S2 Lecture 2 Flashcards
What are the two types of biomechanical (motion) analysis?
Qualitative and quantitative
What are the methods of qualitative analysis?
A slow motion video review and discussion about technique
What are the methods of quantitative analysis?
Putting numbers to the video, digitising body landmarks to get kinematic data and measuring forces (kinetics)
What are the stages of a kinematic analysis?
- Record the sports movement- video sequence compromising a series of images.
- Digitise body landmarks in each image and reconstruct the coordinates, e.g. ankle, knee.
- Further processing of this position data.
How is the video camera different from the 3D motion analysis system?
- It is cheaper and more accessible- 1-2 cameras.
- Can use in a field with minimal interference.
- Pictorial record
- Time-consuming analysis
- Uncertainty in displacement data (digitisation random error).
How is the 3D motion analysis system different to the video camera?
- Expensive- costs tens of thousands to set up a lab.
- Can only use them indoors- marker interference.
- Only the markers are recorded.
- Very quick to acquire coordinates.
- Less random error.
General set-up of a camera:
Consideration of sample frequencies, shutter speed, focal length, and aperture/depth of field.
What is the display resolution?
The number of pixels per dimension (width x height) of a monitor. The higher the number of pixels the greater the image clarity.
What is shutter speed?
The duration that the shutter in open measured by 1/s. It is inversely proportional to the amount of light passing through a lens. Faster shutter speeds produce clearer but darker images.
What is the shutter?
The ‘eyelid’ of the camera, a gate that opens and closes exposing the CCD sensor to light.
What is focal length?
(Zoom): the distance between the lens and sensor. Zoom lens allows the focal length to be adjusted manually. Decreasing the focal length (zooming out), increasing focal length (zooming in).
What is aperture?
-The size of the hole created by the iris, often written as f/stop or f-stop (focal length/diameter.
-Aperture of a 200mm lens at f/4 would be 50mm wide.
-Smaller the f/-number the larger the aperture
-Also affects the amount of light that passes onto the sensor and image’s depth of field.
What is depth of field?
The range of distances where objects appear in acceptably sharp focus.
Important when movement takes place towards or away from the camera and/or 3D movement (not so much in sagittal plane). Can be manipulated through aperture and object-to-camera-distance. Landscape mode increases the depth of the camera. Greater aperture creates a smaller depth of field.
What is the lens distortion?
-An error that comes from the design of the lens itself.
-Ideal world there is no distortion, which is very rare.
-Lenses that go from wide angle to telephoto will have each type of distortion at extreme ends.
-Reduce distortion by keeping the object in the centre of field of view.
What is barrel distortion?
Wide angle lens, fov bigger than size of image sensor.
What is pincushion distortion?
Telephoto/zoom (very long) lenses, fov smaller than size of image sensor
What is sampling frequency?
The number of frames collected per second- typically 50Hz, or 50 frames per second.
However, value may vary depending on the activity. Higher sampling frequencies are needed for very fast movements, such as ball- bat impact.
What is the Nyquist sampling theorem?
Sample frequency must be at least twice the highest analogue frequency content.
What is alias?
When the frequency is too low to match our movement (see lecture slides).
What are two examples of forces transducers?
Strain gauges and Peizoelectric crystals
What is a force transducer?
Materials change their physical properties with applied force and can be converted to electrical signal: voltage
Each force plate has 4 transducers, 1 in each corner
What are the 3 ground reaction forces?
Fx- Anterior-posterior
Fy- Vertical
Fz- Medio-lateral
What are the steps of recording force plate data?
- Force plate (transducers)
- Amplifier
- Analogue to digital converter (ADC)
- ComputerWhy
do we need to use the correct resolutions?
To minimise errors in the resulting force data
What are the different types of resolution?
Display resolution (no of pixels), time resolution (s) and amplitude resolution (N)
What is amplitude resolution/ADC precision?
The number of different steps that we can measure. E.g. 8 bit =2^8= 256 steps, 10 bit =2^10= 1024 steps so on
What is the force plate range?
The minimum to maximum force value that can be measured
What is amplitude resolution formula?
Range/precision
What do we interpret on a force trace?
-Assume good force traces have been obtained of a sporting activity
-Magnitude and timing of GRF is important (peak force, rate of force development, resultant force vector and impulse).
What would we expect to see in steady state running, and why?
Areas under the curve would be equal as the force on the force plate would cancel out.
