The most common reason for exercise-induced collapse after the finish of a long-distance event is blood pooling in the legs, which is known as postural hypotension. The longer the race, the greater the risk of collapse.
If you stand around after a race instead of walking, blood collects in the legs decreasing the amount available to the brain and abdominal organs and a blackout can occur. There may be warning signs such as lightheadedness, nausea, and visual disturbances.
The runner must be helped to lie down with the legs elevated while icing the legs and massaging the legs from a down to up (towards the heart) motion. If in 20 minutes the runner is not awake, then they must be transported to the hospital.
After a race, walk around for at least 5 -10 minutes to allow blood to circulate away from the legs.