1. Biomechanics
Centre of Gravity
Craig Harris

2. Lesson Objectives
• Define centre of gravity / centre of mass
• Experience how centre of gravity is altered due
to movement, and the affect that can have on
performance.
• Define stability and realise its importance in the
sporting environment.
• To introduce the methods of calculating COG

3. Centre of Gravity
• Centre of gravity can be defined as the point in a body
or system around which its mass or weight is evenly
distributed or balanced and through which the force of
gravity acts.
• Centre of mass is the point in a body at which the
entire mass may be assumed to be concentrated.
For our purposes they are the same thing!!

4. Centre of Gravity
• The centre of gravity can be considered an
imaginary point in space
▫ It is not a physical entity, we cannot touch or see
it.
▫ Nor can the centre of gravity be fixed or
permanently marked on an object. Why??

5. Centre of Gravity
• The centre of gravity is a useful concept for analysis
of human movement
▫ When using Newton’s Laws of motion, it is the COG of
a body whose motions are ruled by the laws.
▫ Newtons 3 laws
• Therefore we should be able to locate or at least
estimate the location of the COG of an object

6. COG of the human body
• The COG of the human body is determined by the
location of its limbs at any given time.
When Standing in the anatomical
position, the height of your centre of
gravity is :• 55% of your standing height
for women
• 57% of your standing height
for men.
ANY SUGGESTIONS AS TO
WHY???

7. COG of the human body
• Consider standing in the anatomical position, what effect on
the COG would the performance of the following movements
have?

8. Where is the COG for these athletes?

9. Consider the following

10. “Hang In the Air”
• Athletes such as basketball players, gymnasts,
figure skaters, rugby players etc. Appear to hang
in the air.
• During such jumps, they appear suspended in
the air, instead of rising and falling in a
parabolic path as a projectile should
▫ Can we attempt to explain why?

11. “Hang In the Air”
During the jump, the jumpers head stays at the same level, but
the COG follows a parabolic path. Due to the raising and
lowering of the arms and legs

12. Centre of Gravity and Stability
• The mechanical definition of Stability is
“the capacity of an object to return to
equilibrium or to its original position after
having been displaced.”
• In many sports stability is important .
▫ Name 3 sports in which the performers do not want to
be moved from a certain position
▫ Name 3 sports athletes need a stable base in which to
move from a certain position

13. Factors affecting Stability
• Size of the base of support
• The horizontal distance from the edge of the
support and the COG
• Height of the COG
• Mass of the body
• Direction of the base in relation to the force

14. Calculating Centre of Gravity
• There are two methods that we can use to
calculate a persons COG. They are :▫ The Reaction board method
▫ The segmental method

15. The reaction board method
• Involves a subject laying upon a board, with one
end standing on a weighing scales.
• Only the height of a person COG can be
calculated.
• The reaction board method is easily applied to
static position
▫ i.e. The anatomical position, both above hands
above head, etc.

16. The segmental method
• This method involves the calculation of a bodies
centre of gravity from a photograph of human
movement.
• The human body is broken down into segments
(limbs).
• Centre of masses are calculated for each
segment.
• Is a more versatile method as it can be applied to
more dynamic situation

17. To Conclude
• We can now define centre of gravity / centre of mass
• We can now display and understand of how centre
of gravity is altered due to movement, and the affect
that can have on performance.
• We can define stability and realise its importance in
the sporting environment.
• Been introduced to the methods of calculating COG