10.2 Marker placement and data collection protocols

Motion capture is crucial in sports biomechanics, and marker placement is key to accurate data. Proper placement ensures precise representation of the body's structure, enabling reliable analysis of movement patterns and joint angles. It's the foundation for valid research and clinical assessments.

Standardized protocols like the Helen Hayes and Plug-in Gait models guide marker placement on specific anatomical landmarks. These systems help create consistent data across subjects and sessions. Proper setup, subject preparation, and careful marker application are essential for quality data collection in biomechanics studies.

Marker Placement for Accuracy

Importance of Proper Placement

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• Creates accurate representation of body's skeletal structure and joint centers in 3D space
• Prevents errors in joint angle calculations, segment lengths, and overall kinematic analysis
• Allows for reliable comparisons between subjects and across multiple testing sessions
• Creates biomechanical model closely matching subject's actual anatomy and movement patterns
• Minimizes need for extensive post-processing and data cleaning
  • Saves time
  • Improves overall efficiency of motion capture process
• Directly impacts validity and reliability of research findings and clinical assessments in biomechanics studies

Consequences of Improper Placement

• Leads to inaccurate joint angle measurements
  • Can result in misinterpretation of movement patterns (overestimation of knee flexion)
• Causes errors in calculating segment lengths
  • Affects inverse dynamics calculations (incorrect force estimations)
• Introduces artifacts in movement data
  • Creates false motion not representative of actual body movement (apparent knee valgus)
• Reduces repeatability of measurements between sessions
  • Hampers longitudinal studies or comparing pre/post intervention data
• Increases time spent in post-processing to correct errors
  • May require manual adjustment of marker positions in software

Standardized Marker Protocols

Common Marker Sets

• Helen Hayes marker set
  • Utilizes 15 markers for lower body analysis
  • Includes markers on pelvis, thighs, knees, shanks, ankles, and feet
• Plug-in Gait model
  • Expands on Helen Hayes set with additional upper body markers
  • Allows for full-body motion analysis
• Cleveland Clinic marker set
  • Uses cluster markers on segments for improved tracking
  • Reduces skin movement artifacts

Anatomical Landmark-Based Placement

• Pelvic markers placed on anterior and posterior superior iliac spines
  • Defines pelvic coordinate system
• Knee markers positioned on medial and lateral femoral epicondyles
  • Helps determine knee joint center and rotation axis
• Ankle markers placed on medial and lateral malleoli
  • Used to calculate ankle joint center
• Shoulder markers on acromion process
  • Serves as reference for upper arm segment

Segment-Specific Protocols

• Lower limb markers
  • Thigh cluster or individual markers for femur tracking
  • Shank markers to capture tibia/fibula motion
  • Foot markers on calcaneus and metatarsal heads
• Upper limb markers
  • Upper arm cluster or individual markers
  • Elbow markers on medial and lateral epicondyles
  • Forearm markers for pronation/supination tracking
  • Wrist markers on styloid processes
• Trunk and spine markers
  • C7 vertebra marker for upper trunk reference
  • T10 marker for mid-trunk motion
  • Sternum marker for chest expansion/contraction

Data Collection Procedures for Quality

System Calibration and Setup

• Perform daily system calibration
  • Ensures accurate spatial reconstruction
  • Aligns multiple cameras in capture volume
• Set up capture volume
  • Define boundaries of movement area
  • Position cameras to minimize marker occlusion
• Adjust camera settings
  • Optimize frame rate for expected movement speed (120 Hz for walking, 240+ Hz for sprinting)
  • Set appropriate exposure and threshold for marker visibility

Subject Preparation

• Select appropriate clothing
  • Tight-fitting, non-reflective attire
  • Avoid loose fabric that may obscure markers
• Prepare skin for marker adhesion
  • Clean areas with alcohol wipes
  • Shave hair if necessary for better adhesion
• Take anthropometric measurements
  • Record height, weight, and segment lengths
  • Use for scaling biomechanical models

Marker Application Process

• Apply double-sided tape to markers
  • Ensure strong adhesion to skin
• Use additional securing methods for high-movement areas
  • Athletic tape or pre-wrap for markers on feet or hands
• Follow systematic approach for marker placement
  • Start with static markers (pelvis, joint centers)
  • Progress to segment tracking markers
• Verify marker visibility and labeling
  • Perform static capture to check all markers are visible
  • Ensure correct labeling in motion capture software

Capture Protocol Implementation

• Perform static calibration pose
  • Subject stands in T-pose or other standardized position
  • Used to define segment lengths and joint centers
• Conduct range of motion trials
  • Isolated joint movements to verify marker tracking
  • Helps identify any marker placement issues
• Execute dynamic movement trials
  • Task-specific movements (walking, jumping, sport-specific actions)
  • Multiple trials for statistical reliability
• Implement real-time quality checks
  • Monitor marker trajectories during capture
  • Address any marker dropout or mislabeling immediately

Marker Placement Issues and Solutions

Skin Movement Artifacts

• Occurs when markers move relative to underlying bone
  • Particularly problematic in areas with loose skin or adipose tissue (thigh)
• Minimize through strategic placement
  • Avoid areas of high skin deformation
  • Use bony landmarks where possible
• Implement cluster markers
  • Rigid arrays of 3-4 markers attached to segments
  • Reduces individual marker movement relative to bone
• Apply mathematical models for artifact reduction
  • Global optimization techniques
  • Kalman filtering to estimate true bone position

Marker Occlusion Management

• Occurs when markers are blocked from camera view
  • Results in gaps in trajectory data
• Optimize camera placement
  • Use sufficient number of cameras (8+ for full-body capture)
  • Position cameras to cover all angles of movement
• Implement redundant marker sets
  • Place additional markers on segments
  • Allows for reconstruction of occluded marker positions
• Utilize gap-filling algorithms in post-processing
  • Spline interpolation for short gaps
  • Rigid body fills for longer occlusions

Soft Tissue Deformation Considerations

• Affects marker positions during dynamic movements
  • Can lead to apparent changes in segment lengths
• Account for in marker placement
  • Avoid areas of high muscle bulge or fat pad compression
• Consider in data interpretation
  • Acknowledge limitations in areas prone to deformation (abdomen during trunk flexion)
• Use advanced modeling techniques
  • Implement soft tissue artifact models in biomechanical analysis

Joint Angle Cross-talk Reduction

• Occurs when movement in one plane affects angle calculations in another
  • Common in complex joints (shoulder, spine)
• Careful marker placement
  • Align markers with anatomical axes when possible
• Implement advanced biomechanical modeling
  • Use functional joint centers instead of marker-based
  • Apply joint coordinate systems that minimize cross-talk