Kinematic descriptions
- Position
- Velocity
- Accleration
- (ROM changes, timing changes, distances, etc. over time)
Why quantify motion? - Clinical Application
- Facilitate appropriate / prevent inappropriate surgeries
- Provide patients with efficient mobilization
– Maximum energy conservation to minimize fatigue - Evaluate interventions
Why Quantify Motion? - Research
- Identify movement patterns that contribute to injuries
– Chronic: osteoarthritis, stress fractures, tendinopathies
– Acute: falls, ligament injuries (ACL, MCL, UCL) - Better understand the characteristics of a movement disorder
– Velocity? Initiation? Acceleration? Orientation in space? - Provide a detailed description of a movement pattern (Ex: Walking vs Elliptical)
Why observational motion analysis with PT?
Often used to guide clinical decision-making and choose most appropriate instrumentation
Pros and Cons of Observational Motion Analysis
Pro
* Simple, quick, no formal training, no instruments
* Rating systems may facilitate agreement among raters
Cons
* Difficult to observe motion occurring simultaneously at several joints
* Documentation (Hard to upload)
* Record/playback in clinic (HIPPA and confidence)
* Reliability (Subjective interpretation and POOR inter-rater agreement
* Little confidence in transverse plane motion (due to must be directly overhead)
What is soft tissue artifact?
- Wobbles/jiggle we see when the skin changes places with movement
- Doesn’t allow use to see what is truly happening to the true skeleton
- This is always the biggest limitation with motion capture analysis!
2D Video or Still Camera Considerations
- Camera location vs. movement plane
- 2D viewpoint and perspective error
- Camera distance vs. movement plane
- Camera shutter
- Image size
- Markers/contrast
- Image size
- Marker Visibility
- “Digitzing error”
- Scaling rod
Why is camera location important?
- Need to perfectly align plane of motion of the camera to the subject
- Can only measure things specific to that plane you record
What is perspective error?
- Camera perspective changes the results due to improper alignment
- Results in greater degrees of error the more motion that occurs at the limb. Ex: Knee Flexion; More flexion that is actually truly happening
How can we minimize perspective error?
- Place the camera further away and zoom in to get a closer view you would normally get when closer
- Results in less offset angle of the camera
Quiz question
What is one limitation we need to consider with a single plane and human motion?
- Humans don’t naturally move in one plane therefore there will always be out of plan movements affecting 2D angle calculations.
- Camera correctly positioned, subject moving out of plane (swing phase)
- Ex: Walking and IR
2D motion is not a surrogate for ____
3D
Shutter Speed
- how long the shutter is open and exposes the sensor
- The longer it is open the blurrier it gets
- At very high speeds we need a lot of light because as we get faster less light exposure occurs.
- You need high shutter speeds with movement
- Very short shutter speeds (<1/200 sec are required for most functional movements)
Image Size
- We want a large enough image as possible for resolution but also want them to fill the image
- Need cameras with adjustable lenses
Marker Visibility
- Need contrast (black clothes - white marker)
- Need tight clothing
Digitizing Error
- Slower resolution leads to bigger pixels
- This results in a decrease in the ability to see change when pixels are large
- We want high spatial resolution to avoid this error
Scaling Rod
- Object of known length in view
- Get as close to the subject as possible
List accurate 2D capture requirements
- High resolution (many pixels)
- Far away from subject and zoom in (reduce perspective error)
- Aligh axis of person to the camera (reduce perspective error)
- Scaling Rod as close to person as possible
- If high speed motions, high shutter speed required (reduces motion blur of individual frames taken)
