My implants move when I tighten or use my peck muscles. Is that normal?

It is normal to have some muscle movement with sub muscular augmentation especially if you have larger more developed muscles.

This is common after subpectoral augmentation and can range from mild to severe. Scoring of the muscle internally or change to a subglandular position are options to discuss with your surgeon if the problem is severe enough that you would consider a revision.

An interesting analogy, and this is one side effect of submuscular breast augmentation. The breast and implant will move as you flex your chest muscle. This can be modest for some, and a problem for others. A shaped textured implant might do well subglandular if you are having an issue.