While some are able to extract valuable benefit from gyms the vast majority are wasting time and money. In addition to that, most of these articles are very thin on reproducible facts and true research-based data.
This is, by far, the most interesting and useful resource I have found when it comes to fitness:
http://www.amazon.com/Body-Science-Research-Program-Results/...
The author covers cellular biology and debunks ideas such as "cardio workouts", treadmill bunnies, jogging and walking around your neighborhood to loose weight with plain-old science. Here you'll learn about the cellular metabolic process, Krebs cycle, Insulin resistance, fatty acid synthesis, glycolitic cycle, Cori cycle, Bohr effect, glycogenolysis, amplification cascade and whole host of other topics that are important, relevant and reasonably well understood.
What's more important is that everything that is proposed in this book is backed by science and scientific studies. It's like open-source software. If you care to dive deeper into why something works the way it does the scientific references are provided. The book has over 25 pages of listed references (about 10% of the book is reference data).
Anyhow, one of the claims of the book is "12 minutes a week" every seven to ten days. In other words, that's the actual time under load you need every seven to ten days to affect significant changes in your body. This does not include time walking around, watching TV or resting. Time under load.
I have to say that it works pretty much exactly as advertised. After reading the book I tried it and had a friend try it. We'd spend about fifteen minutes under load at the gym once a week. For me it changed to fifteen minutes every 9 to 12 days (you track your data in order to determine frequency). I got stronger with every passing week. Something that I was not able to do without a ton more effort in the past.
If you are interested in learning about this, start with Dr McGuff's (the author) videos:
http://www.bodybyscience.net/home.html/?page_id=2