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