Dynamometry D. Gordon E. Robertson, PhD, FCSB Biomechanics Laboratory, School of Human Kinetics,...
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Transcript of Dynamometry D. Gordon E. Robertson, PhD, FCSB Biomechanics Laboratory, School of Human Kinetics,...
Dynamometry
D. Gordon E. Robertson, PhD, FCSB
Biomechanics Laboratory,
School of Human Kinetics,
University of Ottawa, Ottawa, Canada1Biomechanics Laboratory, uOttawa
Dynamometry
• measurement of force, moment of force (torque) or power
• torque is a moment of force that acts through the longitudinal axis of an object (e.g., torque wrench, screw driver, engine) but is also used as another name for moment of force
• power is force times velocity (F.v) or moment of force times angular velocity (M
• Examples of power dynamometers are the KinCom, Cybex, home electrical meter
2Biomechanics Laboratory, uOttawa
Force Transducers
• devices for changing force into analog or digital signals suitable for recording or monitoring
• typically require power supply and output device
• types:– spring driven (tensiometry, bathroom scale)
– strain gauge (most common)
– linear variable differential transformer (LVDT)
– Hall-effect (in some AMTI force platforms)
– piezoelectric (usually in force platforms)
• Examples: cable tensiometer, KinCom, Cybex, Biodex, fish scale, force platform
3Biomechanics Laboratory, uOttawa
Tensiometer
• measures tension (non-directional force) in a cable, wire, tendon, etc.
Biomechanics Laboratory, uOttawa 4
Strain Gauge Force Transducers
• uses the linear relationship between strain (deformation, compression, tension) in materials to the applied force (stress)
• materials are selected that have relatively large elastic regions
• if material reaches
plastic region it is
permanently
deformed and needs
replacement
5Biomechanics Laboratory, uOttawa
Stress-Strain Measurements
• Instron 5567 (Neurotrauma Impact Science Laboratory, uOttawa) accurately measures stress and strain for a wide variety of materials
Biomechanics Laboratory, uOttawa 6
Strain Gauges
• can be uniaxial, biaxial, multiaxial
• require DC power supply (battery)
• can be wired singly, in pairs, or quartets
• can measure force, torque, or bending moment
Biomechanics Laboratory, uOttawa 7
Strain Link
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Strain Gauge Transducers
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Power Dynamometers
potentiometer
strain linklever arm
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Strain Gauge Lever
Cybex KinCom
Biomechanics Laboratory, uOttawa 11
• use strain gauges to measure normal force• moment is computed by multiplying by lever length
Bending Moment for Moment of Force
12Biomechanics Laboratory, uOttawa
• this knee brace was wired to measure bending moment
• it could therefore directly measure varus/valgus forces at the knee
Strain Gauge Force Transducers
Advantages:– can measure static loads
– inexpensive
– can be built into wide variety of devices (pedals, oars, paddles, skates, seats, prostheses …)
– portable
Disadvantages:– need calibration
– range is limited
– easily damaged
– temperature and pressure sensitive
– crosstalk can affect signal (bending vs. tension, etc.)
13Biomechanics Laboratory, uOttawa
Force Platforms
• devices usually embedded in a laboratory walkway for measuring ground reaction forces
• Examples: Kistler, AMTI, Bertek• Types:
– strain gauge (AMTI, Bertek)– piezoelectric (Kistler)– Hall-effect (AMTI)
• Typically measure at least three components of ground reaction force (Fx, Fy, Fz) and can include centre of pressure (ax, ay) and vertical (free) moment of force (Mz)
14Biomechanics Laboratory, uOttawa
Kistler Force Platforms
standard
in treadmill
clear topportable
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Piezoelectric Force Platforms
Advantages:– higher frequency response
– more accurate
– wide sensitivity range (1 N/V to 10 kN/V)
Disadvantages:– electronics must be used to measure static
forces, drift occurs during static measurements
– expensive, cannot be custom-built
– require 8 A/D channels
16Biomechanics Laboratory, uOttawa
AMTI Force Platforms
small model
standard model
glass-top model
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Strain Gauge Force Platforms
Advantages:– ability to measure static loads suitable for
postural studies
– inexpensive, can be custom-built
– fewer A/D channels required (typically 6 vs. 8)
Disadvantages:– typically fewer sensitivity settings
– poorer frequency response
– less accurate
18Biomechanics Laboratory, uOttawa
Equations for Computing Centres of Pressure
• centre of pressure locations are not measured directly
• Kistler: x = – (a[Fx23 –Fx14 ] – Fx z) /Fz
y = (b[Fy12 –Fy34] – Fy z) /Fz
• AMTI: x = – (My + Fx z) /Fz
y = (Mx – Fx z) /Fz
• Notice division by vertical force (Fz). This means centre of pressures can only be calculated when there is non-zero vertical force. Typically Fz must be > 25 N.
19Biomechanics Laboratory, uOttawa
Impulse
• Force platforms can measure impulse during takeoffs and landings
• When the subject performs a jump from a static position, the takeoff velocity and displacement of the centre of gravity can be quantified
Impulse = ≈ ( F ) t1
0
t
tFdt
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Takeoff Velocity
• To compute takeoff velocity divide the impulse by body mass
• For the vertical velocity, body weight must be subtractedvhorizontal = Impulsehorizontal / m
vvertical = (Impulsevertical – W t ) / m
• where m is mass, W is body weight, and t is the duration of the impulse
21Biomechanics Laboratory, uOttawa
Centre of Gravity Displacement
• Displacement of the centre of gravity can also be quantified by double integrating the ground reaction forces.
• First divide the forces by mass then double integrate assuming the initial velocity is zero and the initial position is zero. Be sure to subtract body weight from vertical forces.
• Care must be taken to remove any “drift” from the force signals.
22Biomechanics Laboratory, uOttawa
Centre of Gravity Displacement
• shorizontal =
• svertical =
• To compensate for drift (especially with Kistler force platforms) high-pass filtering is necessary.
1
0
1
0
2)/(t
t
t
tdtmF
1
0
1
0
2)/]([t
t vertical
t
tdtmWF
1
0
1
0
2)/(t
t horizontal
t
tdtmF
23Biomechanics Laboratory, uOttawa
Squat Jump (BioProc2)
• Example of a vertical squat jump (starts in full squat)
• red is vertical force, cyan is AP force
body weight lineairborne phase
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Centre of Gravity (BioProc3)
• Squat depth was 1.39 cm
• Takeoff height was 79.6 cm
• Jump height was 28.3 cm
25Biomechanics Laboratory, uOttawa