I remember it from 8th grade english book. Story is about a gladiator type setting in which a human is battling a monster and there is a force-field type enclosure. He realises that something inanimate can pass through the force-field, so he smacks himself in the head and knocks himself out and passes through the force-field.

Need help, this has been bugging for about 20 years.

This is not by Bradbury, but if no one here answers I can post it to the usual places.

This sounds like "Arena", by Fredric Brown. I think it was once adapted for the TV series The Outer Limits, and was also used as the basis of a Star Trek episode.

I second that answer. Definitely "Arena", by Frederic Brown.