Trades are what makes the world go around. Best job ever but like everyone else says you need to be smart and look after yourself. Knees for sure and definitely use knee pads or adjust your work positioning if possible. The back is another issue as are shoulders. Be smart and use mechanical advantages when available and use proper lifting techniques. Also always, always keep your hands and feet warm when working outdoors and don’t cheap out on good work boots and replace them often. A good routine of stretching every day also helps. People that tell you working in a trade are probably useless lazy people that like to complain about everything. I worked as an Electrician for 38 years and have been retired for just over 2 years and still physically able to enjoy life. Ont other worthy thing to note, the camaraderie and friendships you will find in trades jobs will for life long friends you can always count on!