What are these stripes of muscle on the chest wall with muscle contraction? (Photos)

Thanks for your question and photos! The stripe you see when your pectoral muscle contracts is what we plastic surgeons call "animation."  This is a common side effect form placing breast implants sub-muscularly, below the pectoral muscle, and especially in thin patients like yourself. This does not mean your implants are damaged or ruptured, it simply means your pectoral muscles are more noticeable in certain positions. If this animation deformity continues to bother you, you can have your implants moved from below the muscle to above the muscle with a surgical procedure. There are side effects to this sub-glandular approach as well, so be sure to chat with your plastic surgeon to determine the best surgical plan for you

If I understand your question correctly, this is muscle contracting. It may well reduce as the months pass, but even if it does not, it is normal.