Biomechanical Measurement and Modelling Flashcards
How do we measure kinematics
-motion capture
Characteristics of motion capture
-accuracy
-capture rate
-measure volume
-easy use
Motion capture accuracy
-how accurately position can be estimated
Motion capture rate
-number of still frames captured per unit time
Motion capture measure volume
-volume of space in which marker position can be accurately determined
Motion capture easy use
-fast to instrument
Motion capture non-optical systems
-mechanical
-inertial
-magnetic
Mechanical motion capture
-electronic goniometers/exoskeletons that sense joint angle
-inexpensive, unlimited capture volume
-cumbersome, few angles possible
Inertial motion capture
-accelerometer and rate gyro signals to determine position
-untethered, unlimited capture volume
-accuracy not always great because of drift
Magnetic motion capture
-sensors that detect position with respect to a source magnetic field
-no optical occulsion
-wires, complex
Motion capture optical systems
-passive marker
-active marker
-markerless
Passive marker motion capture
-light is reflected off markers and captured by cameras
-no wires required
-marker overlap and occlusion can happen
Active marker motion capture
-LED markers emit light in a sequence capture by cameras
-no marker confusion
-wires required and occlusion
Markerless motion capture
-automatically identify body landmarks from images
-no markers, no overlap or occlusion
-early stages
How many cameras are needed for a 2D image
-1 camera
How many cameras are needed for a 3D image
-2 or more cameras with appropriate calibration
How do we measure kinetics?
-strain gauge
-force plates
Strain gauge
-measures strain (change in length) under tension and compression
How does a strain gauge work
-there is a wire inside that can increase (stretched wire) or decrease (compressed wire) resistance
-to estimate level of force you measure the voltage drop across strain gauge
Force plates
-used to measure forces and torques under foot
-uses a network of strain gauges
What measures do we get out of force plates
-3 forces: Fx, Fy, Fz
-3 torques: Mx, My, Mz
How do we measure muscle activation
-electromyography
Electromyography
-measures current generated by ionic flow across membrane of muscle fibres
Types of EMG electrodes
-surface
-fine wire
-needle
Sources of noise from EMG signal
-noise in electronics components in detection and recording equipment
-ambient noise from sources of electromagnetic radiation (radio, TV etc)
-soft tissue artifact
-electric voltage from heart
Forward dynamics
-measure: kinetics
-calculate/estimate: kinematics
Inverse dynamics
-measure: kinematics
-calculate/estimate: kinetics
System identification
-measure: kinematics and kinetics
-calculate/estimate: inertial properties (mass, mass distribution)
Reasons why foot length may change between frames
-soft tissue artifact
-out of plane movement
-body as a non-rigid object
Soft tissue artifact
-the markers are sitting on the skin and fat
-therefore the markers may move during movement
Out of plane movement
-if you are recording in a 2D direction and are walking in a motion falling outside of the sagittal plane it would fall out of range and therefore the length of the foot would be changing
Body as a non-rigid object
-foot length may change because there is different amount of stress being applied to the foot and it an be strained in different ways
-the body is not a rigid object
