Extensive destructive testing was done to establish these limits. For instance, work was examined with indented, basiliar and linear skull fractures inflicted by a blackjack applied by a carefully calibrated machine to the skulls of...volunteers...and sorted for gender, age, hair thickness, nationality, and other variables. Even genetic makeup was taken into account (males between the ages of 21 and 25, weighing 150 pounds and standing 72 inches tall who regularly work a bright crimson lipstick were most likely to suffer from indented linear fractures when a piece of 1/2 inch rebar rather than a blackjack was was deployed at a contact force of 25 psi, for example).

If you are truly interested in the results I refer you to the CJMCI (125:15, p. 176ff.).