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From quadrupedalism into bipedalism: towards a new biology of human balance
John Skoyles CoMPLEX
University College London8th Feb 2006
“to see the familiar as unfamiliar, and the unfamiliar as familiar” Francis Bacon
“Man is a flightless biped” Aristotle
Two quotations
Human bipedality is unprecedented in biology
Human bipedalism has an unique dependence upon balance
(1) The risk of falls drove the evolution of balance mechanisms
(2) Human balance depends upon an exquisite ability to make forward feed body adjustments
Both (1) and (2) need mathematical analysis
Towards a new biology off human balance
Human bipedalism goes far beyond any other biological locomotion
Upper limbs are never used for locomotion but employed in many activities
A platform upon which diverse secondary motor actions are superimposed
Posterior fat deposits aid balance during pregnancy and lactation
Pawlowski B. 2001 Current Anthropology 42 572-754
Human bipedalism is biologically unique due to balance: “brainware”
Totally dependent upon skeletomuscular adjustment
Walking and running takes seven years to become adult-like
Simple bipedalism, however is easy, the problem is not falling and breaking bones
This individual has no cerebellum and yet can walk – in a manner
cerebellar agensis
Titomanlio et al 2005 Neurology, 64, E21
Dysequilibrium syndrome suffer an inability to reliable stand and walk linked to balance
Some do learn to walk but at great delay like “tin solders”
They have problems in falling when they attempt to walk bipedally
When did this balance faculty arise? A clue lies in a link between injuries when tripping and height
The risk of bone injury links to height
The speed with which a large organism hits the ground is proportional to the square root of its height. Since mass is proportional to the cube of its length, momentum is proportional to its length raised to the power 3.5, and kinetic energy to the fourth power
Height and Australopithecine and Homo species
Australopithecus afarensis AL-288-1 (Lucy) was 1.18 m tall
H. erectus KNM-WT 15000 (Nariokotome Boy) would have been as an adult 1.86 m
Momentum on impact increasing with height to the power of 3.5, and released kinetic energy, to the power of 4
The height increase between Australopithecus and Homo required the evolution of new balance mechanisms
Lucy fell with six-fold more kinetic energy than Nariokotome Boy
Proper models of the risk Australopithecus and Homo to injury falls are needed. Back of the envelop calculations: Vogel S. Comparative biomechanics. Princeton University Press, 2003.
Effects of body type – skinner people suffer worse injury than those with fat
The length of hands compared to legs – chimp hands are 110% longer than legs, humans only 72% their length --Austropithecus in between.
The risks of tripping up when running compared to walking.
There was a biological shift from australipithecine to homo species 2 millions ago.
Enlarged brains
Enlarged vestibular organs
Running
Increase in height
Migration from Africa into Asia
Common to them is improved balance abilities
Expanded brain size could link to improved balance abilities
Cannot talk and walk phenomena
Balance impaired when people do spatial tasks and vice versa
Expansion of the cerebellum closely associated with expansion of the cerebral cortex
Enchanced balance depends upon internal motor models that enable adjustment of the body
Human balance depends upon internal models that map actions into balance adjustments
To keep balance every action must be synchronized with a skeletonmuscular adjustment to counteract its effect upon the centre of mass.
Thus the brain must be able to map changes in posture to know the counteracting changes needed to keep balance
Internal motor models are of two kinds
Inverse models: provide the motor command necessary to achieve a desired state
Forward models: predicts the next stage of a movement given the current state and motor command
Inverse balance models keep us upright
For every action – step, hand movement etc the brain must create an inverse balance model so it can counteract its effect on the projection of the center of mass over support
This is not at present a problem being modelled in biomechanics
Human balance presents key problems
for mathamatics
Human balance underlies the origins of our species
Human balance underlies the nature of everyday lives
Studying the mathematics of falls could provide insights into the origin of Homo
Studying the inverse balance problem is needed to understand how the brain keeps us upright
Huysein suffers from dysequilibrium syndrome. He has compensated for lack of bipedality by using all fours.
In spite of being unable to coordinate and balance for walking, Huysein is still coordinated nonbipedality