🏃Sports Biomechanics Unit 10 – Motion Capture and Data Processing

What's Motion Capture?

• Motion capture (mocap) involves recording movements of objects or people and translating that information into digital models
• Uses sensors placed on the body to track the position and orientation of different body segments over time
• Captures 3D motion data by recording the positions of reflective markers placed on specific anatomical landmarks
• Commonly used in sports biomechanics to analyze techniques, identify areas for improvement, and optimize performance
• Provides detailed kinematic data (positions, velocities, accelerations) of body segments and joints during dynamic movements
• Enables quantitative analysis of complex motor skills and sport-specific techniques that are difficult to assess with the naked eye
• Offers high spatial and temporal resolution, allowing for precise measurements and detailed insights into movement patterns
• Can be combined with other data sources (force plates, EMG) to gain a comprehensive understanding of the biomechanics involved

Key Tech and Equipment

• Optical motion capture systems
  • Use high-speed infrared cameras to track the positions of reflective markers attached to the subject's body
  • Cameras are strategically placed around the capture volume to ensure optimal coverage and minimize marker occlusion
• Passive markers
  • Spherical retroreflective markers that reflect infrared light emitted by the cameras
  • Typically placed on specific anatomical landmarks to define body segments and joint centers
• Active marker systems
  • Use markers that emit their own light, eliminating the need for external illumination
  • Markers are powered by small batteries and synchronized with the camera system
• Inertial measurement units (IMUs)
  • Wearable sensors that combine accelerometers, gyroscopes, and magnetometers to measure linear accelerations, angular velocities, and orientations
  • Provide an alternative to optical systems, particularly useful for field-based measurements or in situations where marker visibility is limited
• Force plates
  • Measure ground reaction forces and moments during contact with the ground
  • Provide important kinetic data to complement kinematic measurements from motion capture
• Electromyography (EMG) sensors
  • Record the electrical activity of muscles during movement
  • Help assess muscle activation patterns, timing, and intensity
• Specialized software
  • Used for calibrating the system, capturing and processing data, and performing biomechanical analysis
  • Examples include Vicon Nexus, Qualisys Track Manager, and Visual3D

Setting Up for a Mocap Session

• Calibrate the motion capture system to ensure accurate and reliable data collection
  • Perform static calibration using a calibration object (e.g., L-frame) to define the global coordinate system
  • Dynamic calibration involves moving a wand with markers through the capture volume to calculate camera positions and orientations
• Create a subject-specific marker set
  • Select anatomical landmarks for marker placement based on the specific movement and research question
  • Ensure marker placement is consistent across subjects and sessions to allow for reliable comparisons
• Prepare the subject
  • Explain the purpose of the study and obtain informed consent
  • Take anthropometric measurements (height, weight, limb lengths) for scaling and normalization
  • Attach markers to the subject's body using double-sided tape or elastic bands
• Set up the capture environment
  • Ensure adequate space for the subject to perform the desired movements safely
  • Control lighting conditions to minimize interference with marker visibility
  • Use appropriate flooring or mats to ensure subject safety and comfort
• Configure software settings
  • Set the desired capture frequency (typically 100-500 Hz for sports applications)
  • Define the marker set and label markers according to the anatomical landmarks
  • Create a capture session and specify the number of trials and conditions

Capturing the Data

• Instruct the subject to perform the desired movement or task
  • Provide clear verbal instructions and demonstrations to ensure consistent execution
  • Allow for practice trials to familiarize the subject with the movement and capture environment
• Start the capture session
  • Ensure all cameras are properly connected and synchronized
  • Initiate data recording in the motion capture software
• Monitor data quality during the capture
  • Ensure markers remain visible to the cameras throughout the movement
  • Check for any marker dropouts or mislabeling in real-time
• Perform multiple trials for each condition
  • Collect a sufficient number of trials to account for variability and ensure reliable data
  • Repeat trials if necessary due to marker occlusion, subject errors, or other issues
• Save and backup the captured data
  • Organize trials and sessions in a structured manner for easy access and analysis
  • Store data on secure servers or external hard drives to prevent data loss

Processing Raw Mocap Data

• Import the captured data into the processing software
  • Ensure compatibility between the capture and processing software formats
• Identify and label markers
  • Assign labels to each marker based on the predefined marker set and anatomical landmarks
  • Verify marker labels are consistent across trials and sessions
• Filter the marker trajectories to remove noise and artifacts
  • Apply appropriate filtering techniques (e.g., Butterworth filter) to smooth the data
  • Choose filter cut-off frequencies based on the characteristics of the movement and noise level
• Fill gaps in marker trajectories
  • Interpolate missing marker data using spline or pattern fill techniques
  • Ensure gap-filling does not introduce artifacts or distort the original movement pattern
• Define body segments and joint centers
  • Create a biomechanical model by linking markers to define body segments (e.g., thigh, shank, foot)
  • Calculate joint centers based on marker positions and anatomical landmarks
• Compute kinematic variables
  • Calculate joint angles, velocities, and accelerations using the processed marker data
  • Use appropriate biomechanical conventions (e.g., Euler angles, quaternions) for describing joint kinematics
• Export the processed data
  • Save the processed marker trajectories, body segments, and kinematic variables
  • Export data in formats compatible with further analysis software or statistical packages

Analyzing Movement Patterns

• Visualize the processed motion capture data
  • Create 3D animations or stick figure representations of the movement
  • Overlay multiple trials or subjects to compare movement patterns
• Compute relevant biomechanical parameters
  • Calculate variables specific to the movement or sport, such as joint ranges of motion, angular velocities, or segment coordination
  • Derive performance-related metrics, such as jump height, stride length, or angular momentum
• Perform statistical analysis
  • Use appropriate statistical methods to compare kinematic variables between conditions, groups, or timepoints
  • Identify significant differences or correlations between variables of interest
• Interpret the results in the context of the research question or application
  • Relate the biomechanical findings to performance, injury risk, or other relevant factors
  • Provide practical implications and recommendations based on the analysis
• Integrate motion capture data with other biomechanical measures
  • Combine kinematic data with force plate or EMG measurements to gain a more comprehensive understanding of the movement
  • Investigate the relationships between kinematic, kinetic, and neuromuscular variables
• Report the findings
  • Present the results in a clear and concise manner, using appropriate graphs, tables, and visualizations
  • Discuss the limitations, assumptions, and potential future directions of the study

Common Challenges and Fixes

• Marker occlusion
  • Occurs when markers are obscured from camera view due to body parts, equipment, or other objects
  • Minimize occlusion by optimizing camera placement and using redundant markers
• Soft tissue artifact
  • Movement of markers relative to the underlying bone due to skin movement or muscle contractions
  • Use techniques such as cluster markers or anatomical calibration to minimize the impact of soft tissue artifact
• Synchronization issues
  • Misalignment of data from different sources (e.g., motion capture, force plates, EMG)
  • Ensure proper synchronization using hardware or software triggers and post-processing alignment
• Marker placement variability
  • Inconsistencies in marker placement across subjects or sessions
  • Develop standardized protocols for marker placement and use experienced personnel to ensure consistency
• Data processing errors
  • Mistakes in labeling, gap-filling, or computing kinematic variables
  • Implement quality control procedures and manually verify the processed data
• Limitations of the biomechanical model
  • Assumptions and simplifications in the model may not accurately represent the actual movement
  • Use appropriate models for the specific movement and population, and acknowledge the limitations in the interpretation of results

Real-World Applications in Sports

• Technique analysis and optimization
  • Use motion capture to analyze sport-specific techniques and identify areas for improvement
  • Provide feedback to athletes and coaches to optimize performance and prevent injuries
• Equipment design and testing
  • Evaluate the impact of different equipment (e.g., shoes, rackets, clubs) on biomechanical variables
  • Inform the design and development of sport-specific equipment to enhance performance and safety
• Injury risk assessment and prevention
  • Identify movement patterns or biomechanical factors associated with increased injury risk
  • Develop targeted interventions or training programs to reduce the risk of sport-specific injuries
• Rehabilitation and return-to-play
  • Monitor the progress of injured athletes during rehabilitation using motion capture
  • Assess readiness to return to sport based on biomechanical measures and comparison to pre-injury data
• Talent identification and development
  • Use motion capture to identify key biomechanical characteristics of successful athletes in a specific sport
  • Inform talent identification and development programs to optimize athlete selection and training
• Research and education
  • Advance the understanding of sport-specific biomechanics through research using motion capture
  • Incorporate motion capture technology into educational programs to provide hands-on learning experiences for students and professionals