Start: Grab a barbell with a full, overhand grip that's shoulder width or a little wider. Stand and hold the bar at collarbone level, with your back in its natural alignment and your knees slightly bent.
Finish: Press the bar overhead until your arms are straight but not locked. Slowly return to the starting position.
Simply perform the military-press movement while sitting on a ball, taking extra care not to let your back arch too much.