HOW BIOMECHANICS CAN
IMPROVE SPORTS PERFORMANCE
D. Gordon E. Robertson, PhD
Fellow, Canadian Society for Biomechanics
Emeritus Professor, University of Ottawa
WHAT IS BIOMECHANICS? Study of forces and their effects on
living bodies Types of forces
External forces ground reaction forces applied to other objects or persons fluid forces (swimming, air resistance) impact forces
Internal forces muscle forces (strength and power) force in bones, ligaments, cartilage
TYPES OF ANALYSES Temporal Kinematic Kinetic
Direct Indirect
Electromyographic Modeling/Simulation
TEMPORAL ANALYSES Quantifies durations of performances in
whole (race times) or in part (splits, 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
EXAMPLE: ELECTRONIC TIMING
Donovan Bailey sets world record (9.835) despite slowest reaction time (0.174) of finalists
Reaction timesRace times
KINEMATICS Position, velocity (speed) & acceleration Angular position, velocity & acceleration Distance travelled via tape measures,
electronic sensors, trundle wheel Linear displacements
point-to-point linear distance and direction Angular displacements
changes in angular orientations from point-to-point using a specified system (Euler angles, Cardan angles etc.). Order specific.
KINEMATICS Instrumentation includes:
tape measures, electrogoniometersspeed guns, accelerometersmotion capture from video or other imaging
devices (cinefilm, TV, infrared, ultrasonic, etc.)
GPS, gyroscopes, wireless sensors
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 datae.g., marker trajectories and their
kinematics, segment, joint, and total body linear and angular kinematics, in 1, 2, or 3 dimensions and multiple angular conventions
Are essential for later inverse dynamics and other kinetic analyses
CHEAP: GAIT CHARACTERISTICS OF RUNNING OR SPRINTING
stride length step length
left foot
swing phase,left foot
right foot
stance phase,left foot
left toe-off
one gait cycle
timeleft foot-strike right foot-strikeright toe-off
a
b
running/sprinting
flight phase
Stride velocity = stride length / stride time Stride rate = 1 / stride time
Notice that running foot- prints are typically on the midline unlike walking when they are on either side
CHEAP: VIDEO ANALYSIS OF SPRINTING Hip locations of last 60 metres of 100-m race Male 10.03 s
accelerated to 60 m beforemaximum speedof 12 m/s
Female 11.06 saccelerated to70 m beforemaximum speedof 10 m/s
Both did NOTdecelerate!
40
50
60
70
80
90
100
5 6 7 8 9 10 11Race time (s)
female: 10 m/s
male: 12 m/s
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
Inside headform (below) is a 3D accelerometer and 3 pairs of linear sensors for 3D angular acceleration
EXPENSIVE: GAIT AND MOVEMENT ANALYSIS LABORATORY
Multiple infrared cameras or infrared markers
Motion capture system
Usually multiple force platforms
Subject has 42 reflective markers for 3D tracking of all major body segments and joints
LACROSSE: STICK AND CENTRE OF GRAVITY KINEMATICS
X, Y, Z linearvelocities ofstick head
Forward and vertical velocitiesof centre of gravity
LACROSSE: PELVIS AND THORAX ANGULAR VELOCITIES
Sagittal,transverse, andaxial rotationalvelocities of L5/S1 and hipjoints
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)
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 measuredforces
Instron compression tester for force and deformation measures of bones, muscles, ligaments, etc., under load
Gait laboratory (U. of Sydney) with 10 Motion Analysis cameras and walkway with five force platforms
KINETICS: DYNAMOMETRY Measurement of force, moment of force,
or power Instrumentation includes:
Force transducers strain gauge, LVDTs, piezoelectric, piezoresistive
Pressure mapping sensorsForce platforms
strain gauge, piezoelectric, Hall effect Isokinetic
for single joint moments and powers, concentric, eccentric, isotonic
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 equipmentExamples: bicycle pedals, oars and paddles,
rackets, hockey sticks, and bats
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 visiblein real-time to coachand athlete
Increased impulsemeans betterperformance
Applies to cycling, canoeing, swim or track starts
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
FORCE TRANSDUCERS Piezoelectric:
inexpensive, range of size and applicationpoor static capability, difficult to calibratesuitable for laboratory testing or in sports
arenasExamples: load cells, force platforms
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 especiallypeak force
Peak force is reducedwhen impulse is spreadover time or over largerarea by helmet andliner materials
FORCE PLATFORMS Typically measure three components of
ground reaction force, location of force application (called centre of pressure), and the free (vertical) moment of force
Piezoelectric:expensive, wide force range, high dynamic
response, poor static response Strain gauge:
moderately expensive, narrow force range,moderate dynamic response, excellent statically
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
KINETICS: INVERSE DYNAMICS process by which all forces and
moments of force across a joint are reduced to a single net force and 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
free body diagram with actual muscle forces, ligament forces, bone-on-bone forces and joint moment of force
joint kinetics are simplified as a single force and a moment of force (in blue)
INVERSE DYNAMICS requires linear and angular kinematics
of the segments and knowledge of the segment’s inertial properties
inertial properties are usually obtained by using proportions to estimate the segment’s mass and then equations based on the mass being equally distributed in a representative geometrical solid (e.g., ellipsoid, frustum of a cone, or elliptical cylinder) based on the segment’s markers
head is an ellipsoid, trunk and pelvis are elliptical cylinders, other segments are frusta of cones
INVERSE DYNAMICS generally analyses start with a distal
segment what is either free swinging or in contact with a force platform or force transducer
then the next segment in the kinematic chain is analyzed
process continues to the trunk and then starts again at another limb
KINETICS: JOINT POWER ANALYSIS Net forces add no work nor do they
dissipate energy then can: transfer energy from one segment to
another passively Net moments of force can:
generate energy by doing positive work at a joint
dissipate energy by doing negative work across a joint
transfer energy across a joint actively (meaning that muscles are actively recruited unless joint is fully extended or flexed)
KINETICS: JOINT POWER ANALYSIS Power of the net force is:
Pforce = F · v
Power of net moment of force is:Pmoment = M · w
Work done by net moment of force is computed by integrating the moment power over timeWmoment = Pmoment dt
Work done by net force is zero
EXAMPLE: SPRINTING male sprinter (10.03 s 100-m) at 50 m into
race stride length approximately 4.68 metres
horizontal velocity of foot in mid-swing was 23.5 m/s (84.6 km/h)!
only swing phase could be analyzed since no force platform in track
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
angular velocity
moment of force
moment power
swing phase
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)
SPRINTING: CONCLUSION knee flexors (rectus femoris and
iliopsoas) are NOT responsible for knee flexion during mid-swing
hip flexors 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
EXAMPLE: KARATE FRONT KICK foot lifts at green arrow, impact at red arrow foot velocity at impact was 8.6 m/s (31 km/h)
knee extensors do no work, knee flexors (red) instead do negative work to prevent hyperextension
hip flexors do positive work (green) then extensors do negative work (blue) to create “whip action”
-2000
-1500
-1000
-500
0
500
1000
1500
2000
0.00 0.20 0.40 0.60 0.80 1.00
Time (s)
Knee power
Hip power
INVERSE DYNAMICS Benefits:
can attribute specific muscle groups to the total work done within the body
can exhibit coordination of motion Drawbacks:
net moments are mathematical constructs, not measures physiological structures
cannot validate with direct measurementscannot detect elastic storage and return of
energycannot quantify multi-joint transfers
(biarticular muscles)
ELECTROMYOGRAPHY process of measuring the electrical discharges
due to active muscle recruitment only quantifies the active component of muscle,
passive component is not recorded levels are relative to a particular muscle and
particular person therefore need method to compare muscle/muscle or person/person
not all subjects can perform maximal voluntary contractions (MVCs) to permit normalization
effective way to identify muscle is recruitment
EMG: AMPLIFIERS Types:
cable reliable less expensive encumbers subject
cable telemetry reliable less expensive less cabling
telemetry unreliable more expensive no cabling
EMG: ELECTRODES Types:
surface (best for sports) reliable less expensive noninvasive
fine wire unreliable more expensive invasive
needle (best for medical) unreliable more expensive painful
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
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
• erector spinaequiet at release• ext. obliques highly active• rect. abd. onlyon near release• noticeable left/right asymmetry
ELECTROMYOGRAPHY Benefits
identifies whether a particular muscle is active or inactive
can help to identify pre-fatigue and fatigue states
Drawbacksencumbers the subjectdifficult to interpretcannot identify what contribution muscle is
making (concentric, eccentric, isometric)should be recorded with kinematics
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
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
CONCLUSIONS CONTINUED inverse dynamics and joint power
analysis identifies 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
QUESTIONS, COMMENTS, ANSWERS
School of Human Kinetics,University of Ottawa,Ottawa, Ontario
Canadian beaver in winter,Gatineau Park, Gatineau,Quebec
FINIS
Muchas Gracias