This Sport demands total musculature strength/balance practices! ..Properly done, You should not feel any undue exertion or pain in any one point...its a study in ergonomics! Lots of kinesiology misconceptions out there....
Some real doctors when questioned will contend that the 'strongest muscle' on the Human body is the Heart, but as Homer Simpson once challenged, "The heart? I thought it was the weener? I seen a guy on youtube lift a bucket of paint with his!" Well, be that as it may, we cant go outdoors swinging our weeners around with a metal detector attached, as awesome as that would be, we got enough of a bad reputation as it is, so its just not prudent...
So there you go..all sorts of theories out there...The "7min Abs" Workout video may help you, Google it....
Myself, I use my Gluteus Maximus muscles for most things, not only to hold my pants up, but for a lot of things, just getting out of bed in the morning they come in handy...For Detecting, I sort of lightly grip my rig in my tiny little shriveled T-Rex arm, keeping my elbow tucked in tight, and develop a steady and comfortable full body swing utilizing total body momentum moreso than strength..I rock from side to side wide legged staggering like a Sailor on shore leave......
You gotta concentrate and develop a comfortable ergonomic process that works for you...The NOX is topheavy, making a guy want to really get a grip, but that force translates through your wrist and up your arm and down into your sternum and into your thoractics detrimentally... just dont grip it so tight, just your thumb and forefinger, as if its a pencil...cuff the top if you want, let the whole thing roll over, but concentrate on a form where you dont even know you have a detector in hand....its your grip and style thats doing this to you...
FWIW, I can also lift a bucket of paint with my weener...but I bend at the knees and lift with my legs!