I second what hbiss just said. Especially I want to emphasize that young electricians should stay off their knees as much as possible, especially on hard surfaces like concrete. Balancing the full weight of your body on open floor joists while in a crawlspace will also take its toll. If yu must bear weight on your knees, use knee pads. There's also nothing wrong with making up splices or installing receptacles while sitting on a stool or spackle bucket. The reason I say "young" electricians is that when you're young you never envision future problems. Once you're older you'll wish you had done things differently.
