Make sure you keep your back straight throughout the set by tensing your core muscles and keeping your head and eyes up.
Keep the rep timing slow and controlled.
Dont "rest" at the top of the movement by locking your elbows out.
Instructions
The exercise ball dumbbell extension is the same as a two arm seated dumbbell extension but you're seated on an exercise ball. The ball makes your core work harder to maintain stability. Set up by sitting on an exercise ball holding a dumbbell above your head using two hands.
Keep your feet at a wide stance on the floor for stability.
Tense your core muscles and straighten your back. Keeping your eyes facing and core tight, slowly lower the dumbbell behind your head as far as possible.
Pause, and then slowly extend your arms squeezing the triceps.