motion capture essentials

Motion Capture Essentials

D. Gordon E. Robertson, PhD

Outline

1. Select appropriate marker set and type of markers2. Determine number of cameras and their locations3. 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


Static Marker Sets• Helen Hayes marker set was a early simplified system but is not

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


subject in static posture with marker clusters

Plug-in-gaitMarker 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

uOttawaMarker 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


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


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


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, 2 overhead7 cameras6 cameras12 cameras

• Usually necessary for later kinetic analyses• One plate or multiple plates. How many?• 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, alternating, movable

3. Force Platforms


two side-by-side and one offset

two side-by-side on moveable platform

mounts for up to 6 large plates

plates in 4 stairs and 1 in ground

4. Other Equipment

• VO2 measurements

• EMGs of leg muscles

• torque resistance measured by ergometer


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


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


ViconWorkstation

TrajectoryDigitization

Steps

1. Calibration2. Reconstruction3. Labeling4. Autolabeling5. Defragmentation6. Gap filling


1. Calibration• Static

– origin and axes are defined

• Dynamic– calibration wand is used to calibrate volume

• Accuracy– usually < 1 mm


2. Reconstruction

• Multiple 2D views to one 3D


3. Labeling

• select the static trial• label each

trajectory• save for

autolabelling


4. Autolabelling

• load dynamic trial(s)• apply autolabelling


5. Defragmentation• join line fragments with same name


6. Gap Filling• fill gaps with interpolating spline


DoneReady for Kinematic/Kinetic Analyses


Visual3D

Inverse Dynamics andJoint 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


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


Model Building Screen• create segments

• create landmarks

• enter body mass and height


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


Workspace Screen• static trials

• movement trials

• tags to identify experimental conditions


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


Signal and Event Processing Screen• event button

• signal tree

• body at current event

• signal histories with events marked


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


Pipeline Form• commands • pipeline • options • import signals • 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


Reporting Screen• contents of pages

• report pages

• graphical or tabular reports

• select page viewed here


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


BioProc3: External/Internal/Total Work• external work

• internal work

• total work


BioProc3: Work Done in Bursts• work done

• which moment of force

• peak power

• peak moment

• mean power

• total work done

Questions? Comments?

www.humankineticswww.health.uottawa.ca/biomech/watbiom

Finis

Muchas Gracias

motion capture essentials

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