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week 2

Methods of Measurement in Biomechanics

  • Primary goal: quantify forces and motions in biological systems using a range of sensors and analysis approaches.
  • Core data pipeline (applicable to most biomechanics DAQ setups):
    • Signal identification (type of signal to measure)
    • Calibration (link sensor output to physical units)
    • Data acquisition (sampling, storage)
    • Input/Output (I/O) transfer (cables, wireless, modems)
    • Software programming (data capture, display, processing)
  • Most biomechanics data acquisition devices follow a common process: Data Acquisition → I/O → Transfer the signal → Software processing.

Strain Measurement

  • Strain measuring chain:

    • Force → Deformation → Strain gauge detects change → Electrical signal → Data

  • How strain gauges work:

    • A wire stretches under load; length increases while cross-section decreases.
    • Electrical resistance increases with strain.

  • Practical considerations:

    • Attach securely to the structure under test.
    • Minimise thermal effects (temperature sensitivity).

  • Key equations:

    • Strain: \varepsilon = \frac{\Delta L}{L_0}
    • Resistance change in a gauge: R = R_0\,(1 + GF\,\varepsilon) where GF is the gauge factor.
    • Gauge factor definition: GF = \frac{\Delta R/R_0}{\varepsilon}

  • Direct measurement path:

    • Strain gauge detects deformation; change is converted to an electrical signal (voltage) for data acquisition.

Strain Measurement: Kayaking Example

  • Application: paddling-induced strain in paddle structure.
  • Calibration concept:
    • Predicting load (y-axis) from voltage (x-axis).
    • Example calibration: slope ~ 37.037 (calibration factor). This maps voltage to load; a zero-offset can be applied so that zero voltage corresponds to zero load.
  • Calibration graph concept:
    • Graph: Voltage (V) on x-axis vs Load (kg) on y-axis.
    • Slope = calibration factor; intercept = zero-offset adjustment.
  • Practical outcome:
    • Link voltage to known force via the calibration relationship (e.g., Load = k × Voltage + b).

Force Measurement – Kayaking (General Force-Time Context)

  • Data typically presented as force-time profiles for paddling activity.
  • Example: Elite female kayak paddler shows force data for left vs right sides over time.
  • Interpretation focus:
    • Peak forces, timing of force application, symmetry between sides, and variability across strokes.

Force Plate Measurement

  • Two main force-plate technologies:
    • Piezoelectric plates (e.g., Kistler ECU Biomechanics Lab)
    • Strain-gauge-based plates (e.g., AMTI)
  • Pros/Cons:
    • Strain-gauge plates:
    • Minimal drift over time
    • Lower sensitivity; good for slow, steady forces
    • Simpler and cheaper design
    • Temperature-sensitive (thermally sensitive)
    • Piezoelectric plates:
    • Generally higher dynamic response, more sensitive to rapid changes
  • Key applications:
    • Measuring ground reaction forces in various activities (running, jumping, etc.)
  • Common dynamical relation (for plates in use):
    • Force measurements feed into locomotion analyses (e.g., COP, GRF).

Quiz Question (Conceptual Check)

  • Question: Which of the following devices does NOT directly measure force?
    • A. Strain gauge sensor
    • B. Force plate
    • C. Accelerometer
    • D. Dynamometer
  • Answer: Accelerometer. It measures acceleration; force is inferred via F = m a.

Force Measurement – Example: Centre of Pressure (CoP)

  • Grimshaw et al. (2006) framing:
    • CoP: position of the resultant ground reaction force (GRF) vector in a plane parallel to the plate surface.
    • Localization: two-dimensional coordinates (Ay, Ax) relative to the plate center (origin).
  • Applications:
    • CoP traces used in various activities such as running, archery, shooting, etc.
  • Related concepts:
    • CoP data are widely used to analyze balance, stability, and technique.

Force Measurement – Swimming (Force Plates in Water Sports)

  • Force plates and related force measurements have applications in swimming, including start/turn analyses and kick propulsion assessments.
  • (Note: Specific numerical details from the slide were not provided in the transcript; the takeaway is that force plates are used to assess swimming propulsion and kinematics.)

Acceleration and Motion Analysis

  • Fundamental equations:
    • Displacement and velocity relationships: v = \frac{\Delta s}{\Delta t}
    • Acceleration: a = \frac{\Delta v}{\Delta t}
  • Accelerometers:
    • Directly measure acceleration.
    • Can be used to infer velocity and displacement via integration (with drift considerations).
  • Motion analysis alternative:
    • Differentiation of cine/video displacement data yields velocity and acceleration:
    • Displacement data can be processed to obtain velocity and acceleration via differentiation.
  • Force relation:
    • Newton’s second law: F = m a
  • Inertial Measurement Unit (IMU):
    • Components:
    • Accelerometer: detects changes in speed
    • Gyroscope: detects changes in orientation (angular velocity)
    • Magnetometer: measures magnetic field
    • Sensor fusion combines readings to estimate 3D orientation and motion.

Inertial Measurement Unit (IMU) Details

  • IMU components and purpose:
    • Accelerometer: linear acceleration
    • Gyroscope: angular velocity
    • Magnetometer: heading reference (magnetic field)
  • Practical use:
    • IMUs are used for orientation estimation in space, motion tracking, and sport analytics.

Smartphone IMU Question (Practical Implication)

  • Question: Does your smartphone use an IMU to rotate the screen when you turn your phone?
    • Answer: Yes (smartphones use an IMU to detect orientation changes and adjust the display accordingly).

Dynamometry

  • Dynamometer = torque/power measurement device.
  • Isokinetic dynamometry:
    • Movement velocity is held constant (isokinetic condition).
    • Applies variable resistive torque to the limb to maintain constant angular velocity.
  • What is measured:
    • Muscle torque at a specific joint angle and velocity.
    • Assessment of muscle strength and endurance across different contraction types:
    • Concentric, eccentric, isometric, isotonic torque.
  • Isotonic torque definition:
    • Tension developed remains approximately constant throughout the movement.
  • Practical problems and considerations:
    • Gravity compensation: failure to account for gravitational torque can bias results (some systems compensate for gravity).
    • Maintenance of angular velocity may be challenging; iso-velocity region may be limited.
    • Axis of rotation alignment with the joint is critical for valid torque measurements.
  • Examples:
    • Dynamometry plots showing hamstrings strength and quadriceps strength (Nm/kg) and ratios.

Pressure Measurement

  • Definition:
    • Pressure = Force / Area
  • Sensor principle:
    • Rows and columns of conductive material measure force distribution to determine pressure over a surface.
    • Use of resistive and piezo technologies; wiring to individual sensors; small contact area (<5 mm).
  • Example devices:
    • Tekscan F-scan system for plantar pressure and other pressure mapping tasks.
  • Data interpretation:
    • Pressure maps enable inference of contact forces, contact area changes, and local loading.
  • Reference example:
    • Comparison of direct force measurement with forces calculated from pressure data ( Bartlett 1997 ).

Other Methods of Measurement

  • Sonic distance measurement:
    • Relies on speed of sound in air (~343 m/s at 20°C).
    • Time-of-flight from emitter to receiver gives distance.
    • Sensitivities: humidity and damp conditions affect speed of sound and thus distance accuracy.
  • Electromagnetic displacement measurement:
    • Systems like Fastrak, Flock of Birds rely on electromagnetic propagation.
    • Distance estimation via known electromagnetic wave speed in air; susceptible to ferrous/magnetic interference.
  • Radar displacement/velocity measurement:
    • Uses Doppler effect or time-of-flight to determine distance or velocity.
    • Radar guns can measure velocity directly (e.g., golf ball speed).

Proper Measurements aren’t Easy

  • Core steps for good measurement:
    • Identify signal type
    • Calibration
    • Data acquisition
    • I/O transfer
    • Software programming method
  • Data pathways:
    • Analog-to-digital conversion is required for most sensors.
    • Transfer options include cables/connectors, modems, wireless (WiFi), and Bluetooth.
    • Software is used to grab, display, and process data; it may operate in one-way or two-way communication with instruments.

Principles of Data Acquisition (Biometrics Focus)

  • Signal types in biomechanics:
    • Analog inputs: temperature, pressure, strain, voltage, vibration, force
    • Digital inputs: TTL (0–5 V), pulse timing
  • Data transfer mechanisms:
    • Cables and connectors for direct transfer
    • Modems/WiFi and Bluetooth for wireless transfer
  • Software programming roles:
    • Data collection, live display, preprocessing, and analysis
    • One-way data capture vs. instrument-controlled two-way communication
  • Practical takeaway:
    • Mastery of DAQ concepts is important for sport science and biomechanics work.

Motion Analysis (Kinematics) Context

  • Methods of measuring kinematics beyond force plates include:
    • Acoustic/radar/electromagnetic sensing
    • Potentiometers and rotary encoders
    • Optical methods (video-based, motion capture)
  • These methods complement force measurements by providing position, velocity, and acceleration data for comprehensive biomechanical analysis.

Key Takeaways and Connections

  • Multiple measurement modalities exist for force and motion; selection depends on speed of events, required accuracy, environmental conditions, and cost.
  • Calibration is essential to convert sensor outputs to physical units; example calibration links voltage to force with a slope and offset.
  • Strain-based sensing is robust for small deformations but temperature sensitive; piezoelectric and strain-gauge force plates offer complementary properties.
  • CoP analysis provides insight into balance and contact mechanics in standing and moving tasks.
  • IMUs enable portable, multi-DOF motion tracking, combining accelerometers, gyroscopes, and magnetometers for orientation estimation.
  • Data acquisition requires careful attention to signal type, sampling, transfer, and software integration; both hardware and software choices influence data quality.
  • Real-world measurement challenges include gravity compensation, axis alignment, drift, environmental interference, and the need for appropriate calibration curves.

Notation and Formulas Recap (Biometrics-Specific)

  • Strain and resistance (gauge-based):
    • Strain: \varepsilon = \frac{\Delta L}{L_0}
    • Gauge resistance: R = R_0\,(1 + GF\varepsilon)
    • Gauge factor: GF = \frac{\Delta R/R_0}{\varepsilon}
  • Force, motion, and kinematics:
    • Force: F = m a
    • Pressure: P = \frac{F}{A}
    • Velocity: v = \frac{\Delta s}{\Delta t}
    • Acceleration: a = \frac{\Delta v}{\Delta t}
  • Centre of Pressure: defined by coordinates relative to plate center (Ax, Ay) in the plane of the plate.
  • Dynamometry (torque): torque and angular dynamics relate to muscle function; isokinetic testing maintains constant angular velocity with variable resistance.

Quick Reference: Sources Mentioned in Context (for Study)

  • Dorn et al. (2012) – Force Measurement in Running (example context, force measurement references)
  • Grimshaw et al. (2006) – Centre of Pressure concepts and applications
  • Bartlett (1997) – Pressure data comparison with direct force measurements
  • AMTI and Kistler – Example force plate technologies
  • Tekscan F-scan – Pressure mapping system

Practice and Review Prompts

  • Explain how a strain gauge detects force and how calibration converts voltage to load.
  • Compare and contrast piezoelectric vs strain-gauge force plates in terms of drift, sensitivity, and temperature effects.
  • Describe how CoP is computed and why it is useful in gait analysis.
  • Outline the steps in data acquisition and the importance of signal type identification and calibration.
  • List the main components of an IMU and describe how sensor fusion yields orientation estimates.
  • Discuss potential sources of error in dynamometry and how to mitigate gravity and axis misalignment issues.

Summary

  • The lecture covered broad methods for measuring forces and motions in biomechanics, including strain gauges, force plates, accelerometers, dynamometers, pressure sensors, and various kinematic measurement approaches.
  • Real-world examples included kayaking (calibration of strain gauges), COP analysis, and force-time profiles, as well as general principles of data acquisition.
  • The content emphasized calibration, data transfer, software processing, and the practical challenges inherent in obtaining reliable biomechanical measurements.