Centre of Gravity

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Introduction[edit | edit source]

Human center 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]

Centre of Gravity

The centre of gravity (COG) of the human body is a hypothetical point around which the force of gravity appears to act. It is point at which the combined mass of the body appears to be concentrated[2]. Because it is a hypothetical point, the COG need not lie within the physical bounds 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[edit | edit source]

In the anatomical position, the COG lies approximately anterior to the second sacral vertebra. However, since human beings do not remain fixed in the anatomical position, the precise location of the COG changes constantly with every new position of the body and limbs. The bodily proportions of the individual will also affect the location of the COG. eg The body has moving parts (arms, legs, head, various areas of the trunk), every time we move, the shape of our overall form changes. And if we carry something like a suitcase, grocery bag or if you wear a backpack, this adds weight to some areas, but not others, changing the COG as it does.  This point can and does change based on what we carry and how we carry it, as well as the position you take and the movements you make[1].

Stability and the Centre of Gravity[edit | edit source]

The direction of the force of gravity through the body is downward, towards the centre of the earth and through the COG. 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 reaction is needed in order to stay balanced.

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)
[3]
[4]

Clinical Implications[edit | edit source]

Examples of altered COG

  1. Persons with obesity 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 there is no real reason that a person with obesity should encounter a balance disadvantage, as long as the cente of gravity remains within the base of support, which is the case in a static situation. However, once the center of gravity of an individual affected by obesity falls outside of the base of support, recovering balance may become more difficult than for people with normal weight, because of the greater body weight to be moved, and the individual with obesity's relatively inferior mass specific lower limb power[5]
  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 the center of gravities that are located excessively 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 those with chronic low back pain whose center 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[edit | edit source]

  1. 1.0 1.1 1.2 Very well health COG Available from:https://www.verywellhealth.com/human-center-of-gravity-296568 (accessed 14.4.2021)
  2. Hall SJ. Equilibrium and Human Movement. In: Hall SJ. eds. Basic Biomechanics, 8e New York, NY: McGraw-Hill; . http://accessphysiotherapy.mhmedical.com/content.aspx?bookid=2433&sectionid=191511590. (Accessed June 12, 2019).
  3. mstateathletics. Physics of Football - Center of Gravity. Available from: http://www.youtube.com/watch?v=2WUdHBso3Vk [last accessed 09/03/13]
  4. hplcchmc. Body Center of Mass Example - Motion Analysis. Available from: http://www.youtube.com/watch?v=HSW8gXmOazs [last accessed 09/03/13]
  5. Pataky Z, Armand S, Müller‐Pinget S, Golay A, Allet L. Effects of obesity on functional capacity. Obesity. 2014 Jan;22(1):56-62.Available from:https://onlinelibrary.wiley.com/doi/full/10.1002/oby.20514 (accessed 14.4.2021)