In general, the majority of the breast implants in the U.S. are placed under the muscle for several reasons. These include, decreased risk of capsular contracture, more natural appearance and decreased ability to feel the breast implants.
In a breast lift the additional benefit of placing the implant under the muscle is that it protects the blood supply to the nipple. The muscle can protect some of the blood vessels that go to the nipple in a surgery where you are reshaping the entire breast.
In certain situations, it is helpful to place the implant above the muscle but these should be discussed with your board-certified plastic surgeon. He or she can go over the risk and benefits of both possible locations for placement of your breast implant.