Introduction to Mobility & Stability (Part 2):

Importance of Stiffness

​In the previous article, we introduced the basics of joint anatomy and unpacked the concepts of joint mobility and stability. Now, let’s take this a step further, to explore how a body’s active and passive restraints work within a joint to enable safe movement.

Remember: Stability is essential to injury-free movement. And an important part of stability is our ability to generate stiffness.

Why is stiffness good?

The stiffness created by the tissues around the joint protects from:

  1. Movement past the joint’s anatomical range of motion (which keeps the joint from “falling apart”)
  2. Harmful forces placed on a joint(see below)

Both scenarios can lead to injury and both require the help of the joint’s passive and active restraints to keep them in check.

So—what is it about the various components of a joint that contribute to its stability?

On the one hand, it’s the passive restraints—including the shape of the adhering bones, as well as the ligaments, cartilage and how they fit together—that contribute to a joint’s “passive stability.” But without the active restraint put upon the joint by muscles, creating “active stability” and stiffness in the joint, you might have a failure of one or more elements that can result in injury.

Figure 1: “Shallow vs. deep joint.” The shape of the ends of the bones that make up the joint contribute to its passive stability. A shallow joint (A) will be less inherently stable but may have more mobility. In contrast, a deep joint (B) will be more inherently stable than a shallow joint but will likely possess less available range of motion.

Image captured using Visible Body

Understanding the role of “passive” vs. “active” joint stability in injury-free movement

Passive Stability:

We can look at the hip joint for an example of passive stability (see Figure 1). Based on the “ball and socket” design—where the ball-shaped end of the femur fits relatively snugly into a cup-like socket in the pelvis (1–B)—the physical configuration (including the shape of the ends of the bones and their cartilage) limit the joint’s ability to “fall apart.”

That’s not to say that this type of deep joint couldn’t separate under the right circumstances. Even a deep ball and socket–type joint such as the hip would be left vulnerable once the two bones began to move on one another. As such, additional restraints are needed to keep things stable, including the surrounding ligaments.

As a joint moves towards its end-range and the ligaments get pulled to the point of tautness, they create stiffness, which contributes to stability. Yet, even with a good network of ligaments, the joint wouldn’t stand a chance under body weight and gravity during movement. The stress to the ligaments would be too great and could lead to ligament failure.

This is where muscles come into play: Under strict control of the nervous system, the surrounding muscles not only provide the means for the joints to move; they also actively regulate the amount of motion and stiffness generated at the joint, delivering an adequate degree of active stability to keep the movement within a range that’s safe for the whole joint.

Figure 2: When multiple muscles contract together (i.e., co-contract) with relatively equal force, the result is joint compression, which translates into stiffness and added joint stability.

Figure 3: During movement, a muscle pulls on one bone, which creates motion relative to the other bone. The co-contraction of an opposing muscle will “check” the movement by adding control and creating compression, and therefore stiffness, within the joint. This stabilizes the joint during movement.

Active stability:

Stability is all about creating stiffness, and muscles are great at creating active stiffness! They do this by “co-contracting,” which simply means that the multiple muscles that cross a joint contract at the same time (see Figure 2). This compresses the two bones into each other, creating stiffness and thereby providing active stability to the joint.

If muscles on either side of the joint are contracting with relatively equal force, you get pure compression and no relative movement. This function is essential for completely limiting motion at a joint.Yet, co-contraction is also utilized during movement (more on these concepts below).

Ultimately, safe movement requires co-contraction of multiple muscles or muscle groups—including those that may oppose the desired movement—working together to create a careful balance between forces that create stiffness vs. forces that create movement.

Within the realm of our “simple model,” when a muscle contracts to move one bone relative to another, another muscle—one on the opposite side—co-contracts simultaneously to create some stiffness within the joint, essentially “checking” the movement from going too far in one or the other direction (see Figure 3). This function helps to mitigate potentially harmful forces placed on a joint.

Stiffness limits the effects of harmful forces on the body

Whether we’re moving or at rest, we’re constantly being bombarded by various types of forces(or loads) that stress our bodies. This includes our body weight, the forces of gravity, the pull of our muscles, the momentum of our movements, and the reaction forces from the ground(the forces exherted from the ground on a body in contact with it). However, these forces aren’t all tolerated equally. So, for instance, while the majority of joints are built to withstand the pressure of compression (i.e., the knee experiences compression when thigh bone compresses into the shin bone when we are standing up), this isn’t true of other types of forces (see Figure 4).

Figure 4: Some forces can be harmful when the joint is not properly stabilized by the muscles. The types of forces that are most damaging often involve some form of shearing (one bone moving in one direction while the other moves in a different direction) or twisting force. This can create strain within the joint.

In fact, when forces move through a joint that hasn’t been properly stabilized by the surrounding muscles, damage can occur. Injury may result from small amount of stress and damage over time (“micro-traumas”) or one major traumatic event.

But no matter the mechanism or type of force, stiffness generated by the muscles provides the active stability needed to minimize the stresses on the joint.


Putting stiffness into practice

The lower back is an example of a body region that tolerates pure compression forces relatively well, but has a more difficulty withstanding bending, twisting and/or shearing.

Considering this, we now subscribe to more modern core stability and back rehabilitation principles than we did in the past. So, while repetitive spinal bending and twisting (i.e., sit-ups, crunches) was the traditional go-to approach, the latest core stability principles emphasize exercises that maintain trunk stiffness while limiting spinal or torso movement into any direction.

Any resistance exercise could be considered a “core stability” exercise but only if you perform it while simultaneously focusing on maintaining a stiff, tall and straight torso (as if you’re standing up straight), absent of any bending or twisting.

Conclusion: To limit injury, build and maintain stability

Stability is essential to injury-free movement. And the most important part of stability is the stiffnessthat allows joints to withstand the forces put upon them in our daily ins and outs of life.

In our next post (“Stability Explained (Part 1: Core Stability”), we move beyond the simple joint model, to apply these principles to the more complex dynamics of the human body, including the functioning of the much-talked-about core muscle groups.

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