The traits exist already and were inherited from the parents or resulted from mutations in the germ cells of the parents.The general premise is that these traits are necessitated due to environmental stressors.
That is a bit different than what you are suggesting. An athletic population would have had genetic changes occur over time in the population and if the environment supported those changes, they would become fixed in the population. But an extent population didn't have a community wide series of transformation events that resulted in concurrent genetic changes. The population has the same genes it started with.An athlete's ancestry would very likely show some need and environmental stressors which necessitated the traits and abilities.
We don't consciously alter our genes in preparation for perceived changes in the environment. If some of members of the population have a genome that provides a benefit to a new environment, then that environment will favor them and they will have a greater success at reproducing. Inevitably, some of the genes that were beneficial in the prior environment may be lost. But in neither of those events was the trait willed into existence or lost due to lack of interest.That's the general premise. The hope and what i find most intriguing is our ability to project future environments and to be able to consciously prepare for them in order to help ensure our survival rates.
Having a convincing expectation of an environmental change would not naturally result in the formation of new genes to meet those perceived changes. A group of people could decide to get together out of a knowledge that they possess genes that would be favored in the expected change, but that would be a breeding experiment artificially selecting genes that already would be known to exist based on the phenotypic expression.This could ease the burden and better prepare our descendants for future stressors that would necessitate the changes, ensuring greater survival.
The bottom line is, that while an interesting idea, it is an old idea that has never had the support of evidence. Random mutation and environmental selection acting on populations is still the model supported by the evidence.