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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, – PowerPoint PPT presentation

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Title: How Biomechanics Can measure Performance


1
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

2
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

3
Types of analyses
  • Temporal (times, timing)
  • Kinematic (positions, motion)
  • Kinetic (forces, moments of force)
  • Direct
  • Indirect
  • Electromyographic (muscle activation)

4
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

5
Example Electronic timing
Donovan Bailey sets world record (9.835) despite
slowest reaction time (0.174) of finalists
6
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

7
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

8
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

9
Gait Characteristics - Walking
10
Cheap Gait Characteristics of Running or
Sprinting
Stride velocity stride length / stride time
Stride rate 1 / stride time
11
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!

12
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
13
Expensive Gait and Movement Analysis Laboratory
  • Multiple infrared cameras or infrared markers
  • Motion capture system
  • Usually multiple force platforms

14
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)

15
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

16
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

17
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

18
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

19
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

20
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

21
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

22
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

23
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

24
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

25
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

26
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

27
inverse dynamics
  • In addition, each moment can work concentrically
    (cause increased motion), eccentrically (cause
    motion to be reduced or stopped) or isometrically
    (hold statically)

28
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

29
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

30
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

31
normalized Moments (left) and powers (right) of
level and incline walking
hip
hip
knee
knee
ankle
ankle
Time (seconds, Toe-off to Toe-off)
32
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)
33
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)
34
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)
35
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)
36
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)
37
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)
38
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)
39
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)
40
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

41
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

42
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

43
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)

44
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

45
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)

46
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

47
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

48
emg amplifiers
  • Types
  • cable
  • cable telemetry
  • telemetry

49
emg electrodes
  • Types
  • surface (safest, painless, best for sports)
  • fine wire (better for detecting which part of
    muscle is active)
  • needle (best for medical)

50
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

51
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
52
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

53
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

54
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

55
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

56
Questions, comments, answers
School of Human Kinetics, University of
Ottawa, Ottawa, Ontario
Beaver in winter, Gatineau Park, Gatineau, Quebec
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