We want elongated backs with tummies tucked in and shoulders rolled back. Your feet should be comfortably in front of you at 90 degree angles. For balance and comfort you can adjust that angle. Remember if you want a better core workout, pump some air in the stability ball to challenge your balance.
Choose a higher weight than you normally would for this particular move. You will be working both triceps at the same time, so double the weight: use both your hand weights or just grab a 10-15 lb weight.
Take the weight(s) behind your head–remember your posture! Keep the elbows pointing forward and close to your ears. If you feel them pull away or notice that they are pointing outwards toward the side walls readjust before you continue. Raise the weight straight up, keeping those elbows in place, until your arms are extended.
For an extra challenge, raise a leg with your weight. Alternate the leg extensions.
Do 3 sets of 12-15 reps.