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Human Mechanics of Rotation 1: Background

When we think about the human body rotating through space (for instance to throw a ball), there are biomechanical factors at play both internally and externally influencing this motion. Understanding these factors will influence how we train for and improve our rotation mechanics. Rotation, just like any other movement, is governed by the laws of physics. These laws give us information about forces involved in causing movement to start and stop as well as how we can quantify and measure movement. One thing that is missed when only letting the laws of physics describe motion is the internal mechanics of the human body. We need to understand how the pelvis and ribcage manipulate internal forces and influence the function of our extremities. In order to take a closer look at this, we will discuss characteristics of our anatomy, actions that cause rotation, and how different athletes achieve rotation.


The human body has often been described as a series of levers working together to produce motion. While this is the case, I believe this definition is lacking. If we look at a cadaver, we see that pulling on a certain muscle (contraction) results in a subsequent motion of a body part. However, in living bodies, we cannot reduce motion to a simple one muscle pulling on a body part. There are many other factors at play. We have to consider that the human body contains a suspended compartment right in the middle filled with air, fluid, and guts. When we shift this compartment around, there has to be a change in muscular recruitment otherwise we would fall over. So, this is where we should start our understanding of rotation. As a matter of fact, we spend the first year or two of our lives trying to learn how to deal with this fluid-filled sack suspended in space. As a baby learns to roll over or stand up, he is learning how to manipulate his guts to achieve a movement outcome. As he takes his first step, he realizes that his guts spin to one side as he pushes from one leg to the other. How he learns to deal with this spin will impact his movement for the rest of his life.


Our guts provide instant feedback for movement. As I push off my left foot, my guts are spun to the right and collide with my abdominal wall. As they push into my abs, my ab wall stretches and my brain knows to put my foot down and start pushing against the ground to control gravity and not fall over. This also starts the process of pushing back to the left to keep me moving forward.


Another aspect of movement that we need to understand is the idea of compression and expansion. Dr. Bill Hartman uses these terms to describe any human movement. When I compress or contract and area I am closing off that space. This means that the fluid or guts in that area or pushed out and must take up space somewhere else. If I have compression in one area, there has to be a subsequent expansion somewhere else. Think of our torso as a tube of toothpaste. If you squeeze the top of a tube of toothpaste, all of its contents move to the bottom of the tube. Similarly, if I compress my right chest area, I have expansion on the back right allowing air to move into the expanded area.


Now that we have this as the basis for human movement, let’s talk about actions that cause rotation to occur. Let’s go back to our example of compression of the right chest area. There are a few options I have in order to compress that area. If I reach my right arm in front of me as far as I can, that area has to compress. This causes the right upper back and left chest to expand as well as the left upper back to compress. Let’s also take this back to walking. As I swing my right arm forward, this turns my chest to the left. The left chest as well as right upper back expand and fill with air. This acts as an airbag too slow and control the rotation to the left. I can then reverse this and continue walking by swinging my left arm forward to compress the left chest and expand the right chest and left upper back.


This example has focused on the upper body and its rotational properties, but the same principles apply to the lower body. As I step my right leg forward, I have a compression on the right front of my hip and left posterior hip (backside). Then I push off the right leg back to the left and reverse the compression and expansion strategies. When we put the upper and lower body together, we have rotation occurring up top and in the lower body. This reciprocal rotation creates a stretch reflex that helps spin us back to the other side and make the gait cycle efficient and effective.


Finally, let’s talk briefly about how people with different postures and anatomical characteristics can achieve effective rotation. Athletes can often get stuck in a position of expansion or compression in different parts of the body based on which strategies they use often. For instance, people that lift heavy weights regularly use what we call an extension strategy. This results in an expansion of the chest and compression of the upper back. Picture a person squaring up ready to fight. This position widens the ribcage. So imagine we take a regular person and they are holding a 6 foot plank of wood across their chest. We have artificially widened their ribcage. Getting them to turn is going to take a lot more time and effort comparatively. Now on the opposite side of the spectrum, if we have a person with a more narrow ribcage, creating a turn should be easy.


We have briefly touched on some of these intricate topics. The big takeaways are: -Human movement is a complex phenomenon -Our guts and internal air and fluid impact our movement -Rotation occurs as a combination of compression and expansion -Different shapes of people achieve rotation in different ways In the coming blog posts, we’ll dive deeper into rotation. We’ll take some time to break down different rotation movements such as cutting, throwing a baseball, throwing in lacrosse and more.


Austin Parks

Masters of Science, Human Performance



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