As a union apprentice, your instructor should have already told not to lift anything by yourself that you can lift with two people. As for the demands of the trade, you will see many, many small cuts and scrapes from commercial grade flourescent fixtures, sore ankles from ladder work, sore knees from kneeling, sore back from installing receptacles over kitchen cabinets, pulled muscles in your back from pulling the bigger wire, sore thumbs from pushing wires back into boxes, etc.
My suggestions would be to get the most comfortable work boots you can afford, always wear your PPE (hardhat, safety glasses, gloves when needed)and work smarter rather than harder.
I am a third gen. electrician after my father and great uncle and both of them made it out of the trade relatively unscathed. My dad still has minor problems with his knees but, at his age it's to be expected.
Just take it as easy as is feasible and don't take unnecessary risks. You'll be fine.
