• Keaton Worland

Joint Mechanics: Levers or Hydraulics?

Updated: Dec 31, 2019

The more I consider the "how" of human movement the less I am convinced that we operate as a lever system. If we truly were a lever system our bodies would literally implode during heavy resistance or plyometric training. We simply don't have rigid enough frames to support these activities. There has to be something else facilitating movement. That is why I now see movement as the byproduct of an internal hydraulic system. In this article I want to discuss why we may actually move by hydraulics and not levers.

Debunking the lever system

Levers are simple rigid machines whereas we are complex biological systems that are ever changing. We cannot operate under the same rules. A lever provides a system a mechanical advantage, but requires a rigid body to physically contact and rotate about a fulcrum. If we apply this idea to joints we see an issue: physical contact. Bones do not touch, there is cartilage and fluid between bone surfaces. Cartilage is hydrophilic meaning it loves water and the fluid within a joint, synovial fluid, is largely water. The juxtaposition of the two causes an ordering of water along each joint surface. This in turn creates hydronium ions. Hydronium ions are water molecules with an extra proton. Now this might seem like a small internal chemistry feat, but the implications are huge because the extra hydrogens congregate away from the hydrophilic cartilage surface leaving them at the middle of a joint. Now simple physics states, like charges will repel each other. So in the context of a joint we can see how this could help maintain a joint space and reduce friction. And if water did not order along joint surfaces, bones would touch during movement and it would become incredibly painful and we most certainly would not be able to produce any kind of force seen during athletics.

So if bones do not touch thereby negating the potential for a lever system how do joints move?

Let's consider a different system: Hydraulics

A hydraulic system operates via pressure. Under normal circumstances when a constant pressure is present within a container and acts on a liquid, the liquid exerts a uniformed force back onto the shell of the container because it incompressible. This is Pascal's law. Now think of a joint as the container with fluid (water) bound by the collagen capsule.

But what happens within the joint when we move and create a change in pressure?

All movement results from a change in pressure, which in turn must yield a fluid shift because fluid is incompressible. However this fluid shifts is intriguing when applied towards the well studied arthrokinematics, or movement of joint surfaces. For the past 30 years research has been elucidating how joints follow predictable patterns of motion based on the shape and type of articulation and as a clinician I have found these to be guiding principles as they allow for enhanced mental imagery of what happens within a joint. But what is really happening inside the joint?

Joints roll, slide, and glide in specific directions based on their shape and follow the convex and concave rules.

  1. When a concave joint surface moves on a convex surface the slide and rotation will be in the same direction.

  2. When a convex joint surface moves on a concave surface the slide and rotation will be in opposite directions.

What effects do these arthrokinematics rules have on joint fluid? Given that fluid is incompressible arthrokinematics simply provide direction for fluid shifts. As one side of a joint experiences an increase in pressure and decrease in volume, the other side will have a decrease in pressure and an increase in volume.

Let’s use open chain knee flexion to better understand. This is a movement that requires the

concave tibia surface to move on the convex femur surface. If we apply the above rules and maintain the integrity of the joint there will be a posterior slide and rotation of the tibia. This increases pressure posteriorly in the knee joint causing an anterior fluid shift because pressure and volume are inversely proportional. To simplify this further the more acute a joint angle becomes the higher the pressure is while the more obtuse an angle becomes the higher volume is. It is important to note that movement can only occur in the direction of high pressure strategies because there is less incompressible fluid in the way. (more on this in a later post).

So in the end in order to move well we must better manage internal pressures to allow for optimal joint mechanics.


  1. Joints and muscles are not a lever system because bones do not touch thanks to hydronium ions.

  2. We are a hydraulic movement system that emphasizes the inverse relationship of pressure and volume.

  3. Arthrokinematics provide direction for internal fluid shifts.

  4. We can only move towards high pressure strategies.

Stay Well


#joints #mechanics #water #hydraulics

#movement #arthrokinematics


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