Measurement of forces directly for immediate feedback to athletes and coaches D. Gordon E....

Measurement of forces directly for immediate

feedback to athletes and coaches

D. Gordon E. Robertson, PhD

Emeritus Professor

School of Human Kinetics

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

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

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)

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

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

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

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

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

Auxillary powersupply

Calibrationsignal

Measuredsignal

Inputtransducer

Signalconditioner

Outputtransducer

Input powersource

Feedback

Input signal Output signalTransduced signal

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

Strain Gauge Transducer:Wheatstone Bridge

VDC = constant DC voltage or battery

Vout = output voltage for display or recording

Ra, 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

Ra

Supply voltage (VDC)

Voltage detectoror meter

Wheatstonebridge

Vout

RcRd

Rb

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

schematicof uniaxial

gauge

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

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

Strain Gauge Transducer:Examples

• ski pole (compression)

• knee brace (bending)

• oar lock pin (bending)

Ra RdRcRb

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

Ra Rb

RcRd

VoutVin

RdRb

RcRa

F

Rd

Rb

Rc

RaF

Strain Gauge Transducers:Types

• Strain ring– tension/compression only– all four gauges are active

• S-type– tension/compression only– all four gauges are active

RdRa RbRc

FRdRa Rb

Rc

F

Rd

Rb

Rc

Ra

F

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

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)

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)

strain ringtransducer

cable or chain

holder and clampfor weights

weights

force platform

lift weighs here

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

transducer

force platform

push/pull hereoar/paddle

fulcrum

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

Loading

Unloading

Sensitivity = Rise/Run

Rise

Run

Output signal

Input signal

Line of best fit

Hysteresis

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, …)

Questions? Comments?

www.humankineticswww.health.uottawa.ca/biomech/watbiom

Finis

Muchas Gracias