Hey Aaron, why do you Aussies always seem to be one step ahead of everyone else?
GIRD in throwers & swimmers has been well documented. Up until the last few years, the consensus was a tightness of the posterior shoulder musculature & joint capsule. As some great research coming out of Australia shows, it's not necessarily the case:
"Sports Participation & Humeral Torsion/Whiteley Ginn et al./JOSPT 4-09.
I realized this occurred in older & professional athletes, but this is the first time (as far as I know), where it's documented in adolescents. The authors say humeral retrotorsion is the norm until about 8 years old. They claim participation in throwing or swimming prior to full skeletal maturity may slow the rate of retrotorsion loss. An interesting finding was in swimmers, the dominant hand side shoulder was more retrotorsioned than the contralateral.
So what does this mean to us down on the hs level? Before making the diagnosis of GIRD, be sure the athletes total arc of rotation is at least equal to the contralateral shoulder. That is, as long as the athlete makes up the active/passive IR they've lost with ER gains, the posterior capsule/musculature may not be hypertonic.