Biomechanics Principles & Application. 4 principles for coaching The example worked in the paper was one of maximum thrust (sprinting, jumping, and so

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”


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


Direction of application

Newton’s 3rd law is about reaction This gives us the final principle – if

you want to move forwards, push backwards


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!


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)


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


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)


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?


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.


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...


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


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


Does this answer the cat example?•No...because...


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


Thus...

Get it? http://www.youtube.com/watch?v=uw-FsgMi6m

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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