Motion Capture Flashcards
What is motion capture?
Process of recording anatomical landmarks and track its trajectories to perform kinematic analysis
What are the different types of systems in motion capture
2D, 2D/3D system Manual Digitizing, 2D/3D System Automatic Digitzing, Inertial Measurement Units
Other miscelleanous types of systems in motion capture
Radar gun, time gaits, acceleroemter, speedometer/linear encoder, electrogoniometer
What are the parameters measured in motion capture
Position, time, displacement, speed, acceleration
What are the pros of using motion capture?
Accurate and reliable
Less time consuming data acquisition
Enables immediate feedback through qualitative analysis
Minimal interference to performer
Can use in different environments (water, indoors, outdoors)
What are the cons of using motion capture?
Expensive Time consuming (analysis and handling) Requires dedicated technician with learning curve Software bugs and breakdowns Large dataset to handle
Procedures for Motion Capture? What are the differentiating factors.
2D or 3D kinematics
2D, one camera and coplanar. Calibration frame or pole with minimum 2 points.
3D, two or more cameras and non-coplanar. Calibration volume, L shaped + T shaped calibration kit. Minimum 6 non-coplanar points (recommended 15-20).
What are the suggested sampling rates for different activities? What is the theorem that it’s based on?
Nyquist-Shannon sampling theorem
25-50hz: walking, swimming, stair climbing
50-100: running, shot put, high jump
100-200: sprinting, javelin throw, football kick
250-500: tennis serve, golf swing, fencing parry
Usage of markers in motion capture, how are they used? What types?
Placed according to anthropometrical model CoM segmentation method.
No marker, passive marker (reflective), active marker (emitting light)
How are markers captured or tracked?
Manual (frame by frame)
Semi automatic (kinovea)
Automatic
What are the concerns when doing manual or semi-automatic marker tracking?
Use same operator to digitize all trials, ensure consistency
Consider anatomical landmark being sought
Calibration frame / control points should be carefully digitized
Check that 3D reconstruction errors fall within acceptable limits after completion of calibration
Representative sequence should be digitised several times to establish intra-operator reliability.
How are 3D motion captures calibrated?
Coordinate transformation using Direct Linear Transformation (Abdel-Aziz & Karara 1971), a 3D reconstruction algorithm.
Describe the considerations when transforming coordinates in a 3D construction.
Calibration frame > large enough to include space of motion (too small, danger of extrapolation and hence inaccurate coordinate computation)
Don’t alter camera settings after calibration is done
Include as many control points as possible
Use as many cameras as possible (reduce redundancy for reconstruction)
Set control object (calibration frame) properly to align axes well in relation to direction of motion.
How is data from Motion Capture processed/analyzed?
Apply low pass filter
Determine cut off value - through residual analysis
Optimal cutoff frequency > one where residual starts to change very little (this point considered as filtering mostly noise and minimally signal)
Export the raw data (coordinates+time)
Filter the signal after residual analysis (on acknowledge: transform digital filters IIR
Low pass)
Compute the resultant speed (vR2=vx2+vy2)
Plot the speed-time curves (data displayed vs. data filtered) on a graph:
What are the differences identified between 2D Video Analysis (manual and tracked) and 3D Optoelectronic systems?
Knee angle measured had few diff between systems
3D model produced larger angles at midstance
Possibly due to how 3D model locates hip joint and addition of marker clusters
2D videography similar results to 3D model when manual digitizing as it allowed for skin movement errors to be corrected
What are the applications of Motion Capture in Sprinting?
Sprinting maximum velocity - 3 types resisted training devices
Minor changes in athletes running technique
Although substantial overload > stride length and running velocity reduced
What are the applications of Motion Capture in Swimming
2D: Swimming with constraints (AquaTrainer Snorkel) changes normal biomechanical pattern in breaststroke and front crawl
Kinematic changes in stroke technique due to added drag
Mainly in gliding phase after start and turns
What are the applications of Motion Capture in Basketball
3D: Relationship between distance and shooting kinematics with respect to position in basketball
Release speed increase with shooting distance for guards/forwards (shoulder flexion, elbow extension, increased CoM speed in direction of basket at release)
What are the applications of Motion Capture in Overarm throw
Found that palmar flexion (wrist) contributed 59% to release speed. Forearm extension (elbow) 8% Arm flexion (shoulder) 14% Thigh extension (hip) 6% Shank extension (knee) -2% Plantar flexion (ankle) 15%
What are the applications of Motion Capture in Soccer ACL
3D: age and gender effects on lower extremity motion patterns of soccer players in stop-jump task
Landing with small knee flexion angles > increase load on ACL
Females after 12yo had smaller knee and hip flexion angles during landing
