http://AskTheTrainer.com/stability-ball-exercises.html Personal Trainer Michael Behnken, MS, NASM-CPT-PES, CSCS 2 stability ball chest flies are an advanced chest isolation exercise. They are challenging for your core as well. Keep your hips parallel with the floor if you are looking to work your core. If you are looking to really work your chest, keeping your hips slightly elevated is okay. San Francisco Personal Training http://fitSF.com