In my case it was regaining strength from taking weight-lifting semi-seriously in my 20's. It was actually amazing at how fast muscle can come back even after a decade. But I continued lifting and am stronger now at 38 than I ever have been in my life. What I am saying is that mid-30's is hardly when sarcopenia sets in. I'm sure it's individualized, but I don't seem to have a harder time gaining muscle now than I did back then; for me it's always been joint/tendon issues that have been limiting anyway.