I was plumbing in a faucet in a sink cabinet to day with a 3 by 2 1/2 by 2 1/2 opening; I put towel over the edge so it would not cut into my back, successfully, but when I slid part way out to sit-up my muscles hurt like bad bruise.
By the fifth time I slid out and sat-up, it still hurt but nowhere near as bad
Due to the drought and some other unexpected things this summer, rather than get more and more outside heavy work done, I did not even get the exercise of mowing two lawns every week ( especially as this spring after fertilizing, I was doing it twice week because of above average rain and heat.)
That lack of exercise is probably more the reason for my lack of flexibility and sore muscles than old age ( my cousin's excuse for ALL HIS problems and he is a year younger than me.)
I found out last year when I put in a three foot high by twelve feet long retaining was with 1/2 yard dirt shovel out and 1/2 yard crushed grainte carried in & 82 pound blocks, when I started it was misery, by the time I put the top level on two weeks later , it was work, but far, far easier than when I started.