The Wilks score is used to compare powerlifting scores for lifters of different body weights. A coefficient
500 Coeff = -----------------------------------------, a + b*x + c*x^2 + d*x^3 + e*x^4 + f*x^5
depending on the body weight "x" of the lifter in kilograms is multiplied by his or her total to arrive at a sort of "standardised" score for comparison. (It appears that it may also be used to compare individual lifts.)
There are specific (sex-dependent) values given in the linked article for a,b,c,d,e,f. Is there any online source explaining the theory behind the Wilks score? The linked Wikipedia article does not explain where the denominator polynomial coefficients (a,b,c,d,e,f) come from and why the formula has the specific form given, and Google wasn't of much help.
The quintic polynomial (a+bx+cx2+dx3+ex4+fx5) appearing in the denominator has three real roots. The negative root can be ignored as meaningless, and the two positive roots (roughly 13.5kg and 283kg) are presumably to be considered "out of range". Thus, I would guess this formula was obtained by fitting some collection of data. But what data? Alternatively, perhaps there is a theoretical model explaining these coefficients? (The only, admittedly crude, model I can think of is a multiplier very roughly like x-(2/3), which doesn't resemble the form given for Wilks, though the curves do have roughly the same overall shape on a sensible body-weight interval.) There must be some published literature on this, but I could not find it.