I agree that the biological impacts are uncertain, but that’s not where the divergence starts. It starts when the Glegg hammer says there is enough give in the ground but the players and coaches say there isn’t - when they say things like ‘just walk on it and you’ll see how hard it is’
It’s not to say the instrument isn’t reliable, or that it is impossible to devise a valid, objective measure of how soft a ground is for an AFL player to run on it, but it’s reasonable to ask the question of whether the response of a 2kg weight from half a metre replicates the human experience of a large human running on a ground for two hours.
I alternate sleeping on two different mattresses at the moment: one good one on my bed, and a rubbish skinner one next to my young kid’s bed. If I used a Clegg hammer on both I reckon I’d get a very similar result, but being roughly 80kg and being on them for 7 hours (if I’m lucky) I can tell you the human experience is very different.