1. Measurement of forces directly for immediate feedback to athletes and coaches D. Gordon E. Robertson, PhD Emeritus Professor School of Human Kinetics

2. Forces and Moments of Force (Torques) External forces –Ground reaction forces –Hand & grip forces or forces in sticks, bats, oars, paddles –Foot forces in pedals, foot stretchers (e.g., rowing) –Impact forces from hand, foot, or body Internal forces (not realistic for sports) –In muscles –In bones or other tissues

3. External Forces: Ground Reaction Usually measured by commercially purchased force platforms –Load cells - measure only 1, 2, or 3, components of a force, no centre of pressure, e.g., instrumented starting blocks –Single pedestal - inexpensive, inaccurate, poor frequency response –Three or four columns – expensive, large (3 corners) or largest (4 corners) area of accuracy, higher frequency response

4. External Forces: Ground Reaction Feedback can be real-time or delayed (shown shortly after force application), e.g., posture studies, pistol & rifle shooting, lifting Can be used for later inverse dynamics analysis Directly determine jump heights (vertical jumps, broad jumps) or starting velocity (sprints)

5. External Forces: Hand & Implement Forces Force transducers can be purchased and installed in implements (tennis racquets, bat, hockey sticks) Strain gauge transducers can be affixed to implements (paddles, oars, …) baseball bat walker hockey stick rowing rigger

6. External Forces: Foot & Pedal Forces Crank or pedal force transducers can be purchased commercially or constructed from strain gauge sensors Force platforms can be modified to fit under feet (e.g., rowing) Smaller load cells can be placed under feet

7. External Forces: Impact Forces Many varieties of load cells, piezoelectric transducers, strain gauge transducers are available Install in appropriate site: ground, wall, bag Pressure mapping sensors are possible for lighter forces

8. Internal Forces: Muscle and Bone Forces Types: –Buckle- transducer on tendon –Tendon strain gauge Highly invasive Only one muscle (usually) at a time Not ethical in most countries except on cadavers

9. Measurement Systems for Forces some or all of the following items form a measurement system input transducer can be strain gauges, LVDTs, Hall effect, etc. for strain gauge transducers a bridge amplifier is usually the signal conditioner

10. Strain Gauge Transducer: Characteristics Inexpensive Need external power source, batteries can be used For field use needs portable recording device or telemetry Types: –tension/compression - useful for push or pull forces –bending moment - often used in strength testing equipment –torque - useful for forearm torque or cycling sports Customizable - can be built into existing equipment (racquets, oars, bats, sticks, etc.) Can be synchronized with motion capture for later inverse dynamics analysis Easy to have real-time display for immediate feedback

11. Strain Gauge Transducer: Wheatstone Bridge V DC = constant DC voltage or battery V out = output voltage for display or recording R a, Rb, Rc, Rd = strain gauges or dummy (resistors) Should use full bridges (4 strain gauges) for best temperature compensation Most designs use at least two active gauges. Some designs permit four active gauges. Poisson gauges reduce cross-talk. Need separate circuits for each direction

12. Strain Gauge Transducer: Strain Gauges Characteristics: –resistance: 120, 350, 1000 ohm –size depends on application –many shapes (linear, bilinear (top left), roseate) –preferably with leads schematic of uniaxial gauge

13. Strain Gauge Transducer: Bridge Amplifiers NI four channel bridge amp single channel amp with filtering can be made portable multichannel autobalancing filtering may be included

14. Strain Gauge Transducer: Output multimeter and oscilloscope are helpful for testing and calibration direct to A/D of computer for real-time output use an oscilloscope, monitor, or computer display (latter can be too slow) multimeter storage oscilloscope computer monitor

15. Strain Gauge Transducer: Examples ski pole (compression) knee brace (bending) oar lock pin (bending)

16. Strain Gauge Transducers: Types tension/compression –useful for push or pull forces bending moment –often used in strength testing equipment torque –useful for forearm torque

17. Strain Gauge Transducers: Types Strain ring –tension/compression only –all four gauges are active S-type –tension/compression only –all four gauges are active

18. Strain Gauge Transducers: In Gjessing Rowing Ergometer Cam to simulate rowing stroke Brake to apply constant workload Strain link –measures pulling/pushing force Optical sensor –for counting flywheel rotations

19. Strain Gauge Transducers: Pedal Transducers Crank transducers –problem with wires becoming twisted –need tension/compression and bending moment circuits Pedal transducers –more difficult to construct –need load and sheer circuits 2D force) Torque transducer (bottom bracket)

20. Strain Gauge Transducers: Calibration of Tension/Compression Setup for load cells, strain rings of links Weights should not be lifted off of platform Platform is zeroed with weights on (tare)

21. Strain Gauge Transducers: Calibration of Bending Moment Setup for oars, paddles, etc. Need to measure distance between load and fulcrum Use this to compute actual moment of force sensed by transducer

22. Strain Gauge Transducers: Sensitivity relationship between applied force and output voltage of transducer (newtons/volt) should be linear within expected range of loads hysteresis typically <1% input signal units are newtons output signal is in volts

23. Summary Strengths –relatively inexpensive especially compared to motion capture –portable enough for field research –can be most important result of a performance –direct measure therefore easy to validate and understand –real-time feedback possible Weaknesses –limited information about how a motion was produced –can impede true execution of a performance –requires frequent calibration, breakable –not applicable to all types of skills (aquatics, soccer, wrestling, …)

24. Questions?Comments? www.humankinetics www.health.uottawa.ca/biomech/watbiom

25. Finis Muchas Gracias