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Motion Capture Essentials

Motion Capture Essentials. D. Gordon E. Robertson, PhD. Outline. Select appropriate marker set and type of markers Determine number of cameras and their locations Select appropriate location, number and types of force measuring devices (force platforms, force transducers)

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Motion Capture Essentials

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  1. Motion Capture Essentials D. Gordon E. Robertson, PhD

  2. Outline • Select appropriate marker set and type of markers • Determine number of cameras and their locations • Select appropriate location, number and types of force measuring devices (force platforms, force transducers) • Other demands: EMGs, netting, speed guns, standardized footwear and equipment? Biomechanics Laboratory, School of Human Kinetics

  3. 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

  4. 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 Biomechanics Laboratory, School of Human Kinetics

  5. 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 Biomechanics Laboratory, School of Human Kinetics

  6. 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 Biomechanics Laboratory, School of Human Kinetics

  7. 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

  8. 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

  9. 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 overhead 7 cameras 6 cameras 12 cameras Biomechanics Laboratory, School of Human Kinetics

  10. 3. Force Platforms plates in 4 stairs and 1 in ground • 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 two side-by-side and one offset two side-by-side on moveable platform mounts for up to 6 large plates Biomechanics Laboratory, School of Human Kinetics

  11. 4. Other Equipment • VO2 measurements • EMGs of leg muscles • torque resistance measured by ergometer Biomechanics Laboratory, School of Human Kinetics

  12. 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

  13. 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

  14. ViconWorkstation Trajectory Digitization

  15. Steps • Calibration • Reconstruction • Labeling • Autolabeling • Defragmentation • Gap filling Biomechanics Laboratory, School of Human Kinetics

  16. 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

  17. 2. Reconstruction • Multiple 2D views to one 3D Biomechanics Laboratory, School of Human Kinetics

  18. 3. Labeling • select the static trial • label each trajectory • save for autolabelling Biomechanics Laboratory, School of Human Kinetics

  19. 4. Autolabelling • load dynamic trial(s) • apply autolabelling Biomechanics Laboratory, School of Human Kinetics

  20. 5. Defragmentation • join line fragments with same name Biomechanics Laboratory, School of Human Kinetics

  21. 6. Gap Filling • fill gaps with interpolating spline Biomechanics Laboratory, School of Human Kinetics

  22. DoneReady for Kinematic/Kinetic Analyses Biomechanics Laboratory, School of Human Kinetics

  23. Visual3D Inverse Dynamics and Joint Power Analysis

  24. 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

  25. 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

  26. create segments create landmarks enter body mass and height Model Building Screen Biomechanics Laboratory, School of Human Kinetics

  27. 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

  28. static trials movement trials tags to identify experimental conditions Workspace Screen Biomechanics Laboratory, School of Human Kinetics

  29. 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

  30. event button signal tree body at current event signal histories with events marked Signal and Event Processing Screen Biomechanics Laboratory, School of Human Kinetics

  31. 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

  32. • commands • pipeline • options • import signals • opn/save scripts Pipeline Form Biomechanics Laboratory, School of Human Kinetics

  33. 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

  34. contents of pages report pages graphical or tabular reports select page viewed here Reporting Screen Biomechanics Laboratory, School of Human Kinetics

  35. 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

  36. BioProc3: External/Internal/Total Work • external work • internal work • total work Biomechanics Laboratory, School of Human Kinetics

  37. 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

  38. Questions? Comments? www.humankinetics www.health.uottawa.ca/biomech/watbiom

  39. Finis Muchas Gracias

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