I don't think you can compare different jobs with eachother. It depends on the person what is considered as heavy. If you are a thinker even doing nothing takes a lot of energy. A fact is those doing heavy work in factories,building houses out of stone and so one will not be able to do so till the age of 72. Sitting behind a desk, answering the phone, filling out papers, computer work can be done.
What we need most is finding a job that brings us happiness. A workplace like that will give us energy and even withpainful joints and back we go over to work.