Centre of Gravity

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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. 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

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.

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. 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)

Centre of Gravity and Center of Mass 

Often there is confusion regarding the usage of the terms"Centre of Gravity (COG)" and "Centre of Mass(COM)" . Gravity is one of the most consistent and influential force that human body encouners in posture and movement. [4]If the gravity acting on every segments of the object is same, then the center of gravity and center of mass for that object remains the same. Considering the human body, the force of gravity acting on various segments depends on the position of the body, distance from the ground and the movement taking place. In normal gait, with various phases of gait cycle, the COG follows the sinusoidal curve .   

Recent Related Research (from Pubmed)

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  1. mstateathletics. Physics of Football - Center of Gravity. Available from: http://www.youtube.com/watch?v=2WUdHBso3Vk [last accessed 09/03/13]
  2. hplcchmc. Body Center of Mass Example - Motion Analysis. Available from: http://www.youtube.com/watch?v=HSW8gXmOazs [last accessed 09/03/13]
  3. Classroom Video. The Body in Motion - Influences on Movement (excerpt) Available from: http://www.youtube.com/watch?v=I_0ApUE18DA [last accessed 09/03/13]
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