.
Form tips
- Lead the movement with your chest, not your hands — think about hugging a wide barrel rather than pressing the weights up.
- Keep the soft elbow bend fixed throughout; the elbow angle should not open and close like a press.
- Lower under control and only go as deep as you can feel a comfortable chest stretch, not shoulder strain.
- Keep your knees stacked at 90 degrees and your lower back flat to keep tension on the chest and protect your spine.
- Use a manageable weight — flyes put a long lever on the shoulder, so leave the ego loads for pressing movements.
Common mistakes
- Bending and straightening the elbows to lift the weight, which turns the fly into a clumsy press and takes tension off the chest.
- Lowering the dumbbells too far below shoulder level, which overstretches the shoulder joint and risks injury.
- Dropping your feet or letting your lower back arch up off the bench, which loses the core bracing this position is meant to create.
- Going too heavy and clanging the dumbbells together at the top instead of controlling the arc, which kills the chest contraction.
- Bringing the dumbbells too far overhead toward your face, which shifts the work onto the front delts instead of the chest.
Frequently asked questions
What muscles does the dumbbell fly with knees at 90 degrees work?
It targets the pectoralis major — both the upper (clavicular) and lower (sternal) chest fibers — with the front deltoids and biceps assisting. Keeping the knees at 90 degrees also keeps the core braced and the lower back flat.
Why hold the knees at 90 degrees instead of feet on the floor?
Lifting the feet and bending the knees to 90 degrees flattens your lower back against the bench and engages your core, which reduces lower-back arch and keeps the tension on your chest rather than letting you cheat with your hips.
How heavy should the dumbbells be?
Lighter than you would press. The fly is an isolation move with a long lever on the shoulder, so pick a weight you can lower under full control through a deep chest stretch for 10–15 reps.
Dumbbell fly vs dumbbell press — what's the difference?
The press bends and extends the elbows to move heavy weight and recruits the triceps, while the fly keeps a fixed soft elbow bend and opens the arms in a wide arc to isolate and stretch the chest.
Is the dumbbell fly good for beginners?
Yes, as long as you start light and keep a soft, fixed elbow bend. Master the arc and the stretch with a controlled weight before loading up, since flyes stress the shoulder more than presses do.
