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Centre of Gravity

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Introduction

Human centre of gravity is a term that has implications for all things related to posture, including issues such as swayback, the design of posture exercise programs, and much more. Gravity is a downward pull or force that the earth exerts on your body. Your centre of gravity is the point where the mass of the body is concentrated. [1] Similar to Centre of Mass (COM), centre of gravity is the point in an object at which the entire body of the object is in balance in reference to gravity. The gravitational field affects the centre of gravity since with variance the centre of gravity value alters as well. [2]

Centre of Gravity

Definition

The Centre of Gravity (COG) of the human body is a hypothetical point around which the force of gravity appears to act. It is a point at which the combined mass of the body appears to be concentrated. [3] Since it is a hypothetical point, the COG does not have to lie within the physical boundaries of an object or person. One subjective way (there are objective measures) to approximate the COG of an object is to visualise it balancing on one finger.

Centre of Gravity in the Human Body

In the anatomical position, the COG lies approximately anterior to the second sacral vertebra [1], at the centre of the pelvis. [4] However, since human beings do not remain fixed in the anatomical position, the precise location of the COG is always shifting with each movement of the body and limbs. The proportion of body weight of the limbs, trunk, and head will also affect the location of the COG every time we move body positions. [1] It is seen that in males, the centre of gravity is in a slightly higher position due to larger shoulder mass.

Factors affecting the COG :

  • Body Composition and body shape
  • Posture and Alignment
  • Movement of body
  • External forces acting on the body
  • Age and Gender
  • Carrying loads (e.g backpacks shift the COG posteriorly and superiorly)

Centre of Gravity in different activities

The position of COG varies by individual, especially differing athletes. An athlete with longer and/or larger legs will likely have a lower COG, unless their upper body is also a relatively large structure and balances the mass of the lower body. [4]

It is seen that in runners, the COG is in the lower region (inferior) of the pelvis, and in front of (anterior to) of the body since the runner generally has a forward lean, thus helping with acceleration.[4]

Lowering the position of your COG helps to increase balance and stability, since it needs to be lifted higher prior to moving outside the BOS. This can be seen in some yoga and balancing poses, gymnastics, track and field activities, and even in a handstand. [4]


When something like a suitcase, grocery bag, or a backpack is carried, the COG will change as weight is added to respective areas of the body. This point of COG will change as the weight and position of a carried object changes, and is also based on the position and movement of the body. [1]

Stability and the Centre of Gravity

The direction of the force of gravity through the body is downward, towards the centre of the earth and through the COG. An imaginary vertical line passing from the centre of gravity of an object down to the ground. It is also known as the line of action of the force of gravity or Line of Gravity.

The Base of Support (BOS) refers to the area beneath an object or person that includes every point of contact that the object or person makes with the supporting surface. These points of contact may be body parts e.g. feet or hands, or they may include things like crutches or the chair a person is sitting in.

This line of gravity is important to understand and visualise when determining a person's ability to successfully maintain balance. When the line of gravity falls outside the Base of Support (BOS), then a postural reaction is needed in order to maintain balance.

When the line of gravity is within the BOS, an object or person is said to be stable. When the line of gravity falls outside the BOS, the object or person is said to be unstable. Given that the line of gravity must fall within the BOS in order to satisfy the criteria for stability, the following factors should be considered:

  • A larger BOS increases stability (the line of gravity must move a greater distance to fall outside the BOS)
  • A lower COG increases stability (it's unlikely that the line of gravity will fall outside the BOS)

Here are some videos to help explain COG

[5]
[6]

Centre of Gravity in Objects

Here is a video that explains and shows how to determine the COG in objects.

[7]

Clinical relevance of COG in Physiotherapy

Understanding and application the concept of COG in critical in clinical settings. It plays an important role in assessment, treatment planning and achievement of goals in patients with various conditions.

Examples of altered COG:

  1. Obese individuals have been shown to have a greater risk of falling (related to dynamic, rather than static components). This could be explained by the fact that an obese individual should not encounter a balance difficulty if their centre of gravity remains within their base of support. This is the case in a static situation. However, once the COG of an obese individual falls outside their base of support, maintaining balance may become more difficult than for an individual with normal weight, because of the increased body weight to be moved, and due to their relatively inferior mass specific lower limb power [8]. 
  2. Chronic Low Back Pain: A 2014 study published in the Journal of Back and Musculoskeletal Rehabilitation found that patients with chronic low back pain tend to have a centre of gravity that is located predominantly towards the back. In the study, patients had decreased low back strength upon extension, plus a decreased amount of normal low back curve. The study authors concluded that individuals with chronic low back pain whose centre of gravity is too far back might have strength and balance challenges to overcome in order to re-establish postural control. [1]
  3. Biomechanics in prosthetic rehabilitation

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Verywellhealth. Complexity of Finding the Human Centre of Gravity. (accessed 14/04/2021)
  2. ↑ Geeks for Geeks. Difference between Center of Mass and Center of Gravity. (accessed 26/04/2024)
  3. ↑ Hall SJ. Equilibrium and Human Movement. In: Hall SJ. eds. Basic Biomechanics, 8e New York, NY: McGraw-Hill. (Accessed June 12, 2019).
  4. ↑ 4.0 4.1 4.2 4.3 pdhpe.net - helping you succeed. Center of gravity. (accessed 25/04/2024)
  5. ↑ muscle and Motion. Mastering Balance: The Science of Center of Gravity and Base of Support. Available from: https://www.youtube.com/watch?v=5s1gEi5wfLg&ab_channel=MuscleandMotion (accessed 20/04/2024)
  6. ↑ hplcchmc. Body Center of Mass Example - Motion Analysis. Available from: http://www.youtube.com/watch?v=HSW8gXmOazs [last accessed 09/03/13]
  7. ↑ vt.physics. Centre of Mass and Gravity CGSE Physics Required Practical. Available from: https://www.youtube.com/watch?v=vDqcEIdhJ_8&ab_channel=vt.physics (accessed 20/04/2024)
  8. ↑ Pataky Z, Armand S, Müller‐Pinget S, Golay A, Allet L. Effects of obesity on functional capacity. Obesity. 2014:22(1):56-62.
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