I reckon a Strength vs. Strength roll would be a good test for every unarmed or melee knockdown. HH get a ST vs. ST roll on EVERY hit and without HH it only does the roll on criticals. This would solve most players getting knocked down by ants, rats, manti and centipedes.
even with 10ST it still doesn't work 100% of the time. the last fight my HH char had was when i was mining in my bluesuit at broken hills. another player arrived with a 10mm pistol and started shooting, I won the fight with 8/120 health left. To me that seemed fair, he had multiple chances to get more distance but he didn't take them. Then I got killed by ants, so nobody won.