Plank hold legs on gym ball, knee bends ("Plank on-the-ball" Core/scapular stabilization, bending leg (alternate))

Start in a standing position with a stability ball in front of you.
Place your hands on the floor and put your shins on the stability ball.
Push your arms down onto the floor to create stability in your shoulder blades.
Do not allow your back to arch.
Hold this position, alternately bending one knee and then the other, aiming to keep the ball as still as possible.
Continue the movement for the required duration.