7.7 Stability
Body stability is a crucial component of our ability to move effectively. Without it, our ability to complete daily tasks and participate in physical activity becomes impaired, potentially putting us at risk for injury.
Stability can be defined as the ability of an object to maintain or return to a position of equilibrium when acted upon by external forces. Within the bounds of this course, stability depends on four distinct factors:
- Mass
- Centre of Mass (COM)
- Base of Support (BOS)
- Position of COM
Mass
A person’s mass consists of all the materials that make up their given body. While this includes the physical body and tissue composition of an individual, it can also include anything connected to or layered on top of that body. This includes but is not limited to hand-held objects (e.g., bats, dumbbells, sticks, etc.), protective equipment (e.g., pads or helmets), and clothing (e.g., ski boots).
As discussed in relation to Newton’s Second Law of Motion: The Law of Acceleration, the greater the mass, the greater the force required to move that mass. Using football players as an example, think of the types of bodies you may see in the position of a defensive tackle, the players whose job it is to act as a wall to prevent any movement of the offence down the field. Athletes in this role are typically tall, wide and have a large mass. This mass works to their advantage by making it difficult for them to be moved.
Centre of Mass (COM) and Base of Support (BOS)
Although an increased mass does contribute to bodily stability, your COM and its relation to your BOS play an even greater determining role. As per its name, COM is the imaginary center point at which the mass of an object is equally distributed. While this point is easily imagined in an object such as a ball, where the COM would exist at the centre point of the ball, the COM within the human body is harder to pinpoint. This is because, although the body is generally symmetrical side to side, the human body is not created equal in all orientations, and it is constantly moving, meaning the location of your COM can change.
Your COM is fluid and changes with your body’s movements and weight distribution, and can even exist outside of the body’s bounds. Consider the following image, where you have a human body holding a ball. Because the ball being held has mass, the bodies’ total mass and distribution are altered, shifting the COM to a new position.
Position of COM
Generally speaking, the closer to the ground your COM is, the more stable your body will be. However, this is not always the case. To maintain stability, the BOS also needs to be considered in relation to COM. BOS represents the support area beneath an object or body that is defined by all points of contact with that surface (feet, hands or any other body part). Like COM, your BOS changes both as you move and depending on the number and positioning of your points of contact.
How we maintain stability is this: if your COM falls over or within your BOS, you will remain stable, but if your COM falls outside your BOS, stability will be lost.
Movement in Action
Have you ever wondered why individuals who are crouched, with their feet far apart and their bodies hunched forward, look intimidating? Maybe, but there is also a biomechanical explanation for it. Martial arts professionals, who practice Karate or Taekwondo, modify their COM and BOS to their advantage. By lowering themselves closer to the ground, they lower their COM, making them less vulnerable to instability or being pushed off their feet. And by standing with their feet far apart, they increase the size of their BOS whilst keeping their COM within it.
Definition of stability: OpenAI. (2025). ChatGPT. [Large language model]. https://chat.openai.com/chat Prompt: Define stability in biomechanical terms.
“COM” images: OpenAI. (2025). ChatGPT. [Large language model]. https://chat.openai.com/chat Prompt: Silhouette of a soccer player holding a ball and facing forward. The ball is under their left arm. Image of the same soccer player, but not holding the ball. Image of just the soccer ball.
