The Iron Bird Dog exercise works your shoulders, back, abs, obliques, butt and hamstrings.
-Beach Body Now: Iron Bird-Dog. Keep your foot flexed and really squeeze the dumbbell behind your knee at all times to keep it in place. When you tuck your knee close to your chest, concentrate on engaging your abs. Do 10 reps, switch sides, and repeat. Do 2 to 3 sets total.