How Biomechanics Can Improve Sports Performance

1. How Biomechanics Can Improve Sports 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 • 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 (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 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 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 • 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 forces 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 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)

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 = Pmomentdt • 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 23.5 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. 2000 Knee power 1500 Hip power 1000 500 0 -500 -1000 -1500 -2000 0.00 0.20 0.40 0.60 0.80 1.00 Time (s) 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”

35. 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 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 • reliable • less expensive • encumbers subject • cable telemetry • reliable • less expensive • less cabling • telemetry • unreliable • more expensive • no cabling

38. 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

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 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 • 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 • 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

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 Beaver in winter, Gatineau Park, Gatineau, Quebec