749KDV,
may be you'll find useful to take a look at MSS standard practice SP-91 (1992 Edition, Reaffirmed 1996), "GUIDELINES FOR MANUAL OPERATION OF VALVES".
Just some FYI in case anyone else is looking for similar information, there is a european standard, DIN 33411 titled, Human Physical Strength (maximum static action moments applied by male operators when actuating hand-wheels, that I found very helpfull also.