* We went from heavy leather shoes to nylon shoes.
* T&F used to be 90% plus North Americans, Europeans and those from Australia and New Zealand. By 1960's, the entire world began to participate.
* We went from cinder tracks to red rubber tracks to Mondo tracks.
* 1950's, we began running 100 plus miles per week for some 800m & 1500m athletes to most 3000mSC to Marathoners.
* 1960's & 1970's, athletes began cross-training.
Look at 800m. We went from 1:46.6 in leather shoes on cinder track in 1939 to today. No one can break 1:42 anymore.
Thoroughbred horse racing has been a thing since 18th Century. Thoroughbred horses reached their peak about 50 years ago.
There are thousands of sub-4 milers &/or sub-3:42.2 1500m men because there have been thousands of men who have been able to race sub-1:48 800m while training for 1500m or one mile. Until women can race 1:53.0 800m on 1500m/one mile training, sub-4 one mile for women will not occur.