3D Kinematics Flashcards
Advantages of 3D vs 2D
- Every movement is 3D in nature; No out of plan movements in 3D
- No perspective error
What is a passive system?
- High speed camers emit infrared light that goes to a spherical marker and marker sends it directly back to camera
- Multiple 2D coordinates obtained simultaneously from 2 or more cameras to create 3D coordinates (Large collection volumes)
- Computer software Direct Linear Transformation (DLT) computes everything
Retroreflective markers send light ____ back to the camera it came from.
directly
If a marker is not showing up as spherical on the motion capture, what could be wrong?
- It is not properly emitting light back. Shows up as a gray blur
____ marker coordinates from each camera solved for 3D coordinates using DLT (after calibration)
2D
We then assign identifiers of each marker on the person’s body.
Once we have 3D coordinates, what do we do now?
- Assing markers to segments of the body (rigid bodies)
- Calculate orientation of one segment relative to another (joint angles)
3D Rigid Body Models:
What are two types of Marker Sets
- Helen Hayes Marker Set (Minimal # markers, near joint axis, 3 per segment)
- Cleveland Clinic Marker Set (Cluster-based, 3+ per segment, near middle of segment)
What big assumption does Helen Hayes make?
- That the marker is actually the joints center. Susceptile to poor placement of marker
- Soft tissue moves when the body moves.
Disadvantages of 3D Passive System Kinematics
- Cost ($) – Due to lots of cameras; large upfront cost
- Cameras sometimes not synchronized – tough to synch (may lead to motion blur)
- Camera bumped or changed focus – loses calibration/lose focus
– Markers not seen by enough cameras (line of sight) - Need two cameras or more to see a marker to get 3D position in space; biggest problem
- Anatomical/marker placement errors – lots of error doesn’t properly track underlying skeleton
- Accuracy/Inaccuracy of different planes? –
– long axis rotation is difficult for systems to pick up because it’s a small movement. Sensitivity to change is very very low with this movement. Ex: pronation - Time to set up and analyze data – labor, trained personnel
– Labor intensive process to track markers.
What are some 3D Active Systems?
Active System: Provides it’s own light
- Optoelectric instruments
- Electromagnetic Instruments
- Inertial Measurement Units
Optoelectric instruments
- Series of unique infrared light emitting sensors
- LEDs light up one at a time
- 2D coordinates create 3D with DLT
- 3+ markers per segment used to calculate joint motion
Optoelectric Advantages
-
Automatic marker tracking/labeling
– No guess work; low labor - 3D coordinates displayed in real time
- Very small markers
- High sampling rate on newer systems (up to 4000 Hz); Can track really fast movements
Optoelectric Disadvantages
- Interference from wires attached to subject
– Need to make sure it doesn’t constrain movement - Decreased accuracy near edge of collection area
– If you get further away from camera the LED gets dimmer. Too far away leads to no light. Big loss of accuracy with more distance.
Electromagnetic Instrument
- Series of electromagnetic sensors attached to subject
- Each sensor measures the magnetic field produced by a fixed transmitter and the Earth’s magnetic field
- 3D orientation & 3D position of each segment determined from 1 sensor/segment = 6 DOF information by each receiver (Single sensor replaces need for 3 markers)
Electromagnetic Tracking allows you to measure ____ areas of the body without needing to marker the entire person!
specific
Electromagnetic Advantages
- 3D joint angles truly displayed in “real time”
- Very little labor (e.g., tracking markers) involved
– Immediate information (no marker tracker needed) - Movement-based feedback
– Can use motion analysis as a training tool for movement education of workers, patients, or clinicians. - No marker occlusion (no line of sight issues!)
Electromagnetic Disadvantages
- Sensors are susceptible to magnetic and electrical interference from metal objects in the environment
– Rebar in concrete floors, metal force platforms
– Wiring
– Electrical sources (AC, monitors, lights, cables, and computers) - Highly decreased accuracy near the edges of the capture volume
- Wiring from the sensors limit examination of many functional activities
- Capture volumes are smaller than for video systems
– < 8 feet diameter - Slower sampling rates (< 250 Hz)
What do inertial measurement units do (IMUs)?
Get Angular Measures: Acceration, Velocity, Position/Orientation
IMU Advantages
- Lower cost
- Small size
- Do not restrict movement
- Wirelessly transmit data
- No line of sight issues
IMU Disadvantages
- Numerical integration errors = DRIFT!
- Susceptible to metal in environment
- Limited sampling frequency (200 or less)
- Short battery life (1 hr)
- Only angular data, not 3D linear position
Triaxial Accelerometer
- Linear accelerometry
- Measure of force experiences at the sensors location if mass is knonw
- Whole body measurement force (Placement: Sacrum or Tibia for Running gives GRF)
Estimation of muscle forces based on motion and force platform data allows us to…
Estimate joint loading
How do you determine what instrument to use?
Depends on:
* What are you looking for? Which have clinical meaning?
* “Normal” movement pattern vs. pathological?
* What discrete variable is most valid with respect to the clinical question (peak, average, at a specific time event)?
* When during the movement (stance, swing?)?
Look in the research!
What is a Discrete Variable
- Need to identify a single value of these time-series data that will be used to represent each participant’s performance.
- Ex: Max, Min, Range, Avg, Slope, Area, etc.
How do I determine the discrete variable I should use?
What is meaningful will depend on the movement, the patient, and the question
