i just found this online:
"Q. Should I have implants under the muscle or over the muscle?
A. Many women who bodybuild are worried about the impact of lifting on the implant if it's under the muscle. Will it move? Will it deflate or pop? Obviously these things can happen, but it is not typical. In general, I recommend placement of implants under the pectoralis muscle, since the appearance of the implant is better, especially in patients with very low body fat. In those patients, the implants tend to be very obvious when placed over the muscle, including features such as rippling. By placing the implant under the muscle, there is more tissue between the implant and the outside world, thus making the implant less obvious.
Q. Will the implant under the muscle be squeezed by the muscle?
A. Well... yes, it will. But that's not necessarily a problem since the implants are made to stand up to lots of abuse, such as a band of Miami Dolphin linebackers. The implants are compressible, and in many cases you will be advised to massage the implants to keep them soft. The motion of the muscle will not normally hurt the implant. On the other hand, the motion of the pectoralis muscle can cause "bouncing" of the implant or flattening, when the muscle is flexed. This isn't a problem either. It's just something that you should be aware of. I've seen a clever exotic dancer who used this ability in her dancing routine. Rather remarkable.
Q. Well, if it's so painful, will I ever be able to work out again?
A. Of course. In fact, I allow my patients to start cardio in about a week, and they can start lifting with light weights after two weeks. I recommend that they don't do any chest exercises for up to six weeks, generally cautioning that if it hurts, don't do it. In spite of this, most women who bench press tell me that they are able to bench press the same weight as they did pre-operatively by eight to twelve weeks post-operatively. In a number of my patients who are very serious about their training, they have continued to increase their bench press strength as though they never had implants placed."
sounds like the muscles will be fine....