Will Under the Muscle Implants Damage Pectoral Muscles Permanently?

Yes, you can have implants under the muscle without losing any muscle strength or ability. As long as the surgery is performed by a board certified plastic surgeon who has experience with this procedure and you are in good health which sounds like you are, you should run a very low risk of any complications.
You will need to refrain from any heavy lifting for several weeks, but can return to light activities after about a week. Most patients, especially those who do heavy lifting, recover with the same amount of muscle strength. Discuss any concerns with your plastic surgeon as always.
Best regards,
Dr. Speron

Most breast augmentation procedures involve a subpectoral placement of the implant. This positioning of the implant is done through what we refer to as a "muscle spliting technique". The origin of the muscle which is found along the inferior aspect of the breast along the fold, is usually released to some extent to allow for better positioning of the implant.

The muscle is still connected to the breastbone and its insertion at the shoulder. So it is injured to some extent through the dissection alone, however the relationship of the mucscle in regards to its mechanics is not noticebly affected .