From the little I know about muscle mass, the less you have, the less you will lose. If you are slender before HRT, I would not expect to notice much of a difference. For me, most of the muscle I lost happened before HRT. I have only lost about 1 to 1 1/2 inches in my arms. I had lost 3 inches in each arm during a few months prior to HRT. I figure, the reason I have lost so little on HRT is because I have a lot less to lose. Hope this makes sense.