Transcript Motor Control Theory 1

Biomechanics
Principles & Application
4 principles for coaching
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The example worked in the paper was
one of maximum thrust (sprinting,
jumping, and so on)
• Over the next few slides, we’ll summarize
the example, and mention Newton’s laws
Summation of joint force
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1st of Newton’s laws is about motion requiring
force
2nd is about the larger the force, the greater
the change in motion
Thus we get the 1st principle...the more
force, the greater the motion
• So where can we get the force from?
• In a sprint, the hip flexors, knee flexors and ankle
flexors
• So simply put, this principle is about maximizing the
contribution of these 3 joints so that the overall
force is maximized
Remember, the problem is to produce maximum thrust
Continuity of joint forces
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Trickier
• In order for the full force to be delivered at
the end point (foot on ground), any force
contributed by the hip must be fully
transferred to the knee, and then to the
ankle and so on.
•This is achieved through the best “timing” of the
movement
•It is this that makes it seem as though experts are
achieving a lot of force with minimum effort –
nothing is “lost in translation”
Remember, the problem is to produce maximum thrust
Impulse
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Total force applied equals size of force
at any unit time multiplied by the total
time for which it’s delivered
• So all of the joints’ contributions are a
product of force x time
• So it’s no good if your strongest joint
produces a huge force, but only for a short
amount of time
Remember, the problem is to produce maximum thrust
Direction of application
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Newton’s 3rd law is about reaction
This gives us the final principle – if you
want to move forwards, push backwards
Remember, the problem is to produce maximum thrust
Michael Johnson
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Short range of motion (“choppy
strides”)
• Problem?
• Think of impulse
• A shorter range of motion might not be a
problem if, for the range of time you are
working, the peak force is significantly
higher (and this is continuous across the time
of the race)
• Need more? Ask!
Remember, the problem is to produce maximum thrust
Michael Johnson
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Example of short stride length being
advantageous:
• .5s stride length, w/100N p/sec = 50N p/stride
• .25s stride length, w/160N p/sec = 40N p/stride
• But, the .25s stride length will have twice as many
strides per unit time. So, for 10s racing:
• The .25s stride length runner will perform 40 strides.
(total force in 10s = 1600N)
• The .5s stride length runner will perform 20 strides.
(total force in 10s = 1000N)
Remember, the problem is to produce maximum thrust
So what should coaches look for?
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Error Detection
• Identify the biomechanical purpose
• Observe the movement
• Assess cause of error
• Observe again, check on supposed cause
• Refine assessment
• Attempt correction
Remember, the problem is to produce maximum thrust
Other principles to be elaborated on
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Stability
• Base of support & center of gravity
•Keep the line of action of the second inside the
first!
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Summation of body segment speeds
• Analogous to summation of joint forces, but
for throwing, striking, kicking
•Speed of end part is the sum of the speeds
achieved in the preceding parts
•Provided you have continuity (timing)
Other principles to be elaborated on
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The basketball shot...
• It’s propulsion
•So you’d clearly expect summation of joint speeds
to come into play
•Anything else?
•How about action-reaction?
Other principles to be elaborated on
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Rotational motion
Conservation of momentum
Rotational inertia manipulation
Body segment momentum manipulation
Resultant forces
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Projectile motion
• When you throw a ball, why does it do this...
• Instead of this?
Resultant forces
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So, the way balls and bodies move in
the air is a result of more than one
force, and the combination resolves
itself as a curve
• With us, the curve is followed by the center
of gravity (or center of mass)
• E.g. Fosbury flop
• http://www.youtube.com/watch?v=Id4W6VA0uLc
•
http://www.youtube.com/watch?v=_bgVgFwoQVE&mode=related&search=
Inertia
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Reluctance to change what one is doing
• Measured by the mass of an object
• More massive things have greater inertia
(reluctance to change current activity)
• So more massive things require greater force
to overcome inertia
Momentum
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A moving body has mass, and velocity
Multiply them together, and you have
momentum
• Think of the rugby tackle
Lots of
momentum
No
momentum
Attacker, 250lbs, moving at 20mph
Defender, 160lbs, static
Try line
Momentum
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A moving body has mass, and velocity
Multiply them together, and you have
momentum
• Think of the rugby tackle
Defender, 160lbs, deceased
Try line
Score!
Attacker, 250lbs, celebrating
Conservation of momentum
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If two or more bodies/objects collide,
the momentum stays constant (ignoring
friction and air resistance)
• Think of balls on a pool table when breaking
•Total energy dissipated by all balls after the break
is totally determined by the momentum of the
cue ball
Angular versions of all this
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Eccentric forces and moments
Imagine pushing a book
What happens in each case?
Eccentric forces and moments
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So the further off center a force acts,
the less it makes the object move in a
straight line, and the more turning
force is applied
• So where would you want to hit someone
when you tackle them (rugby/football)?
Angular stuff
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Can you generate rotation in the air?
Can a cat?
How do you do it?
• How do you increase speed of rotation about
an axis when in flight?
• Or decrease it?
• Demo...
Angular momentum
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Angular velocity x moment of inertia
Moment of Inertia
maximum (around
somersault axis)
Moment of Inertia
minimum (around
somersault axis)
Conservation of angular momentum
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Simply put, when a body is in the air
it’s angular momentum doesn’t change
unless it’s subjected to external forces
• So how the heck does the cat do this then?
The gymnastic cat...
Frames 1
through 5
take 1/8
second, and
the remaining
fall is four
feet – a
further ½
second.
Nasty
biomechanist
The gymnastic cat...
Frames 1
through 5
take 1/8
second, and
the remaining
fall is four
feet – a
further ½
second.
The gymnastic cat...
Frames 1
through 5
take 1/8
second, and
the remaining
fall is four
feet – a
further ½
second.
The gymnastic cat...
Frames 1
through 5
take 1/8
second, and
the remaining
fall is four
feet – a
further ½
second.
The gymnastic cat...
Frames 1
through 5
take 1/8
second, and
the remaining
fall is four
feet – a
further ½
second.
The gymnastic cat...
Frames 1
through 5
take 1/8
second, and
the remaining
fall is four
feet – a
further ½
second.
Explaining the gymnastic cat...
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Think about moments of inertia about
the 3 axes of rotation...
Explaining the gymnastic cat...
...the moment of inertia
about the twisting axis.
The moment of
inertia about the
somersaulting axis is
a lot bigger than...
Explaining the gymnastic cat...
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Linear and angular
momentum
• Both are conserved
• In the linear case, this
means velocity is
fixed after take-off
• But in the angular
case, this is not so
• Angular velocity and
moment of inertia can
vary, as long as their
product remains
constant
Explaining the gymnastic cat...
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Linear and angular
momentum vectors
• In the linear case the
•
•
velocity and momentum
vectors are parallel
Again, in the angular
case, this is not
necessarily so
The momentum will stay
the same, but the
velocity can be divided
between axes and will
be determined by the
inertia about each axis
Explaining the gymnastic cat...

Linear and angular
momentum vectors
• So, suppose you have
angular momentum
about the somersault
axis
AMT  AVS .MIS  AVC .MIC  AVT .MIT
Moving a part of your body in a
direction other than somersaulting
might initiate twisting, but the total
angular momentum will stay the same
Explaining the gymnastic cat...
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Does this answer the cat example?
• No...because...
Explaining the gymnastic cat...

Does this answer the cat example?
• No...because...
• The cat had zero angular momentum
• These ideas are developed for moves where
you are shifting momentum from one axis to
another
• If you have zero angular momentum, then
you have nothing in any axis...so now what?
Explaining the gymnastic cat...
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Remember the body is multiple linked parts
• Momentum of each part added together is zero
• So if you start one part twisting in one direction,
then the other must twist in the other, to maintain
overall zero
• But you can change moment of inertia, too...
• So twist one half with little inertia (relative to the
axis of rotation), and the other half with a lot of
inertia will hardly move
• Then repeat with other part of body, and you get an
overall twist of the body
• Trampolinists do it all the time in tuck drops
Explaining the gymnastic cat...
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Thus...
Get it?
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http://www.youtube.com/watch?v=uw-FsgMi6m4