Implants under or over the muscle?

I think it is in the best interests of most patients seeking breast augmentation surgery to have implants placed in the “dual plane” or sub muscular position. This includes athletic patients.
The submuscular positioning allows for more complete coverage of the breast implants leading to generally more natural feel/look  of the implants in the long-term. This position will also decrease the potential for rippling and/or palpability  of the implants (which may increase with time, weight loss, and/or post-pregnancy changes).

The submuscular positioning  also tends to interfere with mammography less so than breast implants in the sub glandular position. The incidence of breast implant encapsulation (capsular  contraction)  is also decreased with implants placed in the sub muscular position.
On the other hand, sub glandular breast implant positioning does not have the potential downside of “animation deformity” (movement/ distortion of the breast implants  seen with flexion of the  pectoralis major muscle)  that can be seen with breast implants placed in the sub muscular position. 
Overall,  after considering pros/cons carefully, I find that there are  many advantages to placement of breast implants in the sub muscular ( dual plane) position. I hope this, and the attached link, helps.

The ideal postion for implant placement depends on a number of factors, some of which include shape of breast , amount existing  breast tissue, degree of Ptosis(droop), level of physical activity etc..It is difficult to give you an answer without formally examining you however from what you have described in your question under the muscle is probably going to be your best option.