Five factors determine stability and mobility

When standing, we typically have two feet in contact with the ground. If our feet are close together, we feel less stable than when the feet are spread apart. Increasing the distance between the feet increases what is termed our base of support, defined as the area within an outline of all ground contact points. Several bases of support are illustrated in figure 6.6. The importance of the base of support in determining our stability and ability to move effectively is discussed in this section and again, in more detail, in chapters 8 and 9, where we explore movements such as walking, running, jumping, and throwing.

As we prepare to move in any situation, with little or no conscious thought we place ourselves along what might be termed a stability-mobility continuum. In situations of imminent contact, we try to enhance our stability; when we want to move quickly, we try to increase our mobility. In preparation for impending contact by an opponent, for example, an American football player will try to brace himself by widening his base of support and bending his knees. If, on the other hand, the player decides to run away from the collision, he would adopt a different body posture that would enhance his mobility.

From a mechanical perspective, five factors determine our levels of stability and mobility.

• Size of the base of support in the direction of force or impending force: In general, increasing the size of the base of support increases stability. In preparation for an impact, we tend to spread our feet apart. We do so, however, in the direction of the force. If you were about to be struck from the front, would you widen your base of support by abducting at the hips to spread your feet apart to the side? Probably not. Most likely you would increase your base of support by staggering your feet front-to-back, or in an anterior-posterior orientation. Merely increasing the size of your base of support will not necessarily make you more stable. The increase must be made in the direction of force or impending force. Increases in base of support can be made by placing the feet in a certain position, as in the previous example, or by adding ground contact points. Additional contact points can be added by using other body parts, as when a baby creeps along the ground on hands and knees or when an athlete assumes a three-point or four-point stance (figure 6.7). Older or injured persons also can enhance their stability by using a cane or crutch to add contact points to the system, thereby increasing their bases of support (see figure 6.6c).

• Height of the center of gravity above the base of support: When you squat down to improve your stability, you lower your center of gravity, or decrease the height of the center of gravity above the base of support. Conversely, standing up straight raises the center of gravity above the base of support and decreases stability.

• Location of the center of gravity projection within the base of support: Imagine that you drop a plumb line (i.e., a string with a weight on the end) straight down from your center of gravity. That line is referred to as the vertical projection, or projection, of your center of gravity within the base of support. If the projection moves outside the base of support, you become very unstable and will fall without corrective muscle action. In normal standing, when the center of gravity projection lies at or near the center of the base of support, you are more stable than when the projection lies near the edge of the base of support. When another body is about to collide with yours, you tend to lean toward the colliding body. This lean moves the projection near the edge of the base of support so that at impact, the center of gravity has a greater distance to travel before leaving the base of support on the opposite side and causing you to fall.

• Body mass or body weight: A body’s mass (or weight) contributes to stability. Simply stated, heavier bodies are harder to move and hence are more stable. Lighter bodies are moved more easily and are less stable.

• Friction: The amount of frictional resistance at the interface between the ground and any contact points (e.g., foot or shoe) contributes to stability and mobility. A young basketball player trying out her new shoes on a freshly polished gymnasium floor would encounter relatively high friction that would improve her stability. A teenager running on an icy sidewalk in the middle of winter, in contrast, would have much lower stability because of the low friction and would be more likely to slip and fall.

In summary, high stability (low mobility) is characterized by a large base of support, a low center of gravity, a centralized center of gravity projection within the base of support, a large body mass, and high friction at the ground interface. Low stability (high mobility), in contrast, occurs with a small base of support, a high center of gravity, a center of gravity projection near the edge of the base of support, a small body mass, and low friction.