There are a number of books by John Little that followed Bruce's routines, diet, equipment, etc.
In fact, for "Enter the Dragon" was when people noticed that Lee really exploded and was more ripped than ever. Lee got some equipment which was new at the time but was basically a cross-trainer.
Anyway I have all in the series and they proved to be a good read. To answer your question, in my opinion, it was his diet, cross-training, and the fact that running was a form of "relaxation" for him that he was shredded. Well, plus the fact the man was obsessed with training, he would curl a weight while reading a novel.
http://www.amazon.com/exec/obidos/A...9521268/sr=2-3/ref=sr_2_3/002-4416740-6099211
