"prime movers"?

[REG]

In Mr. Kurz's articles and in his book, the Science of Sports Training, he explains that at the beginning of your long-term sequence of strength exercises, you should start strengthening your trunk (core muscles) before the "prime movers." He also says to start squats without weights right from the beginning of your strength training. However, wouldn't doing squats at the beginnning of your strength training, mean that you would actually be working the "prime movers" at the beginning of your long-term sequence of strength exercises? I am very confused?

Also, which muscles actually are precisely the "prime movers"?