Dealing with Compression, Dealing with Tension
- Compression occurs when you apply pressure to something as if trying to flatten it.
- Tension occurs when you pull on something.
A key point in either case is the idea of resistance.
- For something to deal with compression, it has to resist the forces compressing it.
- Likewise, for something to deal with tension, it has to resist the forces that act to pull it apart.
When dealing with a range of possible compressive and tensioning forces, a flexible approach may result in less effort and less wear and tear.
An overview of joints, bones and muscles
Joints connect bones and and the bones they connect are controlled by muscles.
Working with joints, muscles control the relationship between bones and changes in those relationships either causing bones to move relative to each other or resisting the tendency of bones to move relative to each other.
When muscles are relaxed, bones will move freely within the constraints of their joints given any forces they encounter.
Examples of the range of compressive forces our joints deal with
As a whole our body and its joints can be subject to a wide range of forces.
For example, standing on both legs our knees can share the weight of our body. But standing on one leg, the knee of the supporting leg will have to support double (or more) the amount of weight compared to when both legs are sharing the load.
While walking, at each heel strike, the knee will be subject to a certain amount of force. That can vary depending on the style of walking. For example, deliberately striking the floor with the heel (as if kicking the floor) will result in more force than simply striding forwards.
While running the force of a heel strike will be even greater than while walking because of the greater acceleration (or if you like, deceleration).
Running down hill, the heel strike force can be even greater.
Then there's lifting weights.
Take as an example a weighted squat. Warming up with a lighter weight, say a 20kg bar, the knees will be subject to the weight of the upper body plus the weight of the bar.
Add more weight, say 20kg per side, and now the knees have to deal with 40 kgs of greater force that gravity exerts. Add even more weight, say another 10kg per side, and the knees have to deal with that much more force.
Out joints also deal with tensioning forces
Now suppose instead of supporting the compressive weight of your upper body each knee is supporting the weight of a lower leg. Say you are hanging from your hands to do a pull up or chin up, or you are at the dip machine. Your knees are now supporting, as mentioned, the weight of your lower legs.
The weigh of the lower leg is hanging from the knee joint.
This is a tensioning force, one that tends to pull the bones of the knee apart rather than compressing them towards each other.
Add ankle weights and that force is even greater.
Alternatively, hang upside down from your knees. The back of your shins can actually hook the bar. Now your knees are supporting the weight of the rest of your body. And that weight is trying to pull your knees apart.
Our joints have to withstand a range of pulling and pushing forces
The point of the above examples is that our knees, and any other joint of the body, has to be able to withstand a wide range of forces pushing or pulling in different directions. And not only that, our knees have to be able to deal with these forces through a range of positions and while changing positions.
What sort of mechanism allows the knees and other joints of the body to do this without wearing out?
The basic components of a skeletal joint
Our joints could be thought of as fluid filled bladders, albeit, very thin bladders.
The joint capsule of a joint is a connective tissue envelope that connects two bones. In the case of the elbow joint and possibly the shoulder joints (collar bone to shoulder blade and shoulder blade to upper arm bone) the joint capsule may connect three bones.
The parts of the bones that actually "meet" within the joint capsule are covered with cartilage. These parts can be referred to as "articulating surfaces".
Within the joint capsule is synovial fluid. This synovial fluid can create separation between articulating surfaces.
Under small enough loads, surface tension may be enough to maintain separation between articulating surfaces. However, in cases where surface tension fails, synovial fluid may need to be pressurized to prevent articulating surfaces from rubbing or impacting against each other.
And so the "bladder" of our joint is made up of the joint capsule and the synovial fluid that it contains.
When talking about a joint, generally what is being referred to is this fluid filled joint capsule.
Controlling joint capsule tension
The joint capsule itself connects to tendons and ligaments. These in turn are acted on (or directly affected) by muscles.
Changes in muscle activation cause changes in tendon and ligament tension. These changes in tendon and ligament tension in turn cause changes in joint capsule tension. And that in turn can affect synovial fluid pressure.
Further mechanisms for controlling joint capsule tension
Closely related to joints are bursae. Bursae lie beneath or between tendons and/or ligaments. In some cases they communicate directly with joint capsules.
Where bursae lie between tendons and/or ligaments, they can transmit tension from one to the other.
Where bursae communicated with joint capsules then tension in an overlying tendon or ligament can cause that bursae to inject (or try to inject) more fluid into the joint capsule.
Before going on it's worth looking at some analogies.
How tires handle bumps and differences in weight
Analogy one is a car or bicycle tire (or tires in general). They tend to be filled with air.
The nice thing about tires is that when you ride or drive over bumps, they help to absorb the shock.
While shock absorbers, in the case of the car, help to make the car body ride smoother as bumps and dips are encountered, the tires themselves help prevent damage to the rims. They prevent the rims from impacting the edges of bumps or dips.
On a bicycle, assuming it doesn't have shock absorbers, low pressure tires help to provide a smoother ride. On that same bike with higher pressure tires, the ride can be a lot harder. The more pressurized a tire is, the greater its resistance to change. As a result, when encountering a bump or dip in the road, a higher pressure tire will deform less than a lower pressure tire.
Note that the tires of a bike with a light weight rider will depress a smaller amount than the same bike (with the tires at the same pressure) with a heavier rider.
Riding over a dip in the road, the wheels of the bike with a lighter rider are less likely to bottom out against the rims than the same bike with a heavier rider.
And this is a clue to how our joints can resist changes in load and even deal with the shock of impact.
One thing to note about tires is that the tires themselves are flexible and air is compressible. But something that tires have to do is provide grip.
The compressibility of air and the flexibility of the rubber of the tire itself allow the tire to provide this grip. This is something that a skeletal joint doesn't have to deal with.
The points to note are how tires can act differently given differences in internal pressure and differences in the weight they are dealing with.
Hydraulics
A second analogy is the hydraulic pumps used in Cranes and other heavy lifting machinery. These use telescoping hydraulic cylinders to apply force.
Pressurized fluid helps to extend a cylinder or retract it under control.
As fluid is pumped in, the inner cylinder will be pushed out. As fluid is pumped out, the inner cylinder retracts. The idea to take in here is that fluid can be pumped in or out of a device.
Since fluid is incompressible, fluid can be pumped in to extend the cylinder and pumped out to retract it.
For faster and more controllable retraction (that isn't reliant on vacuum), a second cylinder that acts in the opposite direction could be used.
Two mechanisms for controlling the pressure of synovial fluid
With the joints in our body, we have two possible mechanisms for varying fluid pressure.
- We can vary tension of the joint capsule itself, using it to increase or decrease fluid pressure as required.
- We can also try increasing or decreasing the amount of fluid in the joint.
In either case the idea is to help the joint resist compression across a range of force loads.
Note that in the case of a tire, if when going over a hole in the road, the rim impacts the edge of the whole, the tire isn't doing its job. And the reason for that is that it is insufficiently pressurized. And so the solution in such a case would be to add more air to that tire.
With joints, the idea is that joint capsules can pressurize synovial fluid sufficiently so that bone ends do not contact each other. Or fluid pressure cna be maintained or increased by trying to increase the volume of fluid within the actual joint capsule.
Lifting weights, the greater the weight, the greater the force a joint is subjected to and so the greater the required tension of the joint capsule to maintain joint gap.
An important point is that our joints don't need a huge gap between them.
If the gap at a joint between bones is too great, then the joint will lose some stability.
Just as importantly, as long as there is a gap, that means that the articulating surfaces aren't touching or rubbing against each other (or at the very least, not rubbing against each other with excessive pressure).
This gap isn't filled by empty space. There isn't a vacuum there. Instead it is filled with and maintained by synovial fluid.
Muscle activation controls synovial fluid pressure
One way to increase joint capsule tension and thus increase fluid pressure is to increase tension of the joint capsule itself. This can be achieved by muscles acting via ligaments and tendons.
Another way to increase fluid pressure, and this may be more true in the case of the knees, is by increasing the amount of fluid in the joint capsule.
The knee joint has a number of connecting passageways which connect to bursae and these bursae, which are fluid filled sacs, can be affected by overlying tendons or ligaments. When the muscles of these overlying tendons and ligaments activate, they can press on the bursae which then injects fluid into the knee joint, or at the very least tries to do so. The result is greater fluid pressure within the knee.
And so a very simple idea is that in order for joints to resist compression, muscle tension is utilized to either vary tension of the joint capsule and thus cause changes in fluid pressure or muscle tension is utilized to inject more fluid into a joint, thus increasing fluid pressure.
In either case, the idea of variable tension joint capsules (some with "injectable" joint capsules) is to help withstand changes in compressive force thus preventing bone articular surfaces from impacting, touching or rubbing.
Variations in tension and fluid pressure are controlled by changes in muscle activation.
The elegance of all of this is that the same changes in muscle activation that drive movement or stability can also be used to drive changes in joint capsule tension and joint capsule fluid pressure.
Resisting Decompression
Note that in terms of resisting tensile forces (forces that try to pull a joint apart), part of this resistance can come from the natural vacuum that will occur within a joint as forces try to pull a joint apart.
But in addition, muscle activation can also be used to add tension to tendons and ligaments to help resist tensile forces.
Published: 2021 11 21
