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Biomechanics
Principles & Application
4 principles for coaching
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 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
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
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
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
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 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?
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
Stability•Base of support & center of gravity
•Keep the line of action of the second inside the first!
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
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
Rotational motion Conservation of momentum Rotational inertia manipulation Body segment momentum
manipulation
Resultant forces
Projectile motion•When you throw a ball, why does it do
this...
•Instead of this?
Resultant forces
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=Id4W6VA0uL
c• http://www.youtube.com/watch?v=_bgVgFwoQVE&mode=related&search=
Inertia
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
A moving body has mass, and velocity
Multiply them together, and you have momentum•Think of the rugby tackle
Try line
Defender, 160lbs, static
Attacker, 250lbs, moving at 20mph
Lots of momentum
No momentum
Momentum
A moving body has mass, and velocity
Multiply them together, and you have momentum•Think of the rugby tackle
Try line
Defender, 160lbs, deceased
Attacker, 250lbs, celebratingScore!
Conservation of momentum
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
Eccentric forces and moments Imagine pushing a book
What happens in each case?
Eccentric forces and moments
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
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
Angular velocity x moment of inertia
Moment of Inertia maximum (around somersault axis)
Moment of Inertia minimum (around somersault axis)
Conservation of angular momentum
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...
Nasty biomechani
stFrames 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.
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...
Think about moments of inertia about the 3 axes of rotation...
Explaining the gymnastic cat...
The moment of inertia about the
somersaulting axis is a lot bigger
than...
...the moment of inertia about the twisting axis.
Explaining the gymnastic cat... 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... 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
TTCCSST MIAVMIAVMIAVAM ...
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...
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... 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...
Thus...
Get it? http://www.youtube.com/watch?v=uw-FsgMi6m
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