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