The stability ball is a great tool to strengthen the core. By simply sitting on it, it activates the deep core muscles stabilizing your body. My first choice of exercise for your core is the core rollout. It activates and put your focus on the abs. Contract the abs, pull bellybutton towards the back of your spine and start walking out keeping abs contracted the entire time. Reverse and roll back while still contracting your abs to a full sit up. The ball twist is a great challenge on the core st