It is generally best to place implants in the subpectoral position because of better mammographic visualization of the breasts when compared to subglandular placement. They also look more natural and are less easy to feel because they are covered by more tissue. Finally, the capsular contracture rates seem to be less under the muscle.
That being said, there are many ways to create the pocket and how the muscle is handled will affect the answer to your question regarding the state of the muscle if the implants are removed. In addition, the size of the implant is critical because more muscle release must be done for bigger implants and larger implants will thin out the muscle more over time than more moderate sized implants.
By and large, though, "muscle issues" are not part of what a plastic surgeon ever hears from patients after explantation.
