Wave velocity ball, one side is semi-circular plastic, elastic, and the other side is flat, inelastic.
Sometimes, we will put it flat, step on it, squat down and support it with one leg; Sometimes, we put the semicircle down, put our hands on both sides of our heads, do push-ups and do Bobby.
No matter which side is facing down, it is using the instability of the semicircle to play a role.
Because the semicircle is unstable, it will lead to our body, and we must constantly find balance. In order to control our body balance, we will mobilize more muscles to participate in the action.