Keep your feet together and raise your legs until your body becomes a straight line, then slowly lower them. After 15 reps, raise your legs again and make 15 circles with your feet in each direction. Switch sides, and repeat the sequence. The Payoff: "You're challenging the abs and the back together, with all the possible angles, says McClure. "If you feel it in your back, you're doing it right. Getting your abs iron-strong will keep your entire body aligned on the hill.