Description and usage
The stadium was an open, oval area (frequently including a racetrack) around which was built an enclosed series of tiers of seats for those who came to watch the spectacles.
Translation
In 1CO 9:24 the important feature is not the stadium as a construction, but as a place where races were conducted. Accordingly, the literal phrase “all the runners in the stadium” may very well be rendered as “all the runners who are racing.” The focus here is upon the competition rather than upon the type of place in which the competition took place.