In 1997, fitness trainer Stephanie Jackson published Catflexing: The Catlover's Guide to Weight Training, Aerobics, and Stretching. If you look at the cover and suspect that it's about how to use your own cat as a piece of strength training equipment, then you'd be right!
Once I saw this book mentioned by Pulp Librarian, I had to get it. I filed an interlibrary loan request and was able to read it. This book was everything I had hoped for. Yes, Jackson really does use her cats, who are named Bad and Masi, as fitness equipment.
Her exercise plan does require an extremely tolerant and trusting cat. I would need to visit the hospital after attempting this with my own cat.
This book is not a gag, although it would make for a fine gag gift for a catlover. It covers pre-workout stretching and nutrition (the latter for both humans and cats). It could actually be an excellent beginner's guide to fitness for people who find fitness training manuals intimidating.