The training, equipment etc improvements themselves follow population. More natural and designed experiments, more alternatives available, more accumulated know

The training, equipment etc improvements themselves follow population. More natural and designed experiments, more alternatives available, more accumulated know-how and theory. But any given combination of factors is itself part of the athletic trial. The Soviet Russian system of weightlifting carefully husbanded the outstanding weightlifters they had to work with and so selected for athletes who responded well to that training. The Bulgarian system was based on masses of warm bodies, enormous drug regimens and ruthless elimination upon injury. It selected for athletes who responded well to drugs and weren't injury prone. Afficionados of the sport argue constantly about which is "better", since both turned out world record upon world record. But such a debate was beside the point: an athletic system selects whatever it is set up to select for. The rest of the effect is then down to numbers.