Line of gravity, base of support, equilibrium and fixation concepts explain why some positions are stable, why others are prone to falls, and how muscles and external supports help maintain or restore stability. In this article, we will define each term in simple language.
Line of Gravity in the Human Body
The line of gravity is the vertical line that passes through the centre of gravity.
As you can see in the figure, there is a solid bar and a human body. So, for this solid bar, the center of gravity is the point where it perfectly balances over a fulcrum. And the verticle line passes through this point is the line of gravity.
Similarly, for our body the center of gravity lies at the verterbal body of S2 level. The verticle line passing through this point is the line of gravity for our body.
Now let’s study the base.
Base of Support in Standing and Sitting
Base is the area on which an object is supported. So, it’s simple. Let me explain it for you.
As you can see in the figure, a chair is supported on its four legs. So, if we draw a line and connect its four legs, the area that is formed is its base of support.
Let me show you another example. You can see the footprints when we’re standing. So, during the standing position, the area of these footprints is our base of support when standing.
Now, next is equilibrium.
Equilibrium: When Forces Are Balanced
Equilibrium results when the forces acting upon the body are perfectly balanced and the body remains at rest. In our body,there are many forces act simultaneously, yet we remain at rest. This is becasue of equilibrium.
For example, suppose you are sitting comfortably right now. There are many forces acting on you like gravity, the force of muscle etc. All these forces perfectly balance each other and you come to rest.
So, just like the example we discussed earlier, when a object is subjected to equal force of pull in the same point and in exactly the opposite direction, the body comes to an state of equilibrium.
So, in both the example, multiple forces are acting on the body, yet, the body remains at rest in its place. That’s called equilibrium. However, when the forces do not cancel out each other, the body becomes unstable which is called unstable equilibrium.
So, there are two types of equilibrium here. These are:
- a stable equilibrium, and there’s an
- unstable equilibrium.
Stable Equilibrium: Conditions and Examples
To body to come in a stable equilibrium, it has to fufill certain conditions. Let us start with how high or low is the level of center of gravity from the ground.
1) Center of gravity at low level
An equilibrium is stable when its center of gravity is at the bottom, as low as possible. For example, if we sit down our body’s centre of gravity goes down. Our body’s center of gravity is at the S2 level around our waist.
But, when we’re standing, it’s higher. So, sitting position is much stable position as compared to standing. And if, instead of sitting, we lie down, our centre of gravity moves even lower. So, we reach a stable state.
2) Large base of support for stability
Another condition is that there must be a large base of support for a stable equilibrium.
For example, when we’re standing, our base of support is small. So this is a slightly unstable equilibrium. But, when we lie down on our back, our entire back become base of support. So, in a way, this is a stable equilibrium.
3) How close is line of gravity to center of gravity
Then the third condition is that gravity should fall near the center of the base. Let me explain this with another example. For example, in the figure you can see that she’s standing, but she’s in a nearly stable position because her center of gravity is falling on the center of the base of support.
Let’s say this girl leans slightly forward. You can see in the image in a side view. So look, the center of gravity here has shifted slightly outside of the base of support. So it becomes unstable.
So to be stable, you have to stand upright so that your base of support falls on the center. So, these three criteria must be fulfilled for stable equilibrium.
Now let’s move on to unstable equilibrium.
Unstable Equilibrium: When Stability Is Lost
In an unstable equilibrium, the center of gravity is as high as possible. As I just told you, when we stand, our center of gravity is slightly higher, so standing slightly unstable. It has a small base of support.
If we stand on both feet, our base of support is slightly larger than when we stand on one foot, as you can see in the figure. So, the smaller the base of support, the more unstable it is.
Finally, we have reached fixation and stabilization.
Fixation and Stabilization in Physiotherapy Practice
A stable fixation is defined as something that is immobile and stable. So, if I were to give a definition, a stable fixation Describes the state of immobility and stabilization.
Fixation are of two types:
- active fixation and
- passive fixation.
Active Fixation by Muscle Contraction
Active fixation is achieved by muscle contraction, and passive fixation is achieved by external support. So, let me explain how this is relevant to our physiotherapists.
Let’s say he’s holding a dumbbell and is in a stable position. So, here, biceps muscle is working to stabilize this position. So, this is active fixation, because the muscles are acting here. The muscles are contracting and fixing.
Passive Fixation Using External Support
This is passive fixation. This is a fracture with a nail plate and a nail. So this is passive fixation. Because there is external support here. You will understand this later as you read further.
Here is a fractured ankle in which this plaster is applied. So in a way, it is also getting external support. Therefore, this is also passive fixation.
The author is a physiotherapist who has been practising for the last 17 years. He holds a Bachelor's in Physiotherapy (BPT) from SVNIRTAR (Swami Vivekananda National Institute of Rehabilitation and Research), one of the prestigious physiotherapy schools in India.
Whatever he learns dealing with his patient, he shares it with the world through blogs and e-books. He also owns a YouTube channel, "Sunit Physiotherapist" with over 8 lakh active subscribers. Here, he shares everything he gets to learn serving the patient.
