I'm trying to wrap my head around HA fillers placement as it relates to muscle vs. connective tissue.

I don't recommend you inject any filler into the muscle itself.  Depending on the type of filler, it should be injected in the dermis, or supraperiosteal (meaning right on top of the bone).  I agree the Autologous Fat Transfer would be a good option for fixing your particular issue.  Best, Dr Giordano

Dermal fillers are meant to be injected into the skin.Depending on the type of filler we  are injecting the depth within the skin varies. The skin sits on top of the muscle. Dermal fillers are not injected into the muscle. Therefore the injection would be in the skin "on top of the muscle". Hope this helps.

The HA fillers can be placed above or below muscles depending on the area of the body. In the cheek for example the muscles, connective tissue and fat are very inter-related and thin layers that often the filler goes where it needs to create the result. The HA in the filler increases HA production in the tissue itself for longer effect in the tissue. As far as making areas that need significant contour correction like the calf example you give, the number of syringes you would need to acquire the result would be ridiculously expensive. A better option would be fat grafting to the area. Hope this helps.