Science fiction, man trapped in a "time box" (?), lots of push-ups and sit-ups

I've been trying to figure this out for years. I recall reading a science fiction novel (or possibly short story) where a main plot element was a man (boy?) trapped in some type of box/container/cell where the flow of time was altered. From his perspective time moved normally, but the outside world was standing still. I feel like he was trapped for at least a year, during which time he had nothing to do but eat some kind of protein bars and do lots of exercises like push-ups and sit-ups. I think he was plotting to get even with someone upon his release. The book was almost certainly published in the 70s or 80s.

I appreciate any clues you might have. Thanks!

Singularity by William Sleator

That's it! Thanks!

Beat me to it. :)

We should probably encourage all Name That Book-ers to take a look at William Sleator's body of work before posting their question. ;)

4: Or at least House of Stairs.