Would pectoral exercise ruin implants and pocket?

I think after 6-8 weeks, a sufficient capsule has formed around a submuscular implant and exercises that recruit the pectoralis major muscle are not a problem. Patients do experience some 'animation deformity' issues when moving the pectoralis major muscle over a submuscular implant - but this is not a functional problem, just a cosmetic one.

Submuscular implants do not limit your ability to exercise after the surgery has healed.  There are many patients for whom push ups are a mandatory part of life (active duty military) who have sub muscular implants.

I allow my patients with sub-pectoral breast augmentations to perform most exercises or activities as soon as they're comfortable doing so, as early as 10-14 days after surgery. HOWEVER, I ask them to avoid exercises that really stress the pectoral muscles for at least 6 weeks. Not only are the muscles stretched when implants are placed beneath them, but portions of the muscle are often cut where the muscle attaches to the lower ribs and chest bone. It's important to allow time for the muscle to heal before subjecting it to the stress of bench press exercises, push ups, etc. Other exercises (legs, abs, arms, cardio etc.) are fine as soon as the patient feels comfortable enough. 

Your concerns are realistic and valid. There is no question
that pectoralis exercises after having sub muscular breast implants place can
cause pocket abnormalities. Some Plastic Surgeons tell patients they should
never return to those exercises after augmentation. It is certainly more of a
problem when implants re below the muscle vs above the muscle but don't fool
yourself. Implants above the muscle can also widen the breast pocket over time.
It is less of an issue performing pectoralis work when implants are above the
muscle. So in body builders I recommend above the muscle. In the average
patient who works out a lot it can go either way. Your anatomy may push a
surgeon to choose one pocket vs the other. That decision is made on a case by
case basis.

Most surgeons recommend refraining from upper body exercise for some period of time after augmentation, until the scar capsule forms around the implant. The pectoralis muscle usually covers only the upper an inner part of the implant, and heavy use can push the implants downward and outward. Still, the vast majority of surgeons prefer under the muscle position for implants. Exceptions are made on a case by case basis, particularly for weight lifters and body builders, in whom large muscles are more likely to actually displace the implants.All the best. 

I ask my patients to refrain from excess arm activity (tennis for example) for about 7-10 days after surgery and then not to really strain the chest muscles (push ups or bench presses) for at least 6 weeks. During that time a capsule forms around the implant which can help hold it in position. Stressing the muscle and thus the pocket created for the implant earlier can potentially lead to stretching of the pocket which can lead to an abnormal position of the implant.Women who are extremely physically active, body builders, cross trainers or weight lifters may do better with a subglandular (on top of the muscle) breast augmentation. The action of the chest muscle will not affect the implant or its pocket in this case. Each case and individual are different and the choices and their implications should be discussed in detail with their plastic surgeon.For the best results seek the consultation of a plastic
surgeon certified by the American Board of Plastic Surgery. They have a website
listing all the certified plastic surgeons. Members of the American Society of
Aesthetic Plastic Surgery will all be board certified in plastic surgery and
have demonstrated an interest and competence in cosmetic plastic surgery as
well.

I require that my patients avoid all pec exercise for 3 months after breast augmentation so that the capusular lining can form in the pocket. After that I let them return to normal activities and haven't seen any implants move out of position. You just need the time to heal first.

I routinely caution patients that have smooth implants under the muscle to reduce pectoralis-specific exercises as I have seen patients push their implants to the side from enlarged pockets. That means they can stay healthy with general fitness exercises but avoid the "body building" exercise. And why would you need that as the operation will have given you a great shape.