Exercising after submuscular breast augmentation
If you are in the immediate post operative period then you must follow the post operative instructions of your surgeon. because if you do not follow your surgeons instructions, you are to blame for any complications.
In the long run patients with submuscular implants can go back to normal exercise routine.
There are some important issues to consider with implants under the muscle and exercise, especially what are called animation deformities. In general, there is no reason why you can't use your pecs but also no benefit in terms of appearance so it may be better to focus on other muscle groups. If you are not a good candidate for implants above the muscle, then a muscle-splitting biplane procedure may be worth looking into.
I personally do not do submuscular breast implants on anyone whom I know will be doing upper body exercises. The reason is that this can cause displacement of the implant, reduction of the breast projection, and, possibly, an increase in rupture rate. If you are going to do this, I would recommend going slow.
After submuscular breast augmentation I generally have my patients wait 6 weeks before performing the chest exercises you have mentioned. I have found that displacement of the implants is less common after this waiting period.