Using a stability ball challenges your balance and is ideal for training your core muscles, abs in particular.  This exercise targets and strengthens buttocks and hamstrings (muscles behind the leg); abdominals and spine extensors work as stabilizers.

• Kneel with torso draped over ball, facedown, then walk hands forward until your in a plank position with front of hips and thighs on top of ball, wrists in line with shoulders.
• Inhale and exhale as you point your toes and lift both legs high as you can without arching your back.
• Separate feet and rotate hips upward.
• Keep hands planted on floor, contract buttocks as you lift legs up.
• Lower to starting position and repeat.
• Do 2-3 sets of 10-15 repetitions.