Annotation in video analysis refers to the practice of adding visual elements such as lines, angles, circles, arrows, and text directly onto video frames to highlight important features, illustrate measurements, or explain findings. This technique enhances communication, supports education, and helps focus attention on specific aspects of movement.
Types of Annotations
Geometric Annotations
Lines
- Connect anatomical points
- Show segment orientations
- Indicate reference horizontal or vertical
- Illustrate trajectories or paths
- Draw attention to specific features
Angles
- Show joint angles (knee, hip, ankle, etc.)
- Display trunk lean angles
- Illustrate foot strike angle
- Demonstrate limb positions relative to vertical
- Often include numerical value
Circles/Ellipses
- Highlight specific body regions
- Indicate areas of interest
- Show joint centers
- Circle points of concern
Rectangles/Shapes
- Box regions for emphasis
- Create zones or areas of interest
- Define measurement regions
Arrows
- Indicate direction of movement
- Point to features of interest
- Show force vectors
- Illustrate corrections needed
Text Annotations
Labels
- Name anatomical landmarks or joints
- Identify phases of movement
- Label trials or conditions
- Provide subject or date information
Measurements
- Display calculated joint angles
- Show distances or heights
- Present velocities or other metrics
- Include units with values
Commentary
- Explain findings or observations
- Provide coaching cues
- Note areas needing attention
- Give context or interpretation
Time Stamps
- Frame numbers
- Time from event
- Phase duration
- Temporal references
Specialized Annotations
Stick Figures
- Connect tracked points to show body configuration
- Abstract representation of posture
- Useful for highlighting overall pattern
- Can overlay multiple timepoints
Trajectory Paths
- Show path of body part over time
- Illustrate center of mass displacement
- Display implement path (ball, bat, etc.)
- Visualize repeated movements
Color Coding
- Different colors for different trials
- Color-code left vs. right limbs
- Use colors to indicate severity (green/yellow/red)
- Distinguish between phases or events
Comparison Overlays
- Reference lines showing ideal positions
- Normal range bands or zones
- Baseline or target positions
- Contralateral limb for symmetry comparison
Common Applications
Clinical Communication
Helping patients understand:
- Their movement patterns
- Areas of concern
- Changes with intervention
- What "normal" looks like
- Goals for rehabilitation
Coaching Feedback
Showing athletes:
- Technical flaws
- Target positions or angles
- Differences from elite models
- Improvements over time
- Specific cues visualized
Educational Use
Teaching students about:
- Biomechanical principles
- Movement analysis techniques
- Normal and abnormal patterns
- Measurement procedures
- Clinical decision-making
Research Presentation
In publications and conferences:
- Illustrate methodology
- Show example trials
- Demonstrate group differences
- Support findings visually
- Create clear figures
Software Features
Modern video analysis software typically includes:
Drawing Tools
- Freehand drawing
- Shape tools (lines, circles, rectangles, polygons)
- Angle measurement tools
- Text insertion
- Arrow and pointer tools
Automatic Annotations
- Auto-display of calculated angles
- Automatic stick figures from tracked points
- Real-time measurement display
- Trajectory generation from tracking data
Customization
- Color selection
- Line thickness control
- Font and text size
- Transparency adjustment
- Style templates
Dynamic Annotations
- Annotations that update each frame
- Follow tracked points automatically
- Persistent across video playback
- Show changing measurements in real-time
Layer Management
- Multiple annotation layers
- Toggle layers on/off
- Organize different annotation types
- Control layer order
Best Practices
Clarity
- Don't over-annotate (cluttered images confuse rather than clarify)
- Use consistent color schemes
- Choose appropriate line thickness (visible but not overwhelming)
- Make text large enough to read easily
Accuracy
- Ensure annotations reflect true measurements
- Double-check angle values before adding
- Verify anatomical landmarks are correctly identified
- Use precision appropriate to measurement accuracy
Consistency
- Use same colors for same features across images
- Maintain similar annotation style in presentations
- Create templates for repeated annotations
- Document annotation conventions
Purpose-Driven
- Annotate only what's relevant to analysis goal
- Tailor annotations to audience (patient vs. clinician vs. researcher)
- Focus on actionable information
- Support primary message
Communication
- Include units with measurements
- Define abbreviations or terminology
- Add legends or keys if needed
- Provide context in captions
Annotation Workflow
1. Identify Features to Highlight: - Determine analysis goals - Select relevant measurements - Decide what audience needs to see
2. Choose Annotation Types: - Select appropriate tools (lines, angles, text, etc.) - Decide on color scheme - Determine text content
3. Apply Annotations: - Add geometric annotations carefully - Position text clearly - Include measurements with units - Layer annotations logically
4. Review and Refine: - Check clarity and readability - Verify accuracy of measurements - Ensure annotations don't obscure important features - Test understanding with intended audience
5. Export and Share: - Capture annotated frames as images - Create annotated video clips - Generate reports with annotations - Share with appropriate parties
Examples by Use Case
Gait Analysis
- Angle measurement at knee during mid-stance
- Vertical reference line showing trunk lean
- Arrow indicating direction of progression
- Text labeling phases (loading, mid-stance, push-off)
Running Technique
- Foot strike angle relative to ground
- Hip, knee, ankle angles at contact
- Trajectory of center of mass
- Stick figure showing body position
Throwing Mechanics
- Shoulder external rotation angle
- Elbow position at release
- Trunk rotation angles
- Ball path trajectory
Rehabilitation
- Current vs. target range of motion
- Asymmetry between limbs (color coded)
- Progress over time (date-stamped comparisons)
- Areas of compensation highlighted
Considerations
File Size
- Annotations increase file size
- Consider exporting key frames rather than entire annotated videos
- Use appropriate resolution for intended use
Professionalism
- Clean, clear annotations for professional reports
- Ensure proper labeling and attributions
- Protect patient/athlete privacy (de-identify if necessary)
- Maintain professional standards in shared materials
Accuracy Claims
- Only report precision appropriate to measurement method
- Acknowledge limitations of 2D analysis if relevant
- Don't overstate significance of small differences
- Be honest about measurement error
Annotation transforms raw video into an educational and analytical tool, making invisible biomechanical principles visible and understandable. When used thoughtfully, annotations enhance communication, support learning, and improve the impact of video-based movement analysis.