Foals need to run. And not just a little hop in a circle, they need to get up to speed, develop lungs, and bone and muscle.
Perhaps it is lack of movement in earlier breeding days that leeds to so many leg malformations.
Because today, in farms where foals do run and play, there is far less of a development problem than places where they grow up in small pens. Then again, they may be destined for desk jobs at a large computer facility and in that case they will be well versed in sitting still.
An athlete, like mother nature intended, starts from day one.