How Biomechanics Can measure Performance

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How Biomechanics Can measure Performance

• D. Gordon E. Robertson, PhD
• Fellow, Canadian Society for Biomechanics
• Emeritus Professor, School of Human Kinetics,
• University of Ottawa,
• Ottawa, Canada

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What is Biomechanics?

• Study of forces and their effects on living
  bodies
• Types of forces
• External forces
• ground reaction forces
• forces applied to other objects or persons
• fluid forces (swimming, air resistance)
• impact forces
• Internal forces
• muscle forces (strength and power)
• forces in bones, ligaments, cartilage

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Types of analyses

• Temporal (times, timing)
• Kinematic (positions, motion)
• Kinetic (forces, moments of force)
• Direct
• Indirect
• Electromyographic (muscle activation)

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Temporal Analyses

• Quantifies durations of performances in whole
  (race times) or in part (split times, stride
  times, stroke rates, etc.)
• Instruments include
• stop watches, electronic timers
• timing gates
• frame-by-frame video analysis
• Easy to do but not very illuminating
• Necessary to enable kinematic studies

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Example Electronic timing
Donovan Bailey sets world record (9.835) despite
slowest reaction time (0.174) of finalists
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Kinematics

• Position, velocity (speed) acceleration
• Angular position, velocity acceleration
• Distance travelled
• tape measures, electronic sensors, trundle
• wheel
• Linear displacement
• point-to-point linear distance and direction
• Angular displacement
• changes in joint angular orientations from
• point-to-point, 3D angles are order specific

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Kinematics

• Instrumentation includes
• tape measures, electrogoniometers
• speed guns, accelerometers
• motion capture from video or other imaging
  devices (cinefilm, TV, infrared, ultrasonic,
  etc.)
• GPS, gyroscopes, wireless sensors

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Kinematics

• Cheap to very expensive
• Cheap yields low information
• e.g., stride length, range of motion, distance
  jumped or speed of object thrown or batted
• Expensive yields over-abundance of data
• e.g., marker trajectories and their kinematics,
  segment, joint, and total body linear and angular
  kinematics, in 1, 2, or 3 dimensions
• essential for later inverse dynamics and other
  kinetic analyses

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Gait Characteristics - Walking
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Cheap Gait Characteristics of Running or
Sprinting
Stride velocity stride length / stride time
Stride rate 1 / stride time
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Cheap video analysis of sprinting

• Hip locations of last 60 metres of 100-m race
• Male 10.03 s
• accelerated to
• 60 m before
• maximum speed
• of 12 m/s
• Female 11.06 s
• accelerated to
• 70 m before
• maximum speed
• of 10 m/s
• Neither
• decelerated!

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Moderate accelerometry

• Direct measures such as electrogoniometry (for
  joint angles) or accelerometry are relatively
  inexpensive but can yield real-time information
  of selected parts of the body
• Accelerometry is particularly useful for
  evaluating impacts to the body

headform with 9 linear accelerometers to quantify
3D acceleration
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Expensive Gait and Movement Analysis Laboratory

• Multiple infrared cameras or infrared markers
• Motion capture system
• Usually multiple force platforms

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Kinetics

• Forces or moments of force (torques)
• Impulse and momentum (linear and angular)
• Mechanical energy (potential and kinetic)
• Work (of forces and moments)
• Power (of forces and moments)

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Kinetics

• Two ways of obtaining kinetics
• Direct dynamometry
• Use of instruments to directly
• measure external and even internal
• forces
• Indirect dynamometry via inverse dynamics
• Indirectly estimate internal forces
• and moments of force from directly
• measured kinematics, body segment
• parameters and externally measured
• forces

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Kinetics dynamometry

• Measurement of force, moment of force, or power
• Instrumentation includes
• Force transducers
• strain gauge, LVDTs, piezoelectric,
  piezoresistive
• Pressure mapping sensors
• Force platforms
• strain gauge, piezoelectric, Hall effect
• Isokinetic
• for single joint moments and powers,
• concentric, eccentric, isotonic

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force transducers

• Strain gauge
• inexpensive, range of sizes, and applications
• dynamic range is limited, has static capability,
  easy to calibrate
• can be incorporated into sports equipment
• Examples bicycle pedals, oars and paddles,
  rackets, hockey sticks, and bats

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Example rowing ergometry

• Subject used a Gjessing rowing ergometer with a
  strain gauge force transducer on cable that
  rotates a flywheel having a 3 kilopond resistance
• Force tracing visible
• in real-time to coach
• and athlete
• Increased impulse
• means better
• performance
• Applies to cycling, canoeing, swim or track
  starts

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force transducers

• Pressure mapping sensors
• moderately expensive, range of sizes and
  applications, poor dynamic response
• can be incorporated between person and sport
  environment (ground, implement)
• Examples shoe insoles, seating, gloves

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Example impact testing

• Helmet and 5-kg headform dropped from fixed
  height onto an anvil. Piezoresistive force
  transducer in anvil measures linear impact
  (impulse) and especially
• peak force
• Peak force is reduced
• when impulse is spread
• over time or over larger
• area by helmet and
• liner materials

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force platforms

• Typically measure three components of the ground
  reaction force, location of the force application
  (called centre of pressure), and the free
  (vertical) moment of force

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Example fencing (fleche)

• Instantaneous ground reaction force vectors are
  located at the centres of pressure
• Force signatures show pattern of ground reaction
  forces on each force platform

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Kinetics inverse dynamics

• process by which all forces and moments of force
  across a joint are reduced to a single net force
  and net moment of force

• process by which all forces and moments of force
  across a joint are reduced to a single net force
  and net moment of force
• the net force is primarily caused by remote
  actions such as ground reaction forces or impact
  forces
• the net moment of force, also called net torque,
  is primarily caused by the muscles crossing the
  joint, thus it is highly related to the
  coordination of the motion, injury mechanisms,
  and performance

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inverse dynamics

• requires linear and angular kinematics of the
  segments and knowledge of each segments inertial
  properties

• inertial properties are from proportions that
  estimate the segment mass, then equations that
  distribute the mass equally to geometrical solids
  based on markers placed on the segment

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inverse dynamics

• At each joint the moment of force may be flexor
  or extensor depending on the instantaneous
  actions of all the structures acting across the
  joint
• Muscles are the major contributors to the moment

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inverse dynamics

• At each joint the moment of force may be flexor
  or extensor depending on the instantaneous
  actions of all the structures acting across the
  joint
• At the end of the range of motion muscles are of
  less importance

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inverse dynamics

• In addition, each moment can work concentrically
  (cause increased motion), eccentrically (cause
  motion to be reduced or stopped) or isometrically
  (hold statically)

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inverse dynamics

• A concentric contraction of a muscle means the
  muscle shortens producing motion or elevation and
  does positive work
• A concentric moment of force means that the
  moment does positive work

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inverse dynamics

• An eccentric contraction of a muscle means the
  muscle lengthens slowing or stopping motion and
  does negative work
• An eccentric moment of force means that the
  moment does negative work

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example walking

• study of walking on level ground and up 3-degree,
  6-degree, and 9-degree ramps
• averages of 12 subjects (6 females/6 males)
• force platform was in floor and midway up ramp
• only left side analyzed
• planar (2D) analysis

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normalized Moments (left) and powers (right) of
level and incline walking
hip
hip
knee
knee
ankle
ankle
Time (seconds, Toe-off to Toe-off)
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normalized Moments (left) and powers (right) of
level and incline walking
hip
hip
knee
knee
Plantiflexor moment during stance first
does negative work then positive work at push-off
ankle
ankle
Time (seconds, Toe-off to Toe-off)
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normalized Moments (left) and powers (right) of
level and incline walking
hip
hip
Knee flexor moment at end of swing does negative
work to decelerate foot prior to heel-strike (so
you wont scuff the floor)
knee
knee
ankle
ankle
Time (seconds, Toe-off to Toe-off)
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normalized Moments (left) and powers (right) of
level and incline walking
hip
hip
Knee extensor moment during early stance does
negative work to allow controlled bending and
soft weight acceptance then does positive work to
extend knee slightly, but more so for the
9-degree incline
knee
knee
ankle
ankle
Time (seconds, Toe-off to Toe-off)
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normalized Moments (left) and powers (right) of
level and incline walking
hip
hip
Knee extensor moment during late stance does
negative work to control amount of knee flexion
but less so for the three inclines
knee
knee
ankle
ankle
Time (seconds, Toe-off to Toe-off)
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normalized Moments (left) and powers (right) of
level and incline walking
hip
hip
Hip extensor moment does positive work prior to
and during landing to extend hip, but more so as
incline increases
knee
knee
ankle
ankle
Time (seconds, Toe-off to Toe-off)
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normalized Moments (left) and powers (right) of
level and incline walking
hip
hip
Hip flexor moment does negative work
during mid-stance to control amount of extension
knee
knee
ankle
ankle
Time (seconds, Toe-off to Toe-off)
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normalized Moments (left) and powers (right) of
level and incline walking
hip
hip
Hip flexor then does positive work to flex hip
prior to and during swing phase
knee
knee
ankle
ankle
Time (seconds, Toe-off to Toe-off)
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normalized Moments (left) and powers (right) of
level and incline walking
hip
hip

• Comparisons among inclines
• no significant differences in peak moments at
  ankle, knee, or hip
• no significant differences in peak powers
  excepting at the knee during early stance

• Comparisons among inclines
• no significant differences in peak moments at
  ankle, knee, or hip
• no significant differences in peak powers
  excepting at the knee during early stance

• Comparisons among inclines
• significant increase in hip extensor work and
  decrease in hip flexor negative work as incline
  increase
• knee extensor work for 9-degree incline
• significant increase in plantiflexor work

knee
knee
ankle
ankle
Time (seconds, Toe-off to Toe-off)
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example sprinting

• male sprinter at 50 m into 100-m race
• race time 10.03 s
• stride length 4.68 metres
• stride time 0.40 s
• stanceswing 25

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example sprinting

• male sprinter at 50 m into 100-m race
• race time 10.03 s
• stride length 4.68 metres
• stride time 0.40 s
• stanceswing 25
• horizontal velocity of foot in mid-swing was 23.5
  m/s (85 km/h)!
• only swing phase could be analyzed since no force
  platform in track

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sprinting knee

• knee extensor moment did negative work (red)
  during first half of swing (likely not muscles)
• knee flexors did negative work (blue) during
  second half to prevent full extension (likely due
  to hamstrings)
• little or no work (green) done by knee moments

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sprinting hip

• hip flexor moment did positive work (red) during
  first part of swing (rectus femoris, iliopsoas)
• hip extensor moment did negative work mid-swing
  (green) then positive work (blue) for extension
  (likely gluteals)

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sPrinting conclusion

• knee flexors (hamstrings, gastrocnemius) are NOT
  responsible for knee flexion during mid-swing of
  sprinting
• hip flexors (rectus femoris, iliopsoas) are
  responsible for both hip flexion AND knee flexion
  during swing
• hip flexors are most important for improving
  stride length
• hip extensors (gluteals) are necessary for leg
  extension while knee flexors (hamstrings) prevent
  knee locking before landing

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normalized Moments (left) and powers (right) of
walking

• gait speed peak hip power
• run 6.1 m/s 9.0? W/kg
• sprint 11.9 m/s 53.0?W/kg
• thus, 8 fold increase in speed requires 175 fold
  increase in hip flexor power!

• Running (v 6.1 m/s)

• Sprinting (v 11.8 m/s)

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Peak Hip flexor power during swing versus speed

• exponential increase in the peak power required
  to cause thigh flexion during swing phase
• likely muscles are rectus femoris and ilio-psoas

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electromyography

• process of measuring the electrical discharges
  due to muscle recruitment
• only quantifies the active component of muscle,
  passive component is not recorded
• levels are relative to a particular muscle and a
  particular person therefore need a method to
  compare muscle/muscle or person/person
• not all subjects can perform maximal voluntary
  contractions (MVCs) to permit normalization
• effective way to identify patterns of muscle
  recruitment

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emg amplifiers

• Types
• cable
• cable telemetry
• telemetry

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emg electrodes

• Types
• surface (safest, painless, best for sports)
• fine wire (better for detecting which part of
  muscle is active)
• needle (best for medical)

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Example lacrosse

• experience male lacrosse player
• release velocity 20 m/s (72 km/h)
• duration from backswing to release 0.45 s
• hybrid style throw
• 8 surface EMGs of (L/R erector spinae, L/R
  external obliques, L/R rectus abdominus, and L/R
  internal obliques)
• four force platforms
• maximum speed throws into a canvas curtain

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Example lacrosse
left erector spinae right erector spinae left
external obliques right external obliques left
rectus abdominus right rectus abdominus left
internal obliques right internal obliques
start of throw
release
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electromyography

• Benefits
• identifies whether a particular muscle is active
  or inactive
• can help to identify pre-fatigue and
• fatigue states
• Drawbacks
• encumbers the subject
• difficult to interpret
• cannot identify contribution muscle is
• making (concentric, eccentric, isometric)
• should be recorded with kinematics

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future

• musculoskeletal models
• measure internal muscle, ligament and
  bone-on-bone forces
• difficult to construct, validate, and apply
• forward dynamics
• predicts kinematics based on the recruitment
  pattern of muscle forces
• difficult to construct, validate, and apply
• computer simulations
• requires appropriate model (see above) and
  accurate input data to drive the model
• can help to test new techniques without injury
  risk

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conclusions

• kinematics are useful for distinguishing one
  technique from another, one trial from another,
  one athlete from another
• kinematics yields unreliable information about
  how to produce a motion
• direct kinetics are useful as feedback to quickly
  monitor and improve performance
• direct kinetics does not quantify which muscles
  or coordination pattern produced the motion

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conclusions continued

• inverse dynamics and joint power analyses
  identify which muscle groups and coordination
  pattern produces a motion
• cannot directly identify specific muscles,
  biarticular contractions, or elasticity
• electromyograms yield level of specific muscle
  recruitment and potentially fatigue state
• electromyograms are relative measures of activity
  and cannot quantify passive muscle force, should
  be used with other measures

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Questions, comments, answers
School of Human Kinetics, University of
Ottawa, Ottawa, Ontario
Beaver in winter, Gatineau Park, Gatineau, Quebec