Let's take a journey through your average morning. You get up and head to the bathroom, maybe taking a peek at your disheveled self in the mirror. You take a shower and after drying off, you probably look in the mirror. As you brush your teeth, you have no choice but to look in the mirror. As you're getting dressed, you look in the mirror. When you have to do your hair and/or do your makeup, what do you need? A mirror. As you head out the door, you take one final glance in the mirror to make sure everything is in place.
Do you see a theme?
It may sound obvious, but our eyes are in the front of our head. This is somewhat unfortunate because every time we look in a mirror all we see is our front. When we greet someone, we are looking at their front. We have become front-obsessed! How many times have you wanted to "work on abs" or have pecs like Ahhhnold? As humans, we are so concerned with our front that we pay little or no attention to our back. Unless of course, you CrossFit.
Posterior chain (http://www.rsdinfo.com/PosteriorMusclesSketchLRes.gif)
If you go back to high school biology, you know that anterior muscles are in the front and posterior muscles are in the back. For our purposes, let's consider the "posterior chain" as the lower back, the glutes, and the hamstrings. In case you didn't know, your gluteus maximus (butt muscles) are the biggest muscles you have. Add to this fact that the hamstrings consist of the biceps femoris, semitendinosus, and semimembranosus, and you have some really big muscles at your disposal.
BIG TO small
So why do we care about the p-chain? Because it helps us DO THINGS! Whether it's picking up a barbell in a clean or picking up a bag of dog food, we use these muscles in our everyday lives. Making them stronger is going to help you live a better and easier life! You know how we talk about using your arms as "straps" on those cleans? Even though your hands are holding the bar, it's your core that does the heavy stuff. Imagine a truck towing a trailer behind it. The trailer hitch isn't doing the work, the engine is! Your hips are your engine. CrossFit teaches an idea called "core to extremity." Another name for this can be "proximal-to-distal." It means that major movements start with the large muscles and finish with the small ones. It's an efficient wave of energy originating from those strong, slower muscles and traveling out to those small, faster ones.
hip extension = HEAVY snatch. (http://www.owresource.com/lifts/snatch.jpg)
THE HIPS DON'T LIE
Think of all that we do in CrossFit involving the p-chain and hips: cleans, push presses, push jerks, snatches, wall balls, pullups, back extensions, lunges, box jumps, rowing, kettlebell swings, air squats, back squats, overhead squats, running, deadlifts, sumo deadlift high pulls, the list goes on. Do these involve a lot of different muscles? Sure! But they start with the opening or closing of the hips to generate power outward. This is why we stress and yell "open the hips!" during a clean. An early pull of the arms is going to waste all of that awesome energy from your p-chain. By having excellent form, you're going to be using your body efficiently and effectively.
So maybe you don't care about being strong and you only care about looking good naked. Or maybe you want to do both. Consider this: the posterior chain, especially the glutes, have a huge amount of fast twitch fibers. Fast twitch fibers use a LOT of energy and when your body uses huge amounts of energy like this, it becomes more efficient at burning fat. Using your posterior chain + proper nutrition = fat loss! Keith Norris talks about this connection in his blog, Theory to Practice.
So whether the motivation is to pick up your kids, pick up a heavy barbell, or just look good naked, let's remember those big ass muscles in the back (pun intended) and work 'em!