When I was doing bicillin injections my husband would inject very slowly..and I mean taking 3-5 minutes each time. I was doing them 3 times a week and never had any muscle pain or any kind of pain from them. We did mix lidocaine with the bicillin and maybe that also helped.
I would walk around for at least 30 minutes after the injection and massage the area to make sure it was all dispersed. Hopefully this helps you. I had them for almost 4 months and would have continued if they had helped me.