D. Gordon E. Robertson, PhD
Fellow, Canadian Society for Biomechanics
Emeritus Professor, University of Ottawa
 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
 Temporal
 Kinematic
 Kinetic


Direct
Indirect
 Electromyographic
 Modeling/Simulation
 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
Donovan Bailey sets
world record (9.835)
despite slowest
reaction time (0.174)
of finalists
Race times
Reaction times
 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-topoint using a specified system (Euler angles,
Cardan angles etc.). Order specific.
 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
 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
a
running/sprinting
stride length
b
stance phase,
left foot
swing phase,
left foot
Notice that running footprints are typically on the
midline unlike walking when
they
are on either side
one gait
cycle
step length
left foot
right foot
flight phase
right foot-strike
left foot-strike
right toe-off
left toe-off
Stride velocity = stride length / stride time
Stride rate = 1 / stride time
time
 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!
100
male: 12 m/s
90
80
70
female: 10 m/s
60
50
40
5
6
7
8
9
Race time (s)
10
11
 Direct
measures such as electrogoniometry
(for joint angles) or accelerometry are
relatively inexpensive but can yield real-time
information of selected parts of the
Insidebody
headform (below) is a
3D accelerometer and 3 pairs
 Accelerometry is particularly useful
forsensors for 3D
of linear
evaluating impacts to the body angular acceleration
headform with
9 linear
accelerometers
to quantify 3D
acceleration
 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
X, Y, Z linear
velocities of
stick head
Forward and
vertical velocities
of centre of gravity
Sagittal,
transverse, and
axial rotational
velocities of
L5/S1 and hip
joints
 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)
 Two

for force and deformation
Direct dynamometry


ways Instron
of obtaining
kinetics
compression tester
measures of bones, muscles,
Use of instruments
to directly
ligaments, etc., under load
measure external and even internal
forces
Indirect dynamometry via inverse dynamics

Indirectly estimate internal forces
and moments
of force (U.
from
Gait laboratory
of directly
Sydney)
measured
kinematics,
body segment
with
10 Motion Analysis
cameras
walkwaymeasured
with
parameters
and and
externally
five force platforms
forces
 Measurement
of force, moment of force, or
power
 Instrumentation includes:

Force transducers



Pressure mapping sensors
Force platforms


strain gauge, LVDTs, piezoelectric, piezoresistive
strain gauge, piezoelectric, Hall effect
Isokinetic


for single joint moments and powers,
concentric, eccentric, isotonic
 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
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
 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
 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
 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
 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
 Instantaneous
ground
reaction force vectors
are located at the
centres of pressure
 Force
signatures show
pattern of ground
reaction forces on each
force platform
 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
free body
joint
kinetics
diagram
are
 the net momentsimplified
of force,
with
actualasmuscle
a also
single called net
forces,
force
and
ligament
a moment
torque, is primarily
caused
by the muscles
forces,
of
forcebone-on-bone
(in blue)
crossing the joint
thus
is highly related to
forces
anditjoint
force
the coordinationmoment
of theof motion,
injury
mechanisms and performance
 requires
linear and angular kinematics of the
segments and knowledge of the segment’s
head is an ellipsoid,
inertial properties
trunk and pelvis are
 inertial properties
are usually obtained by
elliptical cylinders,
other segments
using proportions
toare
estimate the segment’s
frusta of cones
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
 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
 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)
 Power

Pforce = F · v
 Power

of the net force is:
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
 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
 knee
extensor moment
did negative work (red)
during firstangular
half ofvelocity
swing
(likely not muscles)
 knee flexors did negative
moment of force
work (blue) during
second half to prevent
full extension (likely due
moment power
to hamstrings)
 little or no work (green)
done by knee moments
swing phase
 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)
 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
foot lifts at green arrow, impact at red arrow
 foot velocity at impact was 8.6 m/s (31 km/h)

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)
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”

 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)
 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
 Types:

cable




cable telemetry




reliable
less expensive
encumbers subject
reliable
less expensive
less cabling
telemetry



unreliable
more expensive
no cabling
 Types:

surface (best for sports)




fine wire




reliable
less expensive
noninvasive
unreliable
more expensive
invasive
needle (best for medical)



unreliable
more expensive
painful
 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
left erector spinae
• erector spinae
right erector spinae
quiet at release
left external obliques
• ext. obliques
right external obliques
highly active
left rectus abdominus
• rect. abd. only
right on
rectusnear
abdominus
release
left internal
obliques
• noticeable
left/
right right
internal obliques
asymmetry
start of throw
release
 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

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

 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
 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
School of Human Kinetics,
University of Ottawa,
Ottawa, Ontario
Canadian beaver in winter,
Gatineau Park, Gatineau,
Quebec
Muchas Gracias