Knock on wood I am in pretty good shape at 65. I hit the gym 5-6 days a week and work my body in all ways. Strong body and strong mind matters as you get older. Physical and mental exercise is the key.
My amp weighs 5 pounds and puts out enough power to drive most any speaker well. It excels sonically in pretty much every regard as well.
No desire or need for big heavy amps anymore thanks to modern technical innovations.
Lifting heavy objects requires strong legs core arms and upper body. The whole Shebang. I’d advise consulting a personal trainer for guidance on how to exercise and do it properly without causing injury if not sure.