You'll feel the burn — in your abs and shoulders — from this bodyweight exercise, which also adds a boost of cardio.
Begin in an elbow plank position with the abs engaged.
Draw the left knee into the chest. Then step the foot back to the elbow plank position. Now draw the right knee into the chest and then step it back to the elbow plank position.
This counts as one rep. Start with 12 to 15 reps, and do at least two sets.