The Wobble board push up is a great variation for core conditioning as well as an excellent exercise for upper quarter strengthening.
To perform the exercise, begin in the push up position with hands on the side of a wobble board. Tighten the abdominal muscles and perform a push up, while keeping board stable and even and keeping trunk rigid and repeat. Do only the number of repetitions that can be done while maintaining proper trunk position and a stable wobble board.