anvbiomkacc force webversion - aalborg universitet · anvendt biomekanik uwe kersting –minimodule...

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1© Uwe Kersting, 2008

ACCELEROMETRY

and

FORCE MEASUREMENT

Department of Department of Sport Sport and Exercise Scienceand Exercise Science

SPORTSCI 306 SPORTSCI 306 –– Technique AssessmentTechnique Assessment

Uwe Uwe Kersting Kersting –– LectureLecture 0404 -- 20072007

Center for SensoryCenter for Sensory--Motor InteractionMotor Interaction

Anvendt BiomekanikAnvendt Biomekanik

Uwe Uwe Kersting Kersting –– MiniModuleMiniModule 0404 -- 20082008


Objectives

• Basic working principles of accelerometers

• Considerations how to apply them

• How to interpret data gained from

accelerometers

• Basic working principles of force sensors / force

platforms

• Go through a study in which all techniques we

discussed were applied


Contents

1. Acceleration – velocity – displacement

2. Accelerometry: techniques

3. Applications

4. Force and Acceleration – the same thing?

5. Force platform – standard tool in Biomechanics;

how to calculate the point of force application

6. Integration of forces and kinematics (optional)

7. Summary

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dt

dsv = ds/dt = s

.

t

s

∆∆∆∆t

∆∆∆∆s

v = ∆s/∆t

Illustration


∆∆∆∆t

∆∆∆∆v

dt

dv

→

a = dv/dt = v.

t

v

a = ∆v/∆t

Illustration


t

v

s

a

t

v

s

a

acceleration=0

constant velocity

linear displacement

constant acceleration

linear velocity

parabolic displacement

Examples

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Acc notation

Accelerometer:

measures of acceleration

d2s=dv

= a use kinemetrydt2 dt

r = v = a (vector notation)

F = m * a measure F!


Accelerometers

Typical realisations

Strain gauge

Piezo-electric(one dimensional)

m

m

u


Acc mounting

Applications

Equipment mounted

Bone mounted

Bite bar

Skin mounted

Specifications:

range

natural frequency

weight

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Acc mounting

Skin mounted accelerometers

Relative movement of skin and skeleton

light weight

balsa wood

strapping

Impact


Good or bad vibes?


a

s

v

60 ms5 ms

• measuring frequency(signal frequency vs sampling frequency)

CRITERIA TO EVALUATE MEASURING METHODS

A

B

Examples: vibrations

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0.01 s

fsig=100 Hz

fsamp=400 Hz fsamp≥ 4 x fmax

appropriate sampling frequency

Nyquist frequency

‘2 * fmax’


• investigation and assessment of vibrations

frame grip

racket 1 racket 2

- amplitude - frequency - damping

Parameters


Acceleration and ‘shock’

tibia

head

hip

timingAmp

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• synchronisation signal

accEMG

acceleration signal to

identify touch down

Example


• bone acceleration

• mounting interaction

• angular motion

• gravity

Limitations / considerations


Questions to ask (Nigg 1986):

• How well does the measured acceleration actually

represent the acceleration in which one is interested?

• Is the measured acceleration the variable that

answers the question of interest?

• What does the measured acceleration mean

mechanically and biologically?

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The accelerometer which

became a goniometer?

To measure inclination

Applications

Workplace environments

Activity Monitoring

‚Relatively slow movements‘


History

Aristotle (384 - 322 v.Chr.)

De Motu Animalum

Reference:

„Continuing; the forces on that, which produces

movement and on that, which remains have

to be kept the same ... in a way that the

pushing pushes and at the same time gets

pushed – similarly with the same force.“


Newton‘s laws

1. Law: If a = 0

2. Law: F =

3. Law: F12 =Sir Isaac Newton

(1642-1709)

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Motivation

Why measure forces?

• Definition of biomechanics!

• Forces are the cause of movement

• External forces characterise load

• Forces (may) determine/influence

performance


Force MeasurementStrain gauge


Force PlatformPiezo electric effect

Calculation of the point of force

application

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GRF in gait

Walking:

• Magnitude

• Direction

• Shape

• Rate

• …


GRF in gait II

Running:

• Components

• Variability

• Impact

(t < 50 ms)

• Active

maximum

• COM!


Magnitudes in other sports

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What do forces tell us?Quantify, evaluate load on the body

external impact

in-shoe impact

force rate

in this study a force plate and an

in-shoe force sensor under

the heel were used

extremely hard

hard

medium

soft

extremely soft


What do forces tell us?

Calculate/estimate

internal stress

(inverse dynamics)

A B

Fap

Fcomp

Fap(fem)

Fcomp(fem)

Fap

Fcomp

Fap(fem)

Fcomp(fem)

Shear forces tibia

0

400

800

1200

1600

1 2

F [N]

group1 group2Fap Fcom p

1

3

2


Rearfoot movement

and impact forces

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Background

Running injuries are caused by

- excessive rearfoot movement- high impacts at touch down

(Nigg, 1985; ...)

Impact and tibial acceleration are closely related.

dFp/dt correlates with accmax(Lafortune 1981; ...)


Background

Pronation and impacts are mechanically related

Increasing decelerationdistance decreasesimpact force maximum

Used to explain effects of shoesetc.

(Denoth, 1986; Stacoff, 1998)


Purpose

Introduce variations in rearfoot movement

– hold all other factors constant

– check for a relationship between

pronation and impact force

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Methods

Criteria

– GRF

– tibial acceleration

– rearfoot angle

max, -min, -range


Design

Two types of shoes:

running shoe + sandal (L + S)

4 conditions

– OA normal running

– AC with ankle brace

– TP medial tape

– AT ankle brace and tape


Tibial acceleration and GRF

1,6 1,7 1,8 1,9 2,0

0

200

400

600

800

1000

1200

1400

1600

1800

2000

F [N]

t [s]

Fz

S15FT A [g]

0

-4

10

Acc

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Rearfoot angle and GRF

1,60 1,65 1,70 1,75 1,80 1,85 1,90 1,95 2,00

0

200

400

600

800

1000

1200

1400

1600

1800

2000

F [N]

t [s]

Fz

beta

0

-4

10

β [°]



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Summary

Bracing and taping allow for a limitation of rearfoot

movement during running (short term).

Different combinations of both techniques produce

systematically varying effects.

Tibial acceleration remains stable.

Impact force changes to some extent but

differences can’t be explained by rearfoot

motion.


What does it mean?

Rearfoot movement has no effect on impact

forces (?)

… in treadmill running

A possible Hypothesis: The body system

has its individual impact level – and (tries

to) sticks to it.

anvbiomkacc force webversion - aalborg universitet · anvendt biomekanik uwe kersting –minimodule...

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