Horses evolved as social grazers of the plains, group survivalists moving and grazing together most all of the time. During their 60-million-year evolution, horses came to require near-constant movement to maintain health and vigor of wind and limb. Pulmonary health and resistance to EIPH is dependent on near constant movement and walking, in addition to the daily conditioning routines. To keep lungs healthy, American trainers have to better care for their horses, much as the international trainers are required to do.
Full article as it appeared in the NYTimes: