Are you weight bearing?

No matter which exercise philosophy or alignment theory you follow, there will be a point where you’ll come across weight bearing movement. This term is widely used but not really very well defined.

A quick look on-line came up with this:

weight-bearing

The physical state of supporting an applied load. This often refers to the weight-bearing bones or joints that support the body's weight, especially those in the spine, hip, knee, and foot. (www.biology-online.org/dictionary/Weight-bearing)

Katy Bowman defines weight bearing in her Foot pain relief book as: 

weight bearing: when a structure is carrying the full burden of its own mass, in the human case, when the bones are holding the vertical mass of the body.

Weight bearing exercises are usually all exercises in which you support your full body weight as a unit, as in running or walking as opposed to exercises were you don’t. 

Not fully weight bearing:

Swimming: the water supports a lot of your body weight

Cycling: where part of your body weight is pushing against a cycle seat, taking the load off the hip joint - part of the body is supported on the cycle seat, and another part on the pedals - two separate units. In this case the hip joint is not weight bearing.

In Katy’s alignment theory weight bearing is often referring to the hip (joint). If you are aligned, i.e. hip is aligned over knee and heels you are more weight bearing than if your hips are out front aligned over the mid-foot, where you are less weight bearing.  Aligned in this instance means aligned to the graviational pull i.e. straight up and down or vertical.

Described as less weight bearing as hip aligned over front of foot.  (www.restorative exercise.com)

On the other hand, you have descriptions of situations where mis-alignment causes more weight being “borne” e.g. the weight on the spine increasing with mis-alignment, see Figure 1 (as recently pointed out in one of the RES questions). Here the head is described as "getting heavier and the spine has to carry more weight with mis-alignment". The further the head is forward the harder your upper body has to work to keep it in place.

The hip or the head being out of alignment is the same situation. In both, the weight is forward of their supporting bones. So why are we talking about bearing less weight in one scenario and more in the other?

As we all know, the actual mass is not changing and neither is the weight. The weight being the mass pulled towards the ground by a constant gravitational force. So why are we talking about more or less weight being borne?

The answer to this, is in the way the weight is borne and what effects the weight has on the part of the body that is carrying it. The weight can be borne by being balanced on a bone and squashing the bone vertically towards the ground or it can be borne by muscles, which pull the weight towards a bone that can support the weight. Here is a graphic of a simplified leg bone and a foot.

You may not be able to see this clearly as the scale of the image is small, but the overall forces on the bone (blue arrow) are the same in both situation, aligned and non-aligned. The only difference is the angle of the force. Gravitational squash is a little less in the second non-aligned situation where muscular pull occurs to make up the difference. The length of the arrows shows how large these forces are. This gives an overall simplified picture of the forces created by gravitational pull on the leg. 

The same appies to the situation of the head being out-front of the spine. Here it is a little bit more complicated since the spine consists of several vertebra stacked on top of each other. By the way they are shaped and connected they are able to carry weight just like a solid leg bone. The further forward the head is in front of the spine, the more weight is pushing down on thin air, causing rotational forces. These rotational forces are then counteracted by muscles in the neck and upper back. Instead of the head being balanced on top of the spinal column and creating just bone squash in the spine, the muscles have to pull the head back connecting it with the spine, which can carry the weight. 

In Conclusion:

When people are talking about less weight bearing they usually talk about bones not being squashed by all the weight above them as some of the weight is carried by the muscles pulling the bone back to vertical.

When they talk about parts of the body becoming heavier then they talk about the muscles that have to pull on these part of the body and try and stop them form falling on the ground. The further forward you carry a body part to their supporting bones e.g. head forward of the spine, the “heavier” it gets as most of the weight is pushing down on thin air creating larger rotational forces that the muscles have to counteract. 

Weight bearing also depends on which part of the body you are looking at and how you are positioned in space. When sitting down, you hip joint is not weight bearing but your neck is still carrying all the weight is usually does. If you do a head stand the neck will be carrying a lot more weight than usual. In the second example the body is positioned in space in an unusual way and what you define as “fully” weight bearing may need to be defined differently.

Does it matter, how your body weight is carried? 
I have found no evidence in literature that the alignment of bones to the vertical axis is crucial for bone regeneration, it seems to depend more on the impulses that are send through the bones and not the angle they are at and both muscle contraction and gravitational squash can cause bone regeneration (more about this in a later blog). 

What it does affect however is pressure in joints and tension pattern in muscles throughout the body. If you use muscles further down the body to keep you upright, you often end up using muscles further up the body to do the same. This causes tension in muscles that have not evolved to carry body weight. Bones are stronger and have evolved to do this.

In joints, when they are pushed together by bones at an angle that is not optimal, they often wear out quicker. The pressure on joints in these situations tends to concentrate on one part of the joint e.g. the inner knee only. When pressure is distributed over the whole area of the joint, the pressure decreases and causes less wear and tear.