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
hockey stick
walker
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
Calibration
signal
Feedback
Measured
signal
Input
transducer
Input signal
Input power
source
Signal
conditioner
Transduced
signal
Output
transducer
Output signal
Auxillary power
supply
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
Wheatstone
Voltage detector
bridge
recording
or meter
Ra, Rb, Rc, Rd = strain gauges or dummy
Ra
Rb
(resistors)
• Should use full bridges (4 strain gauges)
Vout
for best temperature compensation
• Most designs use at least two active
Rd
Rc
gauges. Some designs permit four
Supply voltage (VDC)
active gauges. Poisson gauges reduce
cross-talk.
• Need separate circuits for each direction
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
Strain Gauge Transducer:
Bridge Amplifiers
•
•
•
•
can be made portable
multichannel
autobalancing
filtering may be included
NI four channel bridge amp
single channel amp with filtering
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)
Strain Gauge Transducers:
Types
Ra
Rb
Ra
• tension/compression
– useful for push or pull forces
Vin
Rc
Vout
Rd
Rc
F
Rb
• bending moment
– often used in strength testing
equipment
• torque
– useful for forearm torque
Rd
F
Ra
Rc
Ra Rb
Rb
Rd
Rc Rd
Strain Gauge Transducers:
Types
• Strain ring
– tension/compression only
– all four gauges are active Ra
• S-type
– tension/compression only
– all four gauges are active
F
Rb Rc
Rd
Ra Rb
F
Rc
Rd
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 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
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%
Output signal
• input signal units are newtons
Line of best fit
• output signal is in volts
Loading
Rise
Hysteresis
Run
Sensitivity = Rise/Run
Unloading
Input signal
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.health.uottawa.ca/biomech/watbiom
www.humankinetics
Finis
Muchas Gracias