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”

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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