The twisting motion of this exercise primarily targets your obliques, but adding the upward lift with a dumbbell will work your shoulder muscles, too. Start seated with your knees slightly bent and a dumbbell in your hands. Your torso should lean back slightly so that your abs are engaged. Keep your entire core tight as you touch the dumbbell down to the floor on your left and then twist to the right as you raise it up above your head until your arms are stretched above you completely. Lower the dumbbell back to the floor on your left by reversing the twist. Preform 10-15 reps on each side.