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Uwe Kersting, 20111

Biomechanics of Swimming

Center for Sensory-Motor Interaction

Sports Biomechanics

Uwe Kersting MiniModule 10 - 2011

Uwe Kersting, 20112

Objectives

Apply sports biomechanics approach toswimming

Be able to differentiate characteristics ofdifferent swimming styles

Review fundamental concepts on fluiddynamics buoyancy, lift vs. drag

Learn about dophin skin and swim suits

Create a comprehension how all workstogether

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Uwe Kersting, 20113

Contents

1. Introduction: four main swim styles

2. What we can see: characterisation of thefour styles

3. Fluidynamics principles- drag- lift- about surfaces and adjacent materials

4. Simple calculations

5. Finetuning of swim style understanding (?)

6. Summary

Uwe Kersting, 2011

Videos

4 swimming styles & ....

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Factorial Model ofSwimming Performance

partial times model

TIMEtotal

Startingtime

TurningtimeStrokingtime

Block time Flight time Glide time

Glidedistance

Averageglide speed

Horiz

speed atentry

Changes in

horiz speedin glide

Horizontalimpulses in

glide

Mass ofswimmer

Horizontalimpulse intakeoff

Movementtime

Reactiontime

Vert Vel

at TO

Height

above water

Verticalimpulse intakeoff

Uwe Kersting, 2011

Starting timestart depends on horizontal and vertical impulses

produced on the blockspeed in air greater than speed in water: optimise time in

the air.however, too much height in the start produces

greater downward speed which must be stopped inthe water appears to slow the swimmer down

grab vs sprint starts: grab is faster off the blocks, butsprint start greater impulse (what is the objectiveof the start?)

- beware of first out of the blocks syndrome- when to start stroking? When your glide speed drops to

your swimming speed

maximum impulse in minimum time

I = m * v

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Uwe Kersting, 2011

Factorial Model of

Swimming, contd

TIMEtotal

Startingtime TurningtimeStrokingtime

Strokingdistance

Averagestroking speed

Average strokelength

Average strokefrequency

Turningdistance

Startingdistance

Racedistance

Propulsiveforces

Resistiveforces

Pulltime

Recoverytime

Wavedrag

Surfacedrag

Formdrag

Propulsivedrag forces

Propulsivelift forces

Propulsiveforces (legs)

Propulsiveforces (arms)

Uwe Kersting, 2011

Stroke length

Propulsive forces:lift forces from sculling actionsdrag forces from pull actionlegs contribute to propulsion in whip and dolphin

kicks, but less so in flutter kick

Resistive forces:form drag X-C area (viewed from the front)surface drag typically small, reduced by flutter

kick. Also by speed suitswave drag caused by lifting water above surface

level(minimise rolling and vertical motion ofthe body)

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Uwe Kersting, 2011

Basic propulsion instructionsnew water

Hand to move into still water and accelerate it(generate a force against it)

hand must move in a 3 dimensional curve

if the hand moves in a straight line backwards,it cannot accelerate as much water!

lift

additional force can be gained by pitching thehand so that it acts as a wing (producing liftas well as drag)

this is called sculling

Uwe Kersting, 2011

Factorial Model ofSwimming, contd

TIMEtotal

Startingtime TurningtimeStrokingtime

Averageturn time

Numberof turns

Glidetime in

Turntime

Glidetime out

Turntechnique

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Uwe Kersting, 2011

Turnsturns take between 20 35% of race time!the longer the race the more important turns

become

Observational research has shown that a pikedturn is faster than a tucked turn

Distance (yd)

Stroking

Turning

Starting

Percentage

oftotalrace

time

Uwe Kersting, 2011

Physics principles needed forswimming

Need to stay at the surface

Need to produce propulsive forces

Need to minimise resisitive forces

Definitions and concepts

Application to swimming

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A fluid is any substance that tends to flow orcontinuously deform when acted upon

Gases and liquids fluids with similarmechanical behavior

-- but compressible vs. incompressible

Definition of a Fluid

Uwe Kersting, 2011

Relative Velocity

Velocity of a body with respect to thevelocity of something else such as thesurrounding fluid

The velocity of a body relative to a fluidinfluences the magnitude of the forcesexerted by the fluid on the body

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Uwe Kersting, 2011

Other important FluidProperties

Fluid Density - mass per unit volume(i.e., 1 g/ccm), = 1 g/cm3

Fluid Viscosity - internal resistance of afluid to flow (oil vs water)

In addition:

Temperature & atmospheric pressure affectboth above

Uwe Kersting, 2011

Forces exerted by fluids: Buoyancy

in water, the buoyant force equals theweight of the volume of water displaced.

the centre of buoyancy is at the centreof mass of the volume of water displaced

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Will a body float or not???

A body will float only if:

Wt of body Wt of an equal amount fluid

This can also be stated as:

Weight of body

Weight of an equal amount of fluid

termed: Specific Gravity of Body

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Uwe Kersting, 2011

Specific gravity of a body

Effects of:

1. Volume of air inlungs

2. Age (very young orvery old)

3. Females vs Males

4. Body composition

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Uwe Kersting, 2011

Forces exerted by fluids: Buoyancy

weight: Alwaysverticallydownward!!!

Archimedesprinciple:magnitude ofbuoyant force ona given body =

weight of thefluid displaced bythe body

Weight

Weight

BuoyantForce

Weight

BuoyantForce

Weight

BuoyantForce

Uwe Kersting, 2011

Pressure approach

Px = * hx

20

h

P

Px

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In a wave ...A circular shape

is a bad bodysurfer...

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A wave is ...

Uwe Kersting, 2011

Wave Drag =energy loss

motion at theinterface of body andfluid causes waves,

takes energy and slowsdown theswimmer/boat/etc.

But you can use waves!In swimming, the lanemarkers are designedto reduce wave motionbetween lanes

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Uwe Kersting, 2011

DragDrag = resistance force - slowing themotion of a body moving through fluid

Drag force:

FD = drag force; CD = coefficient of drag; = fluid density, AP = projected area of body orsurface area of body oriented to fluid flow; v =relative velocity of body with respect to fluid

2

2

1vACF DD =

Uwe Kersting, 2011

Examples of Coefficient of Drag

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Factors affecting Drag

CD = affected by shape & orientation of bodyto relative to fluid flow

= medium density e.g., air densitydecreases with altitude 1968 OlympicGames in Mexico City (2250m) manyworld records set!

v = greatest effect!!! Theoretical squarelaw if e.g., cyclist at double speed; otherfactors remain unchanged: drag forceopposing increases 4 times!!!!

2

2

1vACF

DD

=

Uwe Kersting, 2011

Form Drag

form drag depends on the cross-sectional area presented to theflow

streamlining is an attempt tominimize form drag

sometimes you want to maximize formdrag:

oar blade

sailing downwind

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What is Form Drag???Separation of flow from boundary andsubsequent re-uniting of the divergentpaths causes a pocket to be formedbehind moving body

Pocket has lower pressure versus highpressure resulting from oncoming airflowstriking the front of the body

Whenever a pressure differential exists, aforce is directed from the region of highpressure to the region of low pressure =FORM DRAG ( = CD)

Uwe Kersting, 2011

An example: streamlining(short)

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Laminar vs Turbulent flow

Laminar flow in parallellayers

Turbulent flow withviolent intermixing offluid

Affected by :

1. form of body2. relative velocity3. surface roughnessof body

Uwe Kersting, 2011

Modifying boundary layer turbulence

you can reduce drag by reducing turbulence

a rough patch on the surface will reduce theseparation angle and thus reduce drag

Images of a ball in a wind tunnel. On the right, the ball had sand gluedto the front of it. Notice the separation angle change

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Speed suitsmany sports now use suits which incorporate rough

patches designed to reduce the separation angleand decrease the drag

Uwe Kersting, 2011

What is surface drag?Example: water rushes past an object, layer

of water in contact with object is sloweddown due to forces the objects surfaceexerts on it - that layer of air slows downthe layer of air next to it etc. .

Boundary layer: region within which fluidvelocity is diminished due to shearingresistance caused by boundary of movingbody

Depending on velocity & nature of body, theboundary layer becomes unstable &turbulent i.e., change from laminar toturbulent flow!

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Uwe Kersting, 2011

Surface Dragsurface drag

depends on thesmoothness of thesurface and thevelocity of flow

shaving in swimming big effect???

other examples of

sports to decreasesurface drag.???

Uwe Kersting, 2011

A note on the technologicaladvancement of the swimsuits

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Uwe Kersting, 2011

The Perfect MaterialIn the past: Hairless skin better than suitHuman skin: Too porous, turbulence too high

Shark skin: Scales spaced very closely together

Hydrophobicity, turbulence control > Dragresistance slice the water.

Fastskin I and II developedby Speedo

Coverage: Eventually fromfeet to hands

Oxygen bubbles alongstitches

Uwe Kersting, 2011

The Perfect Shape

Following three years of research thatincluded input from NASA, tests onmore than 100 different fabrics andsuit designs, and body scans of morethan 400 elite swimmers, Speedo haslaunched its most hydro-dynamicallyadvanced - and fastest - swimsuit todate.

- February 14th, 2008

Extreme tight fit: Streamlinebody shape reduce (bad)vibrations.

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Uwe Kersting, 2011

FINA hits the brakes but too lateIntention: Ban all hi-tech suits before WC 2009

Failed to address 136enquiries

-> all suits were allowed

Super materials: 100%Polyurethane, Hydrofoil

A male suit, mind you

Uwe Kersting, 2011

Restrictions by FINA 1st Jan 2010Surface covered: Men swimsuit shall not extend above the

navel nor below the knee and for women shall not cover theneck or extend past the shoulders nor shall extend belowthe knee.

Type of material: The material used for swimsuits can be only"Textile Fabric(s)" defined for the purpose of these rulesas material consisting of, natural and/or synthetic,individual and non consolidated yarns used to constitute a

fabric by weaving, knitting, and/or braiding.Additional rules for: surface treatment, flexibility, variety

of materials, thickness, buoyancy, permeability,construction etc

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Uwe Kersting, 2011

World Records Never

To Be Broken?

FINA: Records set at WC 2009 willstand.

Though the changes won't go into effect at the worldchampionships that begin Sunday in Rome, they will hangover the competition, seemingly wagging a finger at everyworld-record setter wearing a suit that will never beallowed again in a major swimming championship.

- The Washington Post, July 2009

~ 130 WRs broken since launch of high-tech suits

Uwe Kersting, 2011

Magnus Effectif a ball spins in flight, it will drag some of the

air close to the surface with it. This createsan area of high pressure and an area of lowpressure on opposite sides of the ball

this pressure imbalance will make the ball curve

in flight

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Uwe Kersting, 2011

An example: streamlining

Uwe Kersting, 2011

Bernoullis principleConsider a foil shape fluid flows over the

curved side & is accelerated while on the flatside it remains virtually unchanged

This difference in velocity of flow creates low

pressure on curved side & high pressure on flatside

Remember that force is directed to foil fromarea of high pressure to area of low pressure..causing lift = more effective!!!

Now! Do the stroke!

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Uwe Kersting, 2011

freestyle stroke (butterfly is verysimilar)

Uwe Kersting, 2011

breaststroke pattern relative to the a) swimmer and b) pool

b)a)

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Uwe Kersting, 2011

virtually no pull in breaststrokescull out, then scull in

LIFT forces from L & R limbs add to propulsion insculling motion

DRAG forces cancel in sculling motion

direction of movement

DRAG force

LIFT force

direction of movement

LIFT force

DRAG force

direction of movement

DRAG force

Uwe Kersting, 2011

Lift and Drag Forces on a Discus(Wind v = 25 m/s)

Angle of

Attack

(Degrees)

Lift (N) Drag (N) Lift/Drag

0 0.000 0.036 0.000

10 0.135 0.047 2.890

20 0.331 0.128 2.579

30 0.348 0.254 1.371

40 0.264 0.297 0.890

50 0.268 0.380 0.705

60 0.214 0.466 0.459

70 0.148 0.511 0.290

80 0.079 0.525 0.151

90 0.000 0.551 0.000

Angle of attack angle btwlongitudinal axis of a body &direction of fluid flow

Need to take some drag intoaccount to enable lift!

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Uwe Kersting, 2011

Back to swimming reality: Relationships

between stroke length and frequency

SF for freestyle, butterfly and breaststroke aresimilar and greater than for backstroke

as race distance increases, SL increases, SFdecreases and speed decreases

differences in ability are due primarily to strokelength, with better swimmers having greater SLs

to increase speed in the short term (i.e., on the day)increase stroke frequency

to increase speed in the long term (i.e., over theseason) train to increase stroke length video

Uwe Kersting, 2011

Summary

Factor (subjective) model be aware of thecomplexity of mechanical factors in swimming

resistive and propulsive forces what are they, andhow can you maximise propulsive and minimiseresistive forces

starts a case of optimising distance in the air plusdistance in the glide

strokes how do the principles of new water andlift influence stroke shape?

understand the relationships between SL and SF

turns play a much more important part in racesover 100m that most swimmers realise. Turnpractice is essential!

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Uwe Kersting, 2011

Points to rememberForces exerted by fluidsbuoyancy magnitude and location (+ effect on floating position)

Bernoullis Principle increased velocity of flow results indecreased pressure

lift & drag forces

result from objects being in a fluid flow.

The drag force is aligned with the flow and the liftforce is perpendicular to it.

Maximum lift at 45, zero lift at 0 and 90.

Form drag depends on the X-C area presented to the flow andgeometry

important in streamlining and minimizing frontalarea

Surface drag is comparably small but may be decisive

Uwe Kersting, 2011

Acapulco ...

or what I left out...

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