The simple answer is yes the implants can be moved over the muscle. The most common reason to move the implants above the muscle is in active patients that develop an “animation” deformity. An animation deformity is when the implant and breast are moved when the pectorals muscle is flexed. Typically it will cause a crease across the breast as the muscle is flexed.
Like anything in life there are pros and cons to moving the implant over the muscle. Typically the implants are placed under the muscle to keep the breasts soft and natural. If you move the implant over the muscle you have to do several things to keep the breast looking natural and soft. Any time you place the implant over the muscle you will feel more of the implant, as the muscle is no longer present to cover the implant. You also typically need to use a textured implant when the implant is placed above the muscle to prevent excessive scar tissue from forming around the implants. Silicone implants have made placement of the implant above the muscle more reasonable as they are much softer and natural appearing than saline implants were previously. In the past a saline implant placed above the muscle would be very firm, this is not necessarily true for the new silicone breast implants.
Patients with the correct operation can achieve a very attractive breast and avoid the animation deformity with an implant placed above the muscle.
This surgery is possible and occasionally needs to be done in very active women. I would just make sure that you discuss the surgery with a board certified plastic surgeon that can discuss the risks and benefits of the surgery with you.