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How Biomechanics Can Improve Sports Performance

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Presentation on theme: "How Biomechanics Can Improve Sports Performance"— Presentation transcript:

1 How Biomechanics Can Improve Sports Performance
D. Gordon E. Robertson, PhD Fellow, Canadian Society for Biomechanics Emeritus Professor, University of Ottawa

2 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

3 Types of analyses Temporal Kinematic Kinetic Electromyographic
Direct Indirect Electromyographic Modeling/Simulation

4 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

5 Example: Electronic timing
Donovan Bailey sets world record (9.835) despite slowest reaction time (0.174) of finalists Race times Reaction times

6 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.

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 and multiple angular conventions Are essential for later inverse dynamics and other kinetic analyses

9 Cheap: Gait Characteristics of Running or Sprinting
Notice that running foot- prints are typically on the midline unlike walking when they are on either side Stride velocity = stride length / stride time Stride rate = 1 / stride time

10 Cheap: video analysis of sprinting
Hip locations of last 60 metres of 100-m race Male s accelerated to 60 m before maximum speed of 12 m/s Female s 70 m before of 10 m/s Both did NOT decelerate!

11 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 Inside headform (below) is a 3D accelerometer and 3 pairs of linear sensors for 3D angular acceleration headform with 9 linear accelerometers to quantify 3D acceleration

12 Expensive: Gait and Movement Analysis Laboratory
Subject has 42 reflective markers for 3D tracking of all major body segments and joints Multiple infrared cameras or infrared markers Motion capture system Usually multiple force platforms

13 Lacrosse: stick and centre of gravity kinematics
X, Y, Z linear velocities of stick head Forward and vertical velocities of centre of gravity

14 Lacrosse: pelvis and thorax angular velocities
Sagittal, transverse, and axial rotational velocities of L5/S1 and hip joints

15 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)

16 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 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

17 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

18 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

19 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

20 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

21 force transducers Piezoelectric:
inexpensive, range of size and application poor static capability, difficult to calibrate suitable for laboratory testing or in sports arenas Examples: load cells, force platforms

22 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

23 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

24 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

25 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 joint kinetics are simplified as a single force and a moment of force (in blue) free body diagram with actual muscle forces, ligament forces, bone-on-bone forces and joint moment of force

26 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

27 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

28 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)

29 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 time Wmoment =  Pmoment dt Work done by net force is zero

30 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 m/s (84.6 km/h)! only swing phase could be analyzed since no force platform in track

31 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

32 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)

33 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

34 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 -500 500 1000 1500 2000 0.00 0.20 0.40 0.60 0.80 1.00 Time (s) Knee power Hip power

35 inverse dynamics Benefits: Drawbacks:
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 measurements cannot detect elastic storage and return of energy cannot quantify multi-joint transfers (biarticular muscles)

36 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

37 emg: amplifiers Types: cable cable telemetry telemetry reliable
less expensive encumbers subject cable telemetry less cabling telemetry unreliable more expensive no cabling

38 emg: electrodes Types: surface (best for sports) fine wire
reliable less expensive noninvasive fine wire unreliable more expensive invasive needle (best for medical) painful

39 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

40 Example: lacrosse erector spinae quiet at release
left erector spinae right erector spinae left external obliques right external obliques left rectus abdominus right rectus abdominus left internal obliques right internal obliques erector spinae quiet at release ext. obliques highly active rect. abd. only on near release noticeable left/ right asymmetry start of throw release

41 electromyography Benefits Drawbacks
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 what contribution muscle is making (concentric, eccentric, isometric) should be recorded with kinematics

42 future musculoskeletal models forward dynamics computer simulations
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 computer simulations requires appropriate model (see above) and accurate input data to drive the model can help to test new techniques without injury risk

43 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

44 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

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

46 Finis Muchas Gracias

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