Planar Kinematics Flashcards
Factors to consider regarding movement speed
- faster movement requires higher sampling rate
What type of signal does A and B represent?
A. Analog
B. Digital
Which steps of the digitization Process does each letter represent?
A. Physiological Signal
B. Sample Hold Circtuit
C. Digital Encoder
D. Computer Clock
E. Computer Memory
F. Computer Storage
Sampling Rate
speed at which encoder captures samples and converts to digital code (Hz)
Typical frequency of physiological signals
15hz
what are Hz?
hertz: 1 cycle/ second
Motion analysis sampling rate (units)
pictures/sec
force/ muscle activity sampling rate (units)
volts/sec
sampling rate for most non image signals (units)
volts/sec
Fourier Transform Function
Tool for determining the frequency content of a signal
-transforms signal into combo of sine and cosines
-frequency domain NOT time domain
What do graphs A and B represent?
A. Time
B. Frequency
What does A and B represent?
A. Sine Wave
B. Cosine wave
Nyquist Sampling Theorem
A signal must be sampled at a frequency greater than AT LEAST twice the highest frequency present in the signal itself.
-Generally 4-5 times higher
What happens if the sampling frequency is two low?
Aliasing- true signal is not reproduced
what happens if the sampling frequency is too high?
unnecessary data.
Point Mass
Center of mass in kinematics
What does a Link Segment utilize?
Combines rigid body assumption
Wobbling mass
effective impedence
General Considerations for type of imaging used?
- Size of image or model
- whole body or segment
- discrete measure? (foot angle at toe off)
- continuous measure (foot angle throughout stride)
- linear, angular, combination?
2 D Assumptions
- Motion is assumed to be planar
- Saggital plane movement
- Frontal plane movement
2 D motional analysis limitations
-each camera only sees a single 2d image
-Depth cannot be judged (single camera assumes everything is in the same plane)
Critical Assumptions of 2D analysis
-Single Plane
-Calibrated Movement
2 D calibration
- at least 2 points separated by known distance
- calibration in both vertical and horizontal directions is best
- Defines global coordinate system (lab axes)
sfu= L/Lu
sfv= L/Lv
x = sfuu
y = sfv v
Calibration Formulae
2 D Image faults
Camera placement/alignment error
Deformed image
Perspective error
Mitigation of perspective error
move camera as far away as possible
2 D analysis camera settings
sampling frequency
aperture setting
shutter speed
field of view
Marker placement for 2D imaging
at approximate joint centers
What system is used to determine position in biomechanics?
Cartesian Coordinate System
Global Coordinate system
Defines position and translation of body in space
Local Coordinate system
Set of x,y,z axes associated with each node in a model
What do each of the letters represent?
A. Global Y axis
B. Global X axis
C. Local Y segment
D. Local Y Joint
E. Local X Joint
F. Local X segment
How do you compute segment (absolute) angles
measured relative to a fixed reference
how do you compute relative angles?
difference between segment angles
Dot Product
Takes two equal length sequences of numbers and returns one
AB=|A||B|cos theta
theta = cos^-1 {(AB)/(|A||B|)}
Steps to computing thigh angle
- Create two vectors
-right horizontal (a)
-thigh vector (b) (subtract knee marker from hip marker) - Fight Lengths of A and B (pythagorean theorem)
- divide each coordinate of the thigh vector by the length of the thigh vector to get unit vector
4 multiply vectors and take the inverse cosine of the quantity
theta = cos^-1(AxBx+AyBy)
