In patients with enough padding (2 cm pinch is general rule), it is preferable to put the implant over muscle. Implants put under muscle are actually only under muscle across the top of the implant, which hides the upper edge of the implant in thin people, but has the negatives of dynamic deformity (breasts get deformed when you contract your pectoral muscle) and the implants tend to sit a little too lateral (not close enough to the center) in some patients.