As I browse articles on swing mechanics, I notice a lot of statements made that are only based on observation from video., even slow-motion video. The problem is that without a three-dimensional (3D) motion analysis system, or knowledge about biomechanics, the observer is only guessing. Because of this, there are many misconceptions about efficient swing mechanics.
The area I want to address with this post is the action of the hips.
The first thing to understand is that the hips are one solid piece. There are no hinges. Neither side (left or right) of the pelvic girdle can move independently from the other. Anything the "rear" hip is doing, the "front" hip is doing equally, just in the opposite direction. Many people refer to the actions of the rear hip vs. the front hip, as if they are moving independently which is impossible. See the illustration below:
Notice the bowl-shaped bones on each side? The whole purpose of shaping bones like that is to be able to attach sheets of muscle to the edges. That's where the abdominal muscles attach. See the next illustration and notice that the obliques (muscles that help cause the torso to twist) attach along the ridge of that part of the pelvis:
In an efficient swing, the hips serve the hitter by externally rotating before the shoulders, causing a stretch across several muscle groups including the obliques (above), the psoas group (below), and others.
The psoas muscles attach to the base of the spine and the internal ridge of the pelvis, and "anchor" themselves to the back of the femur. You should be able to see how stretching, or pulling, the lower ends of one side of the psoas muscles would cause a "pull" on that side of the spine causing the spine to begin turning.
Now go back to the illustration above that shows the obliques, and where the attach. You can easily imagine when looking at the illustration that when the hips rotate before the shoulders, it causes the obliques to stretch and pull on the rib cage (which, of course, is also attached to the spine).
The stretching of a muscle is a lot like stretching a rubberband. The more the muscle is stretched the more potential energy ("snap back") it creates. So the goal of a hitter who wants to use hips efficiently is to rotate them before the shoulders in order to create a stretch across the core. This is usually referred to as "hip and shoulder separation" or "torso loading." See the photos below:
Essentially, then, the hips only serve as the base, or anchor, of several muscles groups, that when stretched, cause the spine to turn (torso rotation).
Knowing this allows us to address a couple other common misconceptions about the hips. The first is that the hitter should forcefully "fire the hips" to create more power. In reality, the hips are the slowest rotating part of the body during the swing. Most efficient hitters, from youth players to MLB All Stars, rotate the hips somewhere in the neighborhood of 700 degrees per second. That causes stretching across the muscle groups we discussed above which results in the torso being "pulled" around the spine...like when you release a stretched rubberband. Depending on the quality of the stretch and muscle composition of the hitter, the resulting rotation of the torso is usually at least twice as fast as the hips, somewhere around 1,400 degrees per second or more.
The second misconception is that the hips should not rotate early in the swing for a variety of reasons. Knowing that the hips only serve to provide the base of the core stretch, it should be easier to understand why this is not true. In fact, efficient swings involve hip rotation prior to front foot strike! Examples are provided below:
The final misconception I want to address is that this hip rotation occurs on ALL pitches...not just fastballs and not just pitches on the inner half of the plate. Again, the purpose of the hips is to provide the base of a stretch across the core. That stretch leads to higher-velocity torso rotation. So if a hitter wants to hit ALL pitches hard, regardless of location, then he must create that hip and shoulder separation on every pitch. Below are four Xander Bogaerts swings versus pitches in four different locations. The hips begin rotation prior to front heel plant in all four.
Bogaerts isn't unique either. ALL good hitters do this. Look at more video to see for yourself.
Knowing how the hips actually contribute to an efficient swing should help you coach or train smarter.