McGinnis, Peter Merton
Technology in Biomechanics
McGinnis, Peter Merton, (2013) "Technology in Biomechanics" from McGinnis, Peter Merton, Biomechanics of sport and exercise
pp.383-393, Leeds: Human Kinetics Publishers Ltd. ©
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chapter l6
Technology in Biomechanics
objectives
When you finish this chapter, you should be able to do the following:
•
Define quantitative
biomechanical
analysis
•
Discuss how measurement
•
Discuss instrumentation
used for measuring
•
Discuss instrumentation
used for measuring kinetic parameters
of biomechanical
variables may influence the variables themselves
kinematic parameters in biomechanics
in biomechanics
A golfer lines up a chip shot. But the golfer looks different-he's not wearing
normal golf attire. Bright dots are attached to the club and various parts of the
golfer's body. A small box is strapped onto his back, and cables lead out of this
box and into his shoes. More wires attach to devices on his legs. The golfer is
standing on some sort of plate. Bright lights illuminate him. He's not hitting the
ball on a golf course; he's in a laboratory where a variety of devices are recording
his movements, muscle actions, and reaction forces under his feet. What sort of
lab is this? What kinds of devices are used to measure the biomechanical variables
we have discussed in this book? In the previous three chapters we learned about
qualitative biomechanical analyses.This chapter is about the technology used in
quantitative biomechanical analyses.
Quantitative
biomechanical analysis of
human movement involves actual measurement of human
m vement and the underlying causes of the movement.
If human movement or any of its aspect is quantified or
measured (de cribed with numbers), the resulting analysis
based on these mea urements i a quantitative biomechanical analysis. This chapter presents an overview
of the techn logy used to measure the biomechanical
features of human movement.
Quantitative
Biomechanical Analysis
Teachers, coache. , and therapists often perform qualitalive biomechanical analyses, but they rarely perform any
quantitative biornechanicaJ analyses. When is a quantítalive biomechanicaJ analysis warranted? Insport, quantitative biomechanical analyses are usually done only at the
clit or prof ssional level because of the expense and time
involved. These analyses may be done throughout an athI te's season or career to monitor changes in technique,
to monitor ritical changes in biomechanical parameters
r suiting from t.raining improvements, to monitor progress in rehabilitation from injury, to provide data for
biom hanical research about the specific sport skill,
and s on. Ergonomists and human factors specialists
may use quantitative biomechanical analyses of workers
to determine the causes of overuse injuries in workplace
environments and develop solutions for them. Clinical
biomechanists affiliated with hospitals or other medical
establishments may conduct quantitative biomechanical
analyses of patients to determine effects of various medial interventions on gait, to diagnose musculoskeletal
diseases or injuries, to monitor rehabilitation, and so on.
Quantitative biomechanical analyses are warranted
in all of these cases because changes in the biomechanical variables being measured may be indistinguishable
without special instruments. The movements Occur too
quickly to be readily perceived by the human eye, or
the differences in position and displacement are too
subtle to be noticed. In other cases, the biomechanical
variable being measured may be too difficult to perceive
by anyone other than the athlete (or patient or client).
As an observer, how do you detect the magnitude and
direction of the ground reaction forces acting on a
runner? You can't see the forces-you
can see only
their effects. We need special instruments for measuring these variables.
Measurement Issues
Quantitative biomechanical analyses of human movement involve measurements of biomechanical variables.
The variables measured may be temporal (timing), kinematic (position, displacement, velocity, acceleration), or
kinetic (force, energy, work, power). In any case, Some
sort of instrument is used to measure the variable. The
instrument itself and the setting that it is used in may
affect the performance of the athlete, patient, or client.
The process of measuring something influences the
parameter being measured. The validity of the measured parameter is thus threatened by the measurement
process. Measurement technology that minimizes the
measurement effects on the performer is preferred.
:)
!he process of measuring something
Influences the parameter being measured.
Technology
in Biomechanics
385
Laboratory Data Collection
Ideally, the environment in which you measure the performance should be carefully controlled, if possible. Most
data for quantitative biomechanical analyses are collected
in a biomechanics laboratory, where the environment
can be controlled. The drawback is that a biomechanics
laboratory is not the setting in which the athlete, patient,
or client normally moves or performs. The novelty of
the environment may influence the movements being
measured. The laboratory should be set up to duplicate as
closely as possible the environment in which the movements normally occur.
The benefit of data collection in a laboratory is control
of the environment. The cameras, lights, temperature, and
so forth are always the same. The subject thus performs
in the same conditions each time she is evaluated. Much
of the instrumentation is permanently set in position so
the time to prepare for data collection is minimal. In
addition, sensors, markers, or data collection packs can
be attached to the performer.
:)
The benefit of data collection in a
laboratory is control of the environment.
The drawback is that the environment is not the same
as the real-life environment in which the athlete, patient,
or client usually moves. A baseball pitch thrown in the
lab may be very different from the same type of pitch
thrown in a game. The lights and carneras and the technicians watching and measuring the patient's movements
may make the patient self-conscious and alter the movement. The attachment of markers, sensors, or cables to
the performer will have some effect on the movements
being measured. In laboratory data collection, it is very
important for the subject to become familiar with the
equipment and the laboratory environment before data
collection begins.
In-the-Field Data Collection
An actual athletic competition may be the best environment for measuring the biomechanics of an athlete's
performance because it is one in which the athlete normally performs (see figure 16.1). The competition setting, however, may not be the best for the biomechanist.
Most biomechanics technology is not very portable. To
record ground reaction forces, force plates would have to
be mounted in the competition setting. To record muscle
activity, electrodes would have to be attached to the
Figure 16.1 A biomechanist operates a high-speed motion picture camera to record the strides of 400 m sprinters
at the USA Track and Field Championships. Data collection in the field may not have an effect on the performance of
the athletes being studied.
....
Biomechanics of Sport and Exercise
386
athlete's body and signals from these sent to a receiver
for recording. Alternatively, a device for recording the
signals could be attached to the athlete. The type of da~a
regularly collected in athletic competitions is kinematic
data. Technology for measuring kinematic data includes
electronic timing devices, video recordings or motion
picture mm and their computerized analy is systems,
and radar or laser velocity-measuring devices. Most of
the e measurement device are relatively noninva ive.
The perf l'man e of the athlete is minimally affected
by their lise. Th directors of the athletic competitions
will be more likely to grant biomechanist permission
t use the. e non-invasive types of measurement equipment during competition than the more invasive types
f mcasurern nt equipment.
Th major drawback of collecting biomechanical data
during an athletic cornp tition is lack of control of the
environment. The biomechanist has no control over the
performer Or the factors influ ncing the performance,
and the p sitions of the data collecti n instruments may
b restrict d. Th film or video cameras and the radar or
laser vel ity-measuring devices all require direct views
f the p rformance. Th radar or laser velocity-measuring
d vic s r quirc etup along the line of the performer's
motion. fficials, spe tutors, ther athlete, and so On
muy bl k thes views during p riods of data recording.
hanges in lighting may limit th use f film or video
cam ras. In lement weather may also limit the use of the
equipment (c.g., t o cold, to hOL,t o wet). All of these
devi es requir clectri al power. Multiple batteries must
b n hand, or an ace ssible power SOurce at the athletic
venu mu, t be found. Transporting exp n ive and fragile
I troni equipm nt to and from the e mpetilion venue
exp s it to risk, of damage r theft. Th preplanning
and setup time for data collecti n al an athletic comp titi n is also ext nsive. Additionally, it i difficult to
duplicate th li, of the given amera in xactly the same
positi ns fI' !TI on e mp titi n to the next. In spite of
th s drawbacks, sp rt biomechanists r gularly collect
bi m hanic I data at the Olympic ames and world and
nati nul champi nships for a variety of sports.
Sampling Rafe
Most bi mechanical parameters vary with lime, so they
must be measured throughout the movement. A computer
is usually involved in th data collection to store and process the data. Most biome hanical variables ure analog
ignu)s-they
vary . ntinllOllsly with time. Before
being tored and analyzed by the computer, h wever, the
data must be conv rted to digital form. Digital data are
numerical (for th omputer, the data ar repre ented in
binary form-a. J s and Os), To convert an ana] g signal
t digital, the anal g signal is measured al discrete intervals (the signal is sampled), and then the measured value
is converted into binary form. How often the signal is
sampled is referred to as the sampling rate or sampling
frequency.
The sampling frequency of an instrument indicates
how often the instrument records a measurement. The
sampling frequency of some biomechanics instruments,
such as a force platform, may be as high as several thousand samples per second, while that of a typical video
recording is usually 30 or 60 pictures or samples per
second. The sampling rate of most measuring tools in
biomechanics can be adjusted to suit the motion being
measured. For slow, deliberate movements, sampling
rate below 100 samples per second are adequate, but
for movements that involve impacts or quickly changing
states, much faster sampling rates are required.
Tools for Measuring
Biomechanical Variables
Tools for measuring biomechanical variables vary in
sophistication and cost from simple stopwatches to
highly sensitive force platforms and multi-camera motion
capture systems. Measurement techniques are constantly
evol ving as technology improves. The review of measurement tools presented here is only a brief overview of some
of the technologies available for measuring biomechanical variables. The tools have been categorized as tools for
measuring kinematics and tools for measuring kinetics.
Tools for Measuring Kinematics
Kinematic variables are based on position and time or
the changes in each. Popular tools for measuring kinematic variables in biomechanics include timing systems,
velocity-measuring systems (based on radar or laser
light), accelerometers, microelectromechanical systems
(MEMS) inertial sensors, and optical imaging systems
(film cameras, video cameras, and so on). Full-body
motion capture (mocap) systems may use one or more of
these technologies to record and quantify human motion
in two or three dimensions.
Timing Devices
Time is a fundamental dimension in mechanics, so the
measurement of time is important. Watches are the simplest devices for measuring time. If the duration of an
event being timed is long enough, a simple stopwatch
may be an appropriate timing device. If more accuracy
is needed and if the duration of the event being timed is
short, then an automatic timing device is more appropriate.
Most automatic timing devices use electronic clocks in
a computer or other digital device. Electronic or mechanical switches start and stop the clocks. These switches
Technology
in Biomechanics
387
may be triggered by a variety of means. For instance,
pressure-sensitive mats may be used to start or stop the
clock as a person steps onto or off the mat. If light is the
trigger for the device, it is considered a photogate timer.
Photogates may be sensitive to specific wavelengths or to
a wide frequency. In any case, if a light source is shining
on the sensor and this beam of light is broken by a person
or a limb or an implement, the change in intensity oflight
on the sensor triggers the clock to start or stop.
These automatic timing devices obviously measure
time, but they may also be used to measure average speed.
If the triggering sensors are positioned a known distance
apart, then average velocity can be computed from the
distance and time measures. Adding more sensors can
provide a more detailed data set for a movement. For
example, multiple photogates positioned in an array along
a track or runway can provide information about step rate,
step length, stance time, flight time, and velocity over a
number of steps.
Velocity-Measuring
Systems
Timing systems are the simplest kinematic measuring
tools. They are useful for measuring average velocities of
humans or objects, but what about instantaneous velocity?
The radar gun that troopers use to catch speeders on the
highways has been adapted to capture the instantaneous
velocities of objects in sport. A radar gun transmits a
microwave radio signal at a specific frequency and measures the frequency of the signals that are reflected back
to it. A stationary object will reflect the radio signal at the
same frequency that was transmitted by the radar gun. If
the object is moving, the reflected signal will experience
a shift in its frequency-the
Doppler effect. The velocity
of the object is determined by this frequency shift.
Radar guns are limited to measurements of speed (or
components of velocity) directly toward or away from
the radar gun. They are most widely used to measure
the speed of pitched baseballs, but radar guns are also
marketed for use in golf, tennis, hockey, soccer, lacrosse,
and other sports. Their use in measuring the velocity of an
athlete's body is limited unless a radar-reflective marker
is worn by the athlete.
A laser-based velocity-measuring
device is more
effective at measuring the speed of athletes, especially
runners. It is similar in operation to a radar gun, but it
uses a laser and the reflection of the laser light to measure
velocity. Unlike radar, whose signal disperses as distance
increases, the laser in a laser-based velocity-measuring
device is tightly focused. So, if more than one object is
moving toward the device, the laser velocity-measuring
device can measure the velocity of the specific object
of interest whereas a radar gun will receive multiple
reflected signals. The laser velocity-measuring
device
is more accurate for measuring the velocity of a runner.
Optical Imaging Systems
In a qualitative biomechanical analysis, we discern most
of the qualities of a performance using our vision. It
seems appropriate that the technology for recording these
visual images of performance is the most widely used
tool in biomechanics. The most popular optical imaging
systems in biomechanics are video cameras.
Video cameras provide sequential two-dimensional
images of movement at specific time intervals depending
on the speed of the camera. In a single recorded image,
the position of the body and its parts can be measured
relative to each other or to a fixed reference in the field of
view. If an object of known dimensions is also recorded in
the field of view and in the plane of motion, the position
data can be converted to real-life units. In subsequent
pictures, the changes in position or displacements can be
determined. The time elapsed between the exposure of
sequential frames of video can be determined from the
frame rate of the camera. For example, the ti me between
two adjacent frames of video recorded by a camera operated at 30 frames per second is 1/30 second or 0.033 s.
Velocities can thus be determined from the displacement and time measures. Once velocities are computed,
accelerations can be determined from the velocity and
time measures.
One camera may be enough to adequately record twodimensional or planar motion, since the resulting image
is also two-dimensional. Three-dimensional coordinate
data can be obtained if the motion is recorded by two or
more cameras. Specialized software has been developed
that computes the three-dimensional coordinates from
the two-dimensional data from each camera.
How are coordinate data extracted from the images
from the cameras? TItis process is called digitizing. This
is done manually or automatically, but in either case a
computerized
system facilitates the digitizing. First,
the points of interest on the body or the object being
investigated must be identified. If possible, markers are
placed on these points on the object or subject before the
movement is recorded. In the manual digitizing process,
a single frame of the image appears on the computer
monitor, and you digitize (store the coordinate data for)
each point of interest by positioning a cursor over the
projection of a point on the screen. This is done for each
point of interest (for a full human body model, this may
require more than 20 points) and for each frame offilm or
video of the movement. This process is extremely tedious
and time-consuming. It is also prone to human error. You
manually digitized points when you completed the motion
analysis exercises using the MaxTRAQ software in earlier
chapters, although in the exercises only one or two points
were digitized, and not in every frame.
The second method of digitizing is the automatic
method. Several methods of automatic digitizing exist.
Biomechanics of Sport and Exercise
\
388
Tn on method, highly reflective markers are attached
t the object or ubject (athlete, patient, client). These
markers define the points of interest. The subject is
illuminated so that light is reflected off the markers
and int the camera lens. The resulting video image has
bright p ts on the subject's image wherever a marker
was present (see figureI6.2). Specialized computer
oftware identifies these bright spots and their coordinate in every video frame of the movement. In some
automatic sy tem .thc data are proces ed in real timey u see the computer model of the movement n the
. creen as the mov ment i executed. A more advanced
ver. ion of the MaxTRAQ softwar includes automatic
digitizing routines that track high-contrast markers in
the Ii Id of view.
Another automatic digitizing method uses active
markers in e nua t to passive reflective markers. Tnthe e
. ysterns, the markers ar usually lighl-emitting diodes
CL Ds) that light up in a erwin sequence and at a certain
fr qu n y. p cializcd cameras dete t their pre. en e, and
computer s ftwarc d termines their coordinate location .
n drawback of all the optical imaging system is that
th y dcp nd on lin of sight Limb. move past each other
and hid mark rs fr m cameras. Electromagnetic tracking
Figure 16.2
systems overcome this problem by using electromagnetic
markers and specialized sensing devices for detecting
the locations of the markers. These electromagnetic systems do not suffer from hidden points since body parts
that may hide a marker visually do not hide the marker
electromagnetically, so the body parts are invisible to
the sensing device.
Data collection using automatic digitizing systems
is primarily limited to laboratory settings. Cameras,
which are usually fixed, record movement in only a limited volume or space. Another disadvantage is that the
movements of interest must be performed with markers
attached to the subject's body.
Most optical imaging motion measurement systems
are quite expensive. However, several inexpensive (less
thao $500) motion analysis programs are available that
offer basic tools for analyzing human movement in two
dimensions. The MaxTRAQ software that is bundled with
this book is an example of this software. You used the
MaxTRAQ motion analysis software in earlier chapters in
this book to analyze video clips that were provided with
the software. This program can also be used to analyze
any motion that you record on video if you can load the
video onto your computer in .avi format.
Reflective markers enable the computer and camera to automatically identify the marker locations. The
photo on :he le:t sh.ow~ the markers on the subject under normal lighting conditions; the photo on the right shows the
same subject with lighting and camera exposure set to highlight only the markers.
Technology
in Biomechanics
389
Accelerometers
Acceleration measurements can be derived from the data
obtained from laser and radar velocity-measuring devices
or from data obtained through optical imaging systems.
These acceleration measurements may be contaminated
with errors, however. The computations involved in
deriving velocity from position and time data, and then
acceleration from the velocity and time data, lead to error
propagation. A little noise (random error) in the position
data is amplified in the computation of velocity and is
amplified again in the computation of acceleration. The
data must be numerically filtered to eliminate this noise.
In ~ddition ~o ~his limitation, most velocity-measuring
devices are limited to relatively slow sampling rates (the
number of velocity measures per second is usually 60 or
less). Is there a method of measuring acceleration directly
so that these problems are eliminated?
An accelerometer is a device for measuring acceleration directly. These devices can be very light and small
in size (see figure 16.3). The smallest accelerometers are
smaller than the one shown, with dimensions as small as
2 mm x 2 mm x 1 mm. These tiny accelerometers are
usually components of microelectromechanical
systems
(MEMS). Accelerometers attached to the object measure
the acceleration of the object at the point of attachment.
Accelerometers
measure acceleration
in a specific
direction. A uniaxial or one-dimensional accelerometer
measures acceleration in only one direction-along
a
specific axis of the unit. A triaxial or three-dimensional
accelerometer measures three accelerations-along
three
different axes at right angles to each other. The orientation of the accelerometer determines the direction of the
acceleration measured. If the accelerometer is attached
to a limb that changes orientations, then the direction of
the measured acceleration changes when the limb orientation changes. Accelerometers cannot be attached directly
to the rigid framework of the body (bones) but instead
must be attached to the skin. Because of these difficulties, accelerometers are usually not used for analyzing
general whole-body movements.
Accelerometers
have relatively high frequency
response, so they can sample at high rates. This makes
them especially well suited for analyzing impacts. In fact,
accelerometers are used in automobiles as sensors to trigger air bag deployment. In biomechanics, accelerometers
are used to evaluate the impact-reducing
capabilities
of sport safety equipment. The performance of bicycle
helmets and other protective helmets is evaluated with
accelerometers via measurement of the acceleration of
a headform within the helmet during an impact test.
The cushioning performance of materials used beneath
children's playground equipment is evaluated in a similar
way. Accelerometers are also well suited for measuring
vibrations and their effects on the body.
Accelerometers are also the basis for physical activity monitoring devices. A uniaxial accelerometer is the
basis for self-contained pocket-sized devices that can be
attached to a barbell or to an athlete to measure a variety
of mechanical parameters, including the power output of
athletes. The technology used in simple pedometers that
count steps is a crude uniaxial mechanical accelerometer.
More sophisticated pedometers and activity monitors use
electronic uniaxial, biaxial, or triaxial accelerometers
to quantify the steps or physical activity of the wearer.
Inertial Measurement Units
Figure 16.3 An accelerometer and an example of its
output for a start-stop movement.
MEMS inertial sensors or !MUs (inertial measurement
units.) us~ a mi~ro-accelerometer
and a gyroscope to
provide kinematic measurements. These inertial sensor
devices are small ~nough to be attached to subjects. They
measure changes In position relative to an initial reference or starting position. They don't require a camera or
other recording device but do require specialized software for interpretation of their output. Motion analysis
systems that use IMUs are expensive relative to a basic
two-dimensional
video-based motion analysis system,
e~en though the !MUs are relatively inexpensive. This
difference in cost is shrinking, however, and IMU use
in biomechanics is becoming more and more prevalent.
Bíomechanics
of Sport and Exercise
390
You may actually have a MEMS accelerometer or
gyroscope similar to those used in IMUs. MEMS accelrometers and gyroscope are used in numerous personal
ele tronic devices-including most smart phone and tablets, laptops, and netbook computers-and
video games
uch as Nintendo Wii to detect position and changes in
po ilion of the device in which they are located.
Motion Capture Systems
M tion capture (mocap) systems are used to capture in
digital form the three-dimensional movements of the
whole body. omponents of a typical motion capture
system u ually include six or more video cameras, a
marker system, and specialized oftware and hardware
for reducing and organizing the data to produce the digital
rcpresentati n of the m vement. The body i modeled as
a system of rigid links connected at the joints. Marker
ts nsisting f two or more markers are attached to
each b ely s gment to identify the unique location and
dentation of each segment in three dimensions. Some
systems usc mocap suit with built-in markers or inertial
sens rs; other. y terns require the markers to be placed on
the ubjecl.l'ypical full-b dy marker sets con ist of more
than 50 marker. Re nt advances in image recognition
ftwar have led to the d velopment of markerless optical mo ap systems. These systems use advanced image
r ognition software to directly locate the anatomical
segments and j int centers in the video image.
Three-dim nsional m tion capture systems are expensiv .but the e sy t ms may be us d in sport and exerci e.
Th y ar' more likely to be u edin clinical gait analysis
lab and research settings. The most widespread use of
motion capture systems is in the entertainment industry.
The Xbox Kin t game system is a crude but very inexp nsive motion apture : ystcm. It uses multiple optical
sen' rs t detect, capture, and interpret the movements
of a player or play l'S. The players can control the game
with their movement and gestures. No external game
centroll r is n cd d.
More cornpl x and expensive motion capture ysterns
are us dt captur the motion of athlete and actors. The
captured athl tic rnovem nt f prole sional athletes have
be n used to pr vide realistic movement for the animated
play l'sin many popular p rt-related video games and
apps. The m vcrnents and g stures of actor have been
aptured using íhi: techn logy and used to model movem nts of digital character in feature films, television
show, and comm rcials, and music vide s since the late
19905. The Na 'vi hara tcrs in the 2009 fi Im Avatar were
created using motion capture technology. At the 77th
Annual AClId my Awards in 2005, Academy Awards for
Technical A hievement went to Julian MOITis, Michael
Byrch, Paul Smyth, and Paul Tate for their development of
the Vi on motion captur systems; to John O.B. Greaves,
Ned Phipps, Ton J. van den Bogert, and William Hays for
their development of the Motion Analysis motion capture
systems; and to Nels Madsen, Vaughn Cato, Matthew
Madden, and Bill Lorton for their development of the
Giant Studios motion capture system. The Vicon and
Motion Analysis systems were originally developed in
the 1980s for use in biomechanical applications.
Tools for Measuring Kinetics
Kinetic variables are based on force-the cause of change
in motion. Popular tools for measuring kinetic variables
in biomechanics include force platforms, strain gauges,
pressure-sensing devices, and electromyography (EM G).
Force Platforms
Force platforms are the most popular devices for measuring kinetic variables in biomechanics. Force platforms
or force plates measure reaction forces and the point of
application and direction of the resultant reaction force.
Their measuring surface is rectangular and is typically
about the size of a small doormat (about 40 cm by 60
cm). Force platforms are typically used to measure
ground reaction forces in gait (see figure 16.4). The
forces measured include the normal contact force (the
vertical ground reaction force), the friction force in the
anterior-posterior direction, and the friction force in the
medial-lateral direction.
Force platforms are used in clinical gait laboratories to
assess the effectiveness of treatments for neuromuscular
diseases or to assess the progress of rehabilitation from
musculoskeletal.injuries: how the ground reaction forces
have changed following treatment, or what changes have
occurred during rehabilitation. Examining force platform
records is also a way to evaluate the fit and function of
prostheses.
Force platforms are used in athletics to measure the
ground reaction forces exerted by shot-putters and discus
~hrowers during their throws; by long jumpers, triple
Jun:per~, and pole-vaulters during their takeoffs; by
we~gh~llfters during their lifts; by platform divers during
their dives; and so on. The patterns revealed in the forcetime histories give coaches and scientists information
about technique differences that may affect performance.
Some of the larger athletic shoe manufacturers use
force platforms in their biomechanics laboratories. Analysts ~valuate t~e features of various shoe designs and the
mate~lals used in these designs by examining the ground
reaction forces produced by subjects wearing the shoes.
Force Transducers
Force transducers are devices for measuring force. The
force plates just described rely on multiple force transduc-
Technology
in Biomechanics
391
on bones and joints and even tendons and ligaments.
Strain gauges have been attached to implanted artificial
hips to measure the forces at the hip joint in vivo. In
animals, force transducers (buckles with strain gauges
attached) have been used to measure in vivo tendon
forces.
Pressure Sensors
Figure 16.4 Force platforms are typically used to measure ground reaction forces. These figures show a subject
walking over a force platform and the ground reaction
forces produced during a walking step.
ers within them to determine their output. Another type of
force transducer is a strain gauge. Strain gauges measure
strain-change in length divided by original length. If a
strain gauge is attached to a material with a known shape
and elastic modulus, the stress in the material can be
computed, and ultimately the external load that caused
this stress and strain can be determined. Strain gauges
are thus useful for measuring forces.
Force transducers have been used in a variety of
sports to measure the forces applied to implements or
equipment. Strain gauge force transducers have been
used to measure forces on the rings and horizontal bar in
gymnastics, on the wire in hammer throwing, on the oar
in rowing, and on the start handle in the luge.
Clinical uses of strain gauges to measure forces have
been important in improving the understanding of loads
Pressure sensors are usually thin mats with arrays afforce
sensors imbedded in them. A force platform measures a
resultant reaction force, which is really the resultant of
a number of forces acting on the surface in contact with
the force platform. Pressure sensors better represent
the distributed nature of these forces by quantifying the
pressure (force divided by area) exerted on each specified
area of the pressure mat.
As with force platforms, pressure mats are most widely
used in gait analysis. The regions of high pressure beneath
a patient's foot may be identified when a patient walks
barefoot across a pressure mat. The changes in these pressure patterns following treatment for diseases or surgical
intervention and rehabilitation may be monitored.
Pressure mats may not measure the pressures exerted
on the foot when a patient wears shoes, so insole pressuremeasuring devices have been developed. These devices
fit in the shoes between the sole of the foot and the shoe
(see figure 16.5). Pressure measurements from the e
insole pressure-measuring devices are used by podiatrists
and other medical professionals to design more effective
orthotics and shoes. In sport, pressure-measuring insoles
have been used in skiing to measure the pressures exerted
by a skier's foot on the boot during alpine skiing.
Another clinical application of pressure-measuring
devices is to measure bone-to-bone pressures in joints.
Very thin pressure mats (or pressure-sensitive film) have
been used to measure the pressure distributions within
joints in cadavers.
Electromyography
Muscle forces produced during movement may be
indirectly measured using electromyography (EMG).
EMG measures the electrical activity of a contracting
muscle via surface electrodes placed on the skin over a
superficial muscle or vía indwelling electrodes that are
implanted within the muscle. At the very least, EMG
data indicate whether a muscle is contracting or not.
With skillful processing of the EMG signal, the relative
strength of the contraction may be determined. Although
there is clearly a relationship between the magnitude of
the muscle force and the EMG signal, quantifying this
relationship and determining the muscle force from the
EMG data are not yet possible. EMG is still a useful tool
for clinical and sport applications.
Biom chonlcs of Sport and Exercise
392
surgical repairs or prosthetic devices may be evaluated
before the intervention.
mputer simulation models are usually mathematically based, using equations derived from Newton's laws
of motion. Input to the simulation u ually includes inertial properties of the body and its limbs (mass, lengths,
moments of inertia), initial condition at the start of the
simulation (positions and velocities of the body and
limbs), and time histories of the control functions. The
control functions may be relative position of the limbs,
mu ele forces, resultant joint torques, and so on. The
outcome of a simulation is the movement of the body
that results from these inputs.
Simulations of a variety of sport skills have been
developed. The limitation of computer simulation is that
each simulation is specific to one individual and one set
of input parameters. Results of computer simulations
may be applicable only to the person whose parameters
were input into the imulation. Despite this limitation,
computer simulations are a promising way to investigate
"what if" que tians. What if the diver adducted her arm
farther? What if the gymnast kept his tuck position a
little longer? What if the pole-vaulter u ed a slightly
stiffer pole?
Summary
Figure 16.5 Pressur -measuring insoles are useful tools
for me suring pr ssur on th plantar surface of the feet
durins 9 it.
Computer Simulation
and Modeling
t Ius 'cl by biomechanjgj, is computer simulati n and mod lin . This is not really a measur ment to I
but ruth r an analyi is to I. In sport, computer simulati n
muy h li' cl to predi I the utcorne of a rnovern ru based
11 e rtain inputs. In clinical siluations, the effects of
A mal
KEVTERMS
ne lerom
anale
T
t
r (p. 89)
86)
ignals (p.
ucctr myography ( •M ) p,
fore platf I'm (p, 390)
sampling rat p. 386)
strain gouge p. 3 I)
I)
Comprehensive quantitative biomechanical analyses
are usually limited to performances by elite athletes
I' to clinical. situations. How and where biomechanical mea uremcnts are taken may affect the parameter
being measured. A number of tools are used to measure
biomechanical variables. Tools for measuring kinematic
variables in biomechanics include timing ysterns,
velocity-measuring system , optical imaging system,
à ceterometers,
and MEMS inertial sensors. Motion
captur systems are used for capturing three-dimensional
movements of the whole body. Tools for measuring
kinetic variables in biomechanics include force platforms,
strain gauges, pre sure-sensing devices, and electromyography (EM G). Computer simulation is another useful
tool for biomechanical analysis.
REVIEW QUESTIONS
1. What are the advantages of collecting biomechanical
2. What are the disadvantages
of collecting biomechanical
3. What are the advantages of collecting biomechanical
4. What are the disadvantages
data in a laboratory setting?
data in a laboratory setting?
data in the field?
of collecting biomechanical
data in the field?
5. What is an analog signal?
6. What is a digital signal?
7. What is a sampling rate?
8. How can information from a video-recorded
9. What does an accelerometer
performance
be used to compute velocity?
measure?
10. What is the difference between a uniaxial and a triaxial accelerometer?
11. What does a force platform measure?
12. How does a strain gauge measure force?
13. Which tools are best suited for measuring the mechanics of impact?
14. What tool is used to measure the electrical activity of muscles?
15. What two inertial measurement
devices are part of an !MU?
Motion Anal sis Exercises Usin
MaxTRAQ
If you haven't done so already, review the instructions for downloading and using the educational version of the MaxTRAQ motion analysis software at the beginning of this book, then download and install
the software. Once this is done, you are ready to try the following two-dimensional kinematic analysis
using MaxTRAQ.
1. Complete a quantitative biomechanical
analysis of a planar sport skill or human movement.
Before you record the movement, determine what aspects of the movement you want to analyze
and what kinematic parameters you want to measure. Use a stationary video camera to record
the movement. Make sure that the camera is set up so that its optical axis is perpendicular to the
plane of motion and its field of view is large enough to capture the motion, but not so large that
measurement accuracy is compromised. Place an object of known length in the plane of motion
and use it as your reference length for scaling the video. Video record the skill. Download the
video to a computer. If the video is not in .avi format, convertit to .avi format. (You can download
free video conversion programs on the Internet.) Once the video clip is converted to .avi format,
use MaxTRAQ to measure the kinematic parameters you previously identified.
393