Pedro1965, No wonder your shoulder is aching, don't know anyone who detects 11 hours a day?
We are firm believers of positioning bungee cuff on shaft at the position if you pick up the machine by the bungee it hangs in you normal detecting position. Using two loops so bungee pulls from centre of chest from harness connections, connect and tension bungee so the machine hovers just above ground when you loosen grip on handle.
With cuff positioned too low or two high one end of machine becomes load bearing placing undue stress on arm/shoulder and back.
(We removed the back braces and positioning cups from the pro swing harness but do use the cross brace to support bungee and GPZ.)
Generally our detecting days are broken up into 3 X 2 hour sessions, stopping twice to refresh concentration and take in a cuppa/snack. We prospect as a hobby only, gold is the bonus.
Back in early days of Minelab release SD2000 we noted the majority of our gold was found in the first morning session, that's when we surmised it was because our concentration was at its best.