The most common place for implants to be placed is under the muscle. This
is done to reduce the rates of capsular contracture and to provide a more
natural look and feel to the breast, as well as minimizing rippling.
Placing the implant on top of the muscle is sometimes done for professional
bodybuilders, or for women who have a fair amount of breast tissue already,
and just want to fill out a little bit of looseness after child birth.

Breast Implant Placement: Over or Under the Muscle

Doctor Dana Goldberg explains the differences in placement for implants.