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.