Here is an article from 2010 (a horse racing site):
Philadelphia Park horsemen and stable employees are mourning the loss of veteran exercise rider Mario Calderon, who died from injuries suffered during a training accident at the Pennsylvania racetrack on Sunday morning. Calderon, who also maintained a jockeys license though he rode infrequently, turned 55 years old on Saturday.
According to sources, a horse Calderon was aboard at approximately 6:30 a.m. took a bad step or was spooked approaching the seven-eighths pole. The rider lost his balance, got his foot hung up in the stirrups, and was dragged nearly a quarter of a mile. He suffered severe head and chest injuries after being kicked while the horse continued to run. An ambulance was called immediately, Calderon was taken to a nearby hospital for surgery, but he was pronounced dead shortly thereafter.
Calderon had been employed by trainer Pam Shavelson for a number of years and was well known in the Philadelphia Park backstretch community.
http://www.paulickreport.com/news/ray-s-paddock/exercise-rider-dies-at-philadelphia-park/
That's a problem. The ski industry started to address that issue in the 60's.