Two categories of elements and/or factors work to stabilize the bones in a joint and allow them to move without falling apart: the passive and active elements or restraints.

The passive restraints include the actual shape of the articulating bones and their cartilage, as well as the joint capsule and ligaments (Figure 1A–B). They’re responsible for physically restraining the end-ranges of movement—that’s to say, keeping the bones together—while at the same time guiding the available movement in a way that’s mechanically safe for the joint. Muscles, on the other hand, attach to the bones and form the active elements of the joints (Figure 1C). When contracting, they create forces that get transmitted to the bones they attach to. These forces can pull on the bones, rotating or gliding one bone on the other, creating movement. The forces created by these muscles can also compressone bone into the other, creating “active stiffness” that restrains movement across the joint—an essential for safe and injury-free movement.

Muscles are the toolkit that the body uses to “get stuff done”

In order to keep movement safe, multiple muscles must work together to create a careful balance of forces that restrainmovement and create stiffnessacross a joint (vs. forces that create movement). This all starts with the carefully coordinated signals our nervous system sends to the body, to get it to act. Think of muscles as tools that our nervous system uses to move the body, and essentially, to get stuff done. It integrates the information gathered from the body’s various senses (including balance, body positioning, etc.) in order to perform movements that complete required tasks.

Defining flexibility vs. mobility

When it comes to a body’s ability to create movement, it’s important to differentiate between what’s considered “flexibility of joints” vs. “mobility of movement.” Flexibility refers to the amount of range of motion of a joint or set of joints. This is what people are trying to increase when they perform passive stretches, such as placing your foot on a side-rail for leverage while performing a hamstring stretch. Meanwhile, mobility is defined as “the ability to move or be moved freely and easily.” Our bodies require the ability to move to “get stuff done,” and all moveable joints have an inherent range of motion (flexibility) associated with them. Range of motion is dictated by several factors, including the joint’s boney architecture (i.e., the depth of the socket) and the ability of the joint’s associated soft tissues (i.e., ligaments and muscles) to stretch and lengthen.

So, mobility differs from flexibility as it refers to our ability to actively move a joint and not simply our ability to stretch a joint to a particular range of motion. And though some level of joint flexibility is important—as we need range of motion to carry out tasks—we really need mobility (which includes joint flexibility, as well as muscle strength and coordination) to complete specific movements or tasks. And to ensure that those tasks are injury-free, we need stability.

While our mobility allows us to move, it’s our stability that keeps it safe.

Stability refers to our body’s ability to create stiffness in order to limit any movement that puts our body at risk of injury. This function is absolutely necessary to keep joints safe, and it requires both the passive and active elements of the joint to achieve it. Check out our next post (“Fundamentals of Joint Mobility and Stability (Part 2): The Importance of Stiffness”) for a more in-depth discussion on stability, and the role of stiffness in delivering injury-free movement.