Intro to Lower Body Biomechanics
An important factor before approaching rehabilitation is an understanding of some basic bio-mechanic guidelines so you can set appropriate goals.
For the purposes of rehabilitation it can be useful to imagine the motion of the lower extremity to be broken down into 4 quadrants: Low back/pelvis, hip, knee, and ankle. These can be further broken down but for now this is sufficient. Between these quadrants there is a give and take relationship.
Quadrant 1: Pelvis
The low back and pelvis provide stability and anchor points for the hip muscles. Spinal engine theory coined by Serge Gracovetsky in the 1980's puts forth the idea that movement of the limbs originates in the spine and trunk.The motion of the limbs is an amplification of motion that begins in the spine.
The hip joint is meant to be capable of a large range of motion, nearly as wide a range as our shoulder joint. As with the shoulder this impressive range of motion can lead to instability issues in some athletes.
The knee joint is a source of a lot of pain in both runners and lifters. In my experience a lot of the time the cause of this pain is from either above or below the knee itself. This is because left to its own devices the knee is a fairly stable joint that only wants to move in a hinge like pattern.
Quadrant 4: Ankle
The ankle is really an amazing joint that is capable of movement in so many planes but like the hip this huge range of motion can put the joint at risk of instability.
Now where things get interesting is in how these quadrants interact with each other to achieve movement. Whether it is to perform a squat, a deadlift, or in locomotion the relationship remains the same. This is best explained using examples with caveats, the biggest one being that we assume the body in the example is a healthy body that is fully able to adapt. I'm also going to be inventing some units of measurement for simplicity.
The Units:
As promised the unit we are using is going to be called a "Range Unit"(RU). A quadrant with a high RU score is considered to have a larger range of motion and higher instability. Low RU scores mean this quadrant has a smaller range of motion and higher stability
So under healthy conditions when quadrants are able to move freely all four quadrants combined will equal 100RU.
Q1(pelvis): 5RU
Q2(Hip):50RU
Q3(Knee):15RU
Q4(Ankle):30RU
Therefore: Q1(5RU) + Q2(50RU) + Q3(15) + Q4(30RU) = 100RU
So now we can play with these numbers.
Problem 1:
Lets say this person has a stationary desk job with tight hips. We are going to minus 15RU from their Q2 leaving 35RU left. Without compensation we would find our four quadrant total only at 85RU. One of our caveats is that healthy bodies will adapt so it will rely on the other quadrants to restore its 100RU total. Most commonly it will demand more RU from the quadrants that are adjacent. Unfortunately for Q1 & Q3 this means increasing their RU score above the healthy range. When more motion is demanded of a stable joint you increase your risk of injury within it. If the RU deficit is made up for with an increased RU at the knee then you are increasing instability and possibly when loading the knee with stress you may experience an injury. This is an example of how an unstable knee can actually be caused by a lack of flexibility in the hip.
Problem 2:
This is a more complicated scenario. Lets say this person has experienced an ACL tear in their knee so now they have increased instability of 10RU. Initially as they return to their activities the hips and ankles will have to make up this surplus RU by decreasing their values appropriately. For a non athlete this may be a sustainable scenario but for someone who is frequently loading the joints this can actually be a significant amount of dysfunction. The decreased RU in the ankle can reduce someones potential stride length in running and their squat depth in lifting. Decreased RU in the hip may interfere with the function of the sacroiliac joints in the pelvis. This is where appropriate physiotherapy to decreased the RU at the knee by increasing its stability while also increasing the RU at the ankle and hip can restore function post injury assuming this is a non-surgical tear.
So what's this all mean?
This means that our bodies are incredible at adapting to imperfect bio-mechanics. Even Usain Bolt does not have perfect joint symmetry. Our nervous system is like a computer taking in information from each joint as it moves (proprioception) and adjusting the suspension on the fly in order to give us the smoothest ride possible. Unfortunately there are limitations of matter and tissues can only be stretched so far until they begin to break down. When we divert too far from normal ranges of motion we overwhelm our ability to adapt to it and sometimes it is the adjacent joints that feel the brunt of it.
-This is a useful tool but not a replacement for bio-mechanical evaluation performed by a licensed professional-
