I just read a great blog post by the great Dan John, whose work I wholeheartedly recommend in general, and realized something about movement that was probably obvious to everybody else but not to me.
I always thought of lower body strength as coming from flexion and extension of the knees. I was raised on bodybuilding magazines, and they always focused on quad development (which is what you want if you're going to stand onstage in a speedo) and not glute or hip development, and quad development comes mostly from action at the knee. So I used to focus on action at the knee joint when I worked out.
If you look at a real squat, though, the knee is only part of the story. A big part of the force in a squat comes from action at the hip. I mean, look at a squat - the knee bends, but the hips flex just as much. If you squat and focus all your attention on flexing your quads and straightening at the knee you won't get nearly as much pop as if you really focus on extending the hips and tightening the glutes (go ahead, try it!)
Different extension exercises for the lower body fall into different categories as far as whether more stress is focused on the knee or the hips. Squats put a little more focus on the knees, for example, while deadlifts put relatively more stress on the hips. Just watch someone do the two and see where more motion occurs. Front squats will be even more knee dominant than back squats (because you'll lean farther forward in a back squat, recruiting the hips more). I think you get the idea. Mike Boyle calls different exercises "knee dominant" or "hip dominant."
None of this is new, not even to me. But reading Dan John's post I realized that it relates to something else I was confused about: jumping. What's the problem? Well, if you think about jumping as forcefully extending a bent knee (which I always have), you run into a couple of problems. First, you might think that to jump high you'd squat down really low and load the knees, but if you try to do that (jump high from a deep squat) you'll see that it isn't very effective. Second, you'd think that jumping off of one foot would be significantly (very significantly) worse than jumping off of two feet - after all, you'd get (roughly) half the force, and a little physics will tell you that you should be able to jump half as high. But many of the biggest jumps we're used to seeing - like somebody dunking a basketball - are done off one foot. Now they're also done from a running start, so we're adding in all kinds of conflating factors, like the transfer of forward momentum and energy temporarily stored in elastic tissues in the leg, but still - guys don't run up, plant both feet, then jump up to dunk. Which you'd think they would if it was going to come anywhere close to doubling their vertical jump!
So what's going on? Well, (and again, this may have always been obvious to everybody else, in which case I apologize for wasting your time), if most of the thrust for a jump comes from hip extension and not knee extension, then all makes sense. As long as your stabilizers are keeping the hips nice and level and the supporting leg isn't overly weak you can transmit all the hip snap force through a single leg. In other words, you'll lose the knee extension contribution of the leg that doesn't touch the ground, but you're not losing half your force output, you're losing half of much less than half, or much less than a quarter of your force. I have a much easier time believing that the transfer of forward momentum is going to make up for a loss of less than a quarter of the force involved.
Take home message? When trying to jump higher (because we all know jumping kicks impress the girls) focus on hip snap, not on knee extension. If you're not sure what hip snap is, I can't recommend kettlebell swings highly enough. I really feel the difference in my hips when I swing regularly. You can also apply these same ideas to kicks. Other than the mawashi geri (roundhouse), most kicks utilize hip extension at impact for either the kicking (yoko geri) or supporting (mae geri) leg. By working hip extension in your conditioning program and focusing on snapping the hip and squeezing the glutes when you kick you can increase the power of your kicks.
One last point - you can snap the hip at the supporting leg in a mawashi geri, but the hip is flexing on the kicking leg... I'm not sure exactly which part of the movement is contributing the most to the force of the kick. Maybe that's fodder for a future post.