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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
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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
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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
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∆∆∆∆t
∆∆∆∆v
dt
dv
→
a = dv/dt = v.
t
v
a = ∆v/∆t
Illustration
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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!
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Accelerometers
Typical realisations
Strain gauge
Piezo-electric(one dimensional)
m
m
u
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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
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Good or bad vibes?
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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’
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• investigation and assessment of vibrations
frame grip
racket 1 racket 2
- amplitude - frequency - damping
Parameters
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Acceleration and ‘shock’
tibia
head
hip
timingAmp
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• synchronisation signal
accEMG
acceleration signal to
identify touch down
Example
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• bone acceleration
• mounting interaction
• angular motion
• gravity
Limitations / considerations
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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‘
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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.“
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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
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Force MeasurementStrain gauge
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Force PlatformPiezo electric effect
Calculation of the point of force
application
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GRF in gait
Walking:
• Magnitude
• Direction
• Shape
• Rate
• …
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GRF in gait II
Running:
• Components
• Variability
• Impact
(t < 50 ms)
• Active
maximum
• COM!
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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
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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
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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; ...)
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Background
Pronation and impacts are mechanically related
Increasing decelerationdistance decreasesimpact force maximum
Used to explain effects of shoesetc.
(Denoth, 1986; Stacoff, 1998)
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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
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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
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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.
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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.