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 1 st of Newton’s laws is about motion requiring force 2 nd is about the larger the force, the greater the change in motion Thus we get the 1 st 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 3 rd 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

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, celebrating Score!

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 biomechanist 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.

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 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?