Motion Capture
Essentials
D. Gordon E. Robertson, PhD
Outline
1. Select appropriate marker set and type of markers
2. Determine number of cameras and their locations
3. Select appropriate location, number and types of force
measuring devices (force platforms, force transducers)
4. Other demands: EMGs, netting, speed guns, standardized
footwear and equipment?
Biomechanics Laboratory, School of Human Kinetics
1. Marker Sets
• Motion capture requires selection of a suitable marker set
• Types:
– individual markers
– clusters of markers (includes wands) on rigid structures
– virtual markers (uses wands to identify body locations)
• Static marker set
– this set is necessary to establish joint centres and their 3D
relationships with the dynamic marker set
– virtual marks can be included
• Dynamic marker set
– this may be a reduced set that with a combination of
individual markers and/or clusters
Biomechanics Laboratory, School of Human Kinetics
Static Marker Sets
subject in static
posture with
• Helen Hayes marker setmarker
was a early
simplified system but is not
clusters
appropriate for full body 3D analyses
• Vicon’s Plug-in-gait marker set is a more robust full body marker
set but requires several anthropometric measurements (e.g.,
mass, height, leg length, knee width, ankle width, shoulder
offset, elbow width, wrist width, & hand thickness)
• uOttawa marker set is a modified version of Plug-in-gait. It uses
hip, iliac crest, medial limb, and ear markers, but no ASIS or
PSIS markers. It also requires fewer anthropometric
measurements (e.g., mass, height, hip offset, & shoulder offset)
• clusters may also be used for some or all segments instead of
non-joint markers
Biomechanics Laboratory, School of Human Kinetics
Plug-in-gait
Marker Set
•No hip markers
•No medial markers at
knees, ankles, elbows,
and wrists
•Head markers not over
ears (location of cofg)
•Pelvis markers (ASIS &
PSIS) are horizontally
located
•Only one markers on
foot
•Potentially arm & leg
markers are linear
avoid placing markers
linearly on a segment
Biomechanics Laboratory, School of Human Kinetics
uOttawa
Marker Set
•Hip markers
•Medial markers at
knees, ankles, elbows,
and wrists
•Head markers over ears
•No ASIS or PSIS
markers
•Two markers for each
foot, hallux optional
•Potentially arm & leg
markers are linear
Biomechanics Laboratory, School of Human Kinetics
Clusters
•Usually 3 or 4 markers
•4th marker can be offset
•cluster must be well
fixated to the skin
•can be difficult to
automatically digitize
•can impede motion
Biomechanics Laboratory, School of Human Kinetics
Dynamic Marker Sets
• this marker set that is similar to the static but with extraneous
markers removed
– medial markers
– when clusters are used individual markers can be removed
Biomechanics Laboratory, School of Human Kinetics
2. Camera Setup
• Number of cameras depends on complexity of task and
complexity of the model
• 2 to 4 cameras may be sufficient for analyzing planar type
motions of one side of the body
• 6 or more cameras for full-body models of small volumes
• 10 or more for large volumes such as walkways, cricket pitches
10
cameras,
7 cameras
6 cameras
212
overhead
cameras
Biomechanics Laboratory, School of Human Kinetics
3. Force Platforms
plates in 4 stairs
•
•
•
•
Usually
for later
twonecessary
side-by-side
two side-by-side
on kinetic analyses
and 1 in ground
One plate
or
multiple
plates.
How
many?
moveable
and
platform
one offset
Imbedded into floor or into stairway
Size:
– small (40x60): good for starts or static jumps, easy to miss
for running studies
– large (60x90): good for running studies but can result in
multiple feet on same plate (prevents inverse dynamics)
• Location:
– side-by-side, separated,
mountsalternating,
for up to 6movable
large plates
Biomechanics Laboratory, School of Human Kinetics
4. Other Equipment
• VO2 measurements
• EMGs of leg muscles
• torque resistance
measured by
ergometer
Biomechanics Laboratory, School of Human Kinetics
Example: Single Markers, Side Kick
• Single markers one
either side of each joint
and one extra mid
segment
• Three force platforms
• All joints can be
analyzed since there
are no closed loops
Biomechanics Laboratory, School of Human Kinetics
Example: Clusters, Olympic Lifting
• Clusters identify limb
segments
• Two force platforms
• Only ankle, knee, hip,
and pelvis kinetics can
be analyzed
• Arms kinetics are not
possible because of the
bar connection
Biomechanics Laboratory, School of Human Kinetics
Vicon
Workstation
Trajectory
Digitization
Steps
1.
2.
3.
4.
5.
6.
Calibration
Reconstruction
Labeling
Autolabeling
Defragmentation
Gap filling
Biomechanics Laboratory, School of Human Kinetics
1. Calibration
• Static
– origin and axes are defined
• Dynamic
– calibration wand is used to calibrate volume
• Accuracy
– usually < 1 mm
Biomechanics Laboratory, School of Human Kinetics
2. Reconstruction
• Multiple 2D views to one 3D
Biomechanics Laboratory, School of Human Kinetics
3. Labeling
• select the static trial
• label each
trajectory
• save for
autolabelling
Biomechanics Laboratory, School of Human Kinetics
4. Autolabelling
• load dynamic trial(s)
• apply autolabelling
Biomechanics Laboratory, School of Human Kinetics
5. Defragmentation
• join line fragments with same name
Biomechanics Laboratory, School of Human Kinetics
6. Gap Filling
• fill gaps with interpolating spline
Biomechanics Laboratory, School of Human Kinetics
Done
Ready for Kinematic/Kinetic Analyses
Biomechanics Laboratory, School of Human Kinetics
Visual3D
Inverse Dynamics and
Joint Power Analysis
Marker and Force Data
• marker data come in a .C3D file created by Vicon, Motion
Analysis, Qualysis or similar motion capture system
• SIMI and APAS may also be used but data must be placed in a
ASCII (.FSV) file by, for example, BioProc2/3
• force data come with the .C3D file but may also be collected and
placed in an ASCII (.FSV) file by BioProc2/3
• data may be streamed directly to Visual3D using newest version
of Visual3D and compatible motion capture system, e.g., Nexus
Biomechanics Laboratory, School of Human Kinetics
Creating the Model
• first a static trial is loaded to create the model (you can use a
movement trial)
• a previous model may be used for a similar marker set
• model is customized for each participant (mass and height etc.)
• ideally markers are located at each joint but these markers do
not have to be included in the movement (dynamic) trials
• should save the model file (.MDH) separately for use with other
trials or experimental conditions of the same person
• new models must be created for each new marker set
Biomechanics Laboratory, School of Human Kinetics
Model Building Screen
•
create segments
•
create landmarks
•
enter body mass
and height
Biomechanics Laboratory, School of Human Kinetics
Movement Trials
• one or more movement trials may be loaded into same
workspace (.CMO), essential if trials are to be averaged
• model is applied to each trial
• trial may be viewed with bones or geometrical solids for each
segment
• ground reaction forces should be checked
• errors in force platform locations and parameters may be
checked and changed in necessary
Biomechanics Laboratory, School of Human Kinetics
Workspace Screen
•
static trials
•
movement trials
•
tags to identify
experimental
conditions
Biomechanics Laboratory, School of Human Kinetics
Event Labeling
• if events were not created by motion capture software they can
be added automatically or manually with Visual3D
• special events used for reports can also be added (e.g., BEGIN,
END, HIT)
• events should be checked for accuracy using GUI
Biomechanics Laboratory, School of Human Kinetics
Signal and Event Processing Screen
•
event button
•
signal tree
•
body at current
event
•
signal histories
with events
marked
Biomechanics Laboratory, School of Human Kinetics
Data Processing Pipeline
• a pipeline may be applied to process the data for inverse
dynamics, energy, or power analyses
• a script may be used to perform these operations repeatedly
• a typical script (.V3S) includes interpolation and data filtering
• special forces may be added such as moving force platforms,
transducer forces, or pedal forces
Biomechanics Laboratory, School of Human Kinetics
Pipeline Form
• commands
• pipeline
• options
• import signals
Biomechanics Laboratory, School of Human Kinetics
• opn/save scripts
Graphical Reports
• reports, including graphs, of the various kinematics and kinetics
are created next
• special tags may be used to display only certain types of trials,
e.g., right vs. left leg starts, barefoot vs. shod gait, loaded vs.
unloaded lifts
• these tags are created in the workspace area
• basic reports (.RGT) may be loaded or created and saved for
later repeated use
• graphs may be exported for presentation
Biomechanics Laboratory, School of Human Kinetics
Reporting Screen
•
contents of pages
•
report pages
•
graphical or
tabular reports
•
select page
viewed here
Biomechanics Laboratory, School of Human Kinetics
Export Data to MatLab or BioProc3
• kinematic or kinetic data may be exported in .C3D or ASCII files
• BioProc3 has capability of computing total, internal and external
work, angular impulses and work done by individual bursts of
power
• script is used to create ASCII file for BioProc3
• MatLab can compute foot clearance
Biomechanics Laboratory, School of Human Kinetics
BioProc3: External/Internal/Total Work
• external work
• internal work
• total work
Biomechanics Laboratory, School of Human Kinetics
BioProc3: Work Done in Bursts
• work done
• which moment
of force
• peak power
• peak moment
• mean power
• total work done
Biomechanics Laboratory, School of Human Kinetics
Questions?
Comments?
www.health.uottawa.ca/biomech/watbiom
www.humankinetics
Finis
Muchas Gracias