Table 1. Parameters of Head Hit Index (HHI) calculations.

HHI	A = 0	A = 1	A = 2	A = 3
(A×B)	(no hits)	(1–4 hits)	(5–20 hits)	(>20 hits)
B = 0 (e.g., negligible, body-helmet or ground-helmet contacts)	0 - no head hits	0- negligible hits	0 - negligible hits	N.A.
B = 1 (e.g., player acknowledges the head hit)	0 - no head hits	1 - e.g., a few head hitsduring game	2 - several butnormal hits	3 - players who had>20 normal hits
B = 1 (e.g., player acknowledges the head hit)	0 - no head hits	1 - e.g., a few head hitsduring game	2 - several butnormal hits	3 - players who had>20 normal hits

A scoring system was used to segregate players based on the number and intensity of head hits experienced during games (see also Methods for details).