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Dynamometry

Dynamometry. D. Gordon E. Robertson, PhD, FCSB Biomechanics Laboratory, School of Human Kinetics, University of Ottawa, Ottawa, Canada. Dynamometry. measurement of force, moment of force (torque) or power

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Dynamometry

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  1. Dynamometry D. Gordon E. Robertson, PhD, FCSB Biomechanics Laboratory, School of Human Kinetics, University of Ottawa, Ottawa, Canada Biomechanics Laboratory, uOttawa

  2. 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 (Mw) • Examples of power dynamometers are the KinCom, Cybex, home electrical meter Biomechanics Laboratory, uOttawa

  3. 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 Biomechanics Laboratory, uOttawa

  4. Tensiometer • measures tension (non-directional force) in a cable, wire, tendon, etc. Biomechanics Laboratory, uOttawa

  5. 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 Biomechanics Laboratory, uOttawa

  6. Stress-Strain Measurements • Instron 5567 (Neurotrauma Impact Science Laboratory, uOttawa) accurately measures stress and strain for a wide variety of materials Biomechanics Laboratory, uOttawa

  7. 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

  8. Strain Link Biomechanics Laboratory, uOttawa

  9. Strain Gauge Transducers Biomechanics Laboratory, uOttawa

  10. Power Dynamometers potentiometer lever arm strain link Biomechanics Laboratory, uOttawa

  11. Strain Gauge Lever Cybex KinCom • use strain gauges to measure normal force • moment is computed by multiplying by lever length Biomechanics Laboratory, uOttawa

  12. Bending Moment for Moment of Force this knee brace was wired to measure bending moment it could therefore directly measure varus/valgus forces at the knee Biomechanics Laboratory, uOttawa

  13. 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.) Biomechanics Laboratory, uOttawa

  14. 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) Biomechanics Laboratory, uOttawa

  15. Kistler Force Platforms portable standard clear top in treadmill Biomechanics Laboratory, uOttawa

  16. 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 Biomechanics Laboratory, uOttawa

  17. AMTI Force Platforms small model standard model glass-top model Biomechanics Laboratory, uOttawa

  18. 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 Biomechanics Laboratory, uOttawa

  19. Equations for Computing Centres of Pressure • centre of pressure locations are not measured directly • Kistler:x = – (a[Fx23–Fx14] – Fxz) /Fz y = (b[Fy12–Fy34] – Fyz) /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. Biomechanics Laboratory, uOttawa

  20. 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 =≈ (SF ) Dt Biomechanics Laboratory, uOttawa

  21. Takeoff Velocity • To compute takeoff velocity divide the impulse by body mass • For the vertical velocity, body weight must be subtracted vhorizontal = Impulsehorizontal / m vvertical = (Impulsevertical – W t ) /m • where m is mass, W is body weight, and t is the duration of the impulse Biomechanics Laboratory, uOttawa

  22. 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. Biomechanics Laboratory, uOttawa

  23. Centre of Gravity Displacement • shorizontal = • svertical = • To compensate for drift (especially with Kistler force platforms) high-pass filtering is necessary. Biomechanics Laboratory, uOttawa

  24. Squat Jump (BioProc2) • Example of a vertical squat jump (starts in full squat) • red is vertical force, cyan is AP force airborne phase body weight line Biomechanics Laboratory, uOttawa

  25. Centre of Gravity (BioProc3) • Squat depth was 1.39 cm • Takeoff height was 79.6 cm • Jump height was 28.3 cm Biomechanics Laboratory, uOttawa

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