Does placing buttock implants inside the muscle affect the function of the muscle?

Indeed the absolute best placement of buttock implants is within the gluteus maximus muscle.  Although this does cause more pain and swelling during the intitial recovery phase, it does yield the best and most natural contour long-term results by far. I have done this procedure on many athletes, beauty, and fitness competitors and they have all been able to get back to their competitive functioning status post-operatively. Glad to be of help. 

As a body builder and board certified plastic surgeon, my experience with buttock implants is that a reasonable size placed in an intramuscular position tends to produce the result with the fewest number of complications. This approach tends to minimize palpability and visibility as well.  Implants placed within the muscle will appear more natural and feel more natural without the risk for sciatic nerve compression found in under the muscle implants. Intramuscular implants can improve volume by 350 cc or so, depending upon the size of the individual. A significant amount above this volume compromises healing by creating more tension on the healing incision.  After buttock augmentation with implants, exercising the muscles with resistance training can improve muscle size. Allow 2 months after the procedure before you gradually return to your exercise regimen.

Kenneth Hughes, MD, Board Certified Plastic Surgeon
Los Angeles, CA

You do not need to cut the muscle transversally  to create a pocket inside the muscle, you rather spread the muscle along its fibres. Usually there is no problem with muscle function at all.In very few cases muscle atrophy can be an issue but that is more an aesthetic issue it would not affect your muscle function itself.