This one is fast and dynamic, and lots of fun!
Get your stability ball. Lie with the middle of your back on the ball, with knees bent and feet flat on the floor. Place your hands behind your head like you’re doing an abdominal crunch.
Squeeze your abs in tight and lift your upper back off the ball. Keeping your shoulders above the ball, trace a counterclockwise oval with your torso. (Like you’re bobbing and weaving between punches…)
Keep rotating for about 10 ovals, then switch and rotate clockwise.
This workout prepares your body to rock back and forth on steep hill climbs 
Looks a bit difficult.