Core Pushup instability strength trainer

A standard push-up will strengthen primarily your upper body, but this steel stability board creates an unstable surface that forces your core muscles, and even your hip flexors, to continuously engage. The secret is the mini instability ball under the platform that you can inflate to increase the difficulty or deflate to make the board more stable. Even when we did a simple plank hold, we immediately felt the difference in difficulty from working out directly on the floor. [$60; sklz.com]