Vanishing strength?

In our yard, there are have two compost tumblers that we began purchasing more than 10 years ago and that we still continually use. 

A few weeks ago, I emptied one of them, spread its black, powdery content over part of our veggie garden, and then attempted to rotate the full and not yet composted one and could barely move it, yet alone get it too spin. 

I felt it was extremely heavy. For years now, I been able spin the 80 gallon device without straining myself too much, but this time, it took me several minute to make it do a 360 degree movement around its axis. A few days ago, 

I was dumping some new vegetal waste in the empty drum next to it, so I decided to give a spin to the full one and, once more, had terrible difficulties getting it to fully turn around. 

Without making any more excuses, I concluded that it wasn’t that the tumbler was overloaded or any other plausible reasons that were opposing my manly strength, it was just that I wasn’t as strong as I used to be!