2D (two-dimensional) analysis is a video analysis approach that uses a single camera perspective to measure and analyze movement occurring primarily in one plane. This is the most common and accessible form of video-based biomechanical analysis, suitable for many clinical, coaching, and research applications.
Fundamental Concept
2D analysis measures movement in:
- X-axis: Typically horizontal (left-right or forward-backward)
- Y-axis: Typically vertical (up-down)
- Plane: The flat surface containing the movement
Movement perpendicular to the camera (toward or away) is not measured in 2D analysis.
When 2D Analysis is Appropriate
Planar Movements
Movements occurring primarily in a single plane:
- Sagittal plane: Walking/running viewed from the side
- Frontal plane: Jumping jacks or lateral movements viewed from front
- Transverse plane: Rotational movements viewed from above
Planar Components
When analyzing one component of a multi-planar movement:
- Sagittal plane knee flexion during a squat (side view)
- Frontal plane hip abduction during gait (front view)
- Arm elevation in sagittal plane during reaching (side view)
Practical Constraints
When 3D analysis is not feasible due to:
- Equipment limitations (single camera available)
- Space constraints
- Time constraints
- Cost considerations
- Sufficient information available from 2D
Advantages of 2D Analysis
Accessibility
- Requires only one camera
- Simpler setup and calibration
- Less expensive equipment
- Faster analysis process
- Easier to learn and implement
Practicality
- Suitable for field use
- Quick turnaround for feedback
- Can analyze archived footage
- Good for screening and monitoring
Sufficient Information
For many applications, 2D provides adequate data:
- Joint angles in primary movement plane
- General movement patterns
- Relative comparisons (pre/post, left/right)
- Gait temporal parameters
Limitations of 2D Analysis
Out-of-Plane Movement
- Cannot measure movement toward/away from camera
- Underestimates true 3D angles when movement includes out-of-plane components
- May miss important frontal or transverse plane motions
Perspective Errors
- If body part moves out of measurement plane, measurements become inaccurate
- Closer objects appear larger than distant objects
- Wide-angle lenses may distort measurements at frame edges
Limited Accuracy
- 3D movement projected onto 2D plane
- True joint angles may differ from apparent 2D angles
- Cannot calculate true 3D positions or velocities
Setting Up 2D Analysis
Camera Positioning
Critical factors:
- Perpendicular View: Camera should be 90° to plane of movement
- Centered: Subject should be centered in frame at mid-point of movement
- Appropriate Distance: Far enough to capture full movement, close enough for detail
- General rule: 2-3 times the height/width of movement
- Minimizes perspective distortion
- Stable Mount: Tripod or firm mounting surface
- Correct Height: Approximately mid-height of movement
- Hip height for gait analysis
- Adjust for specific analysis needs
Camera Settings
- Frame Rate: 60 FPS minimum for walking, 120-240 FPS for running
- Shutter Speed: Fast enough to eliminate motion blur (1/250s or faster)
- Focus: Fixed focus on movement area (not autofocus)
- Zoom: Wide enough to capture full movement, tight enough for detail
- Resolution: Highest available
Calibration
- Place reference object in plane of movement
- Known dimension (typically 1-2 meters)
- Vertical and/or horizontal reference depending on analysis
- Must be flat against plane of analysis
Common 2D Analysis Views
Sagittal Plane (Side View)
Most common for gait analysis:
- Ankle, knee, hip flexion/extension
- Forward/backward trunk lean
- Arm swing
- Stride length
- Vertical displacement
Frontal Plane (Front or Back View)
Useful for:
- Hip abduction/adduction
- Lateral trunk lean
- Step width
- Pelvic drop during single leg stance
- Knee valgus/varus angles
Transverse Plane (Top/Bottom View)
Less common but useful for:
- Rotational movements
- Turning mechanics
- Tibial or femoral rotation
- Shoulder or hip rotation
Types of 2D Measurements
Spatial Measurements
- Joint angles at specific time points
- Distances between points
- Heights and horizontal displacements
- Ranges of motion
Temporal Measurements
- Contact times
- Flight times
- Phase durations
- Event timing (when events occur)
Spatio-Temporal
- Velocities (position change over time)
- Accelerations (velocity change over time)
- Stride length and cadence
- Step frequency
Analysis Workflow
1. Setup and Recording: - Position camera appropriately - Include calibration object - Record multiple trials
2. Import and Calibration: - Import video to analysis software - Establish calibration scale - Set coordinate system
3. Tracking/Digitizing: - Identify anatomical landmarks - Track or digitize points through movement - Review for accuracy
4. Calculate Variables: - Joint angles from tracked points - Distances and displacements - Velocities and accelerations - Temporal parameters
5. Interpret and Report: - Compare to normative values - Assess asymmetries - Identify deviations from optimal patterns - Provide feedback or recommendations
Accuracy Considerations
Maximizing 2D Accuracy
- Ensure movement stays in analysis plane
- Use telephoto lens or longer distance to reduce perspective error
- Calibrate carefully with reference in measurement plane
- Use high resolution and frame rate
- Apply appropriate data filtering/smoothing
Recognizing Limitations
- Acknowledge out-of-plane movement affects results
- Compare only trials with similar camera setup
- Don't over-interpret small differences (measurement error exists)
- Consider 3D analysis when out-of-plane movement is substantial
Applications
Clinical Gait Analysis
- Pre/post surgical or treatment comparison
- Asymmetry detection
- Fall risk assessment
- Rehabilitation progress monitoring
Sports Coaching
- Technique feedback
- Pre/post coaching intervention
- Injury screening
- Performance monitoring
Ergonomics
- Workplace posture assessment
- Repetitive task analysis
- Lifting technique evaluation
- Workstation design
2D analysis, while limited compared to 3D, remains a practical, accessible, and valuable tool for movement assessment. Understanding its appropriate use cases and limitations enables effective application across clinical, coaching, and research settings.