Kelvins 1st rule of instrumentation
The measuring instrument should not alter the event being measured.
Common factors in selection and use of instrumentation that impact validity and reliability of data obtained
- Selected: Which instrument is best for a given situation?
- Acquired: How will my patient end up on my computer screen?
- Processed: What was done to the “signal” generated by my patient?
– Electric; Has someone done this correctly - Interpreted: Are we constraining things too much? Is it measuring what is supposed to measure?
Identify common instrumentation devices and their appropriate selection for answering clinical questions about human movement measurement.
- Dynamometers – Objective Strength Measurements
- Force Platforms – Force Measurements
- Pressure mapping – Postural Sway/Balance Measures
- Electromyography – Muscle Electrical activity
- Ultrasound imaging – Imaging to identify ruptures, tears, cross sectional area, injury
Describe the basic operation of an electrical strain gauge in tension and compression within a force transducer
- Tension = increased resistance, decreased conductance
- Compression = decreased resistance, increased conductance
What are the properties exhibited by a high quality force transducer?
- Low temperature sensitivity – record the same measure at high or low temperatures
- Low electrical interference – prevent as little environmental electrical interference as possible. Therefore, more likely to get true data
- High linearity – sensitivity doesn’t change through strain range
- Low hysteresis – low conductivity change in loading and unloading.
- Low cross talk – Allows for motion to accurately be depicted in the proper plane. Want to minimize data from one plane moving into the other as otherwise all data is mixed which is not good.
- High natural (resonant) frequency – The higher the frequency the better it is at responding to quick changes in force. Wont wobble
- Overload protection – safety mechanisms so if too much force is placed on it. The force plate does not get damaged.
Describe how a force transducer or force platform should be calibrated to ensure valid data
A force platform should be calibrated by taking a known weight (Ex: 5 lb that has been weighed) and placing it in multiple different areas of the force plate. The values should remain the same 5 lbs
Describe the process by which forces are converted into electrical signals, amplified, filtered, and converted into a computer signal by an analog to digital converter
- Force Input -> Tansducer -> Electrical Signal -> Signal Conditioner -> Computer -> A/D conditioner (bits)
- Analog – Continuous
- Digital - Discrete
How do you know when sampling frrequency is not collected properly?
- Too large: No such thing as sampling too much!
- Too small: Leads to aliasing (undersampling and inaccurate signal)
How do you know when amplitude gain is not collected properly?
- Having too large of an amplitude gain leads to the wave being cut off because it is too zoomed in
- Having too small of an amplitude gain, doesn’t allow for differences to be seen because the wave looks like little squiggles.
Having ____ be adjustable is good!
- Gain
- Adjust the volume to get the best range of the picture
How do you determine sampling frequency?
- Nyquist Theorem: Needs to be 2x faster than any frequency of movement
How does sampling frequency affect data quality?
- Allows to get the motion of objects at high speeds and be able to get slow motion movements of fast moving objects
GFR Walking
Vertical: heel and forefoot contact forces
GRF Running
GRF Skip
GRF Jumping
How do you know if you are speeding up or slowing down on a walking GRF?
- Longitudinal: Breaking and propulsion force
- Constant = Even (As seen)
- B > P = Slowing down
- P > B = Speeding up
What are some characteristics of a GRF?
- Impulse: total area under
- Loading Response: Slope of the line initially
- Time to stabilize: onset to peak (time to peak), peak to stabilize
- Loading Rate: Shape of curve (viscoelastic)
Describe the center of pressure of a person’s ground contact and how it is measured by force platforms - Weight Shifting vs Ankle Strategy
- CoP excursion is larger/moves more than the CoM excursion
- Intersect above the CoM = how we position feet and use torque to generate GRF
- Intersect through the CoM = we would keep moving, there would be no correction
Ankle Strategy: AP motion
Weight Shifting: ML Motion
Describe and discuss the types of data that can be collected using a force platform
- Area outline by the COP Path; Ex: 95% ellipse
- Path Length (total distance traveled)
- Maximal Excursion (width of trace in both ML and AP)
- Variability (Ex: SD)
- Velocity (How fast something is moving)
- Time to contact (before falling off balance)
Discuss the sensory contributions to postural control and how their absence or impairment impacts center of pressure data measures
- Absence or impairment of sensory contributions results on reliance of other forms of senses. Ex: somatosensory absence, results in vision to become primary.
- Overall leads to worse balance
Describe uses of posturographic assessment equipment and reports
- Assessment can include Sensory Organization Test
- Static Balance (& Dynamic Balance – Only Motek Medical)
– Shifting side to side
Identify common uses of isokinetic testing machines for measurement of strength, assessment of injury risk, and neuromuscular training
- Outcome measures
– Bilateral comparison
– Pre/post designs - Training
– Endurance, strength, failure - Eccentric Loading (though entire ROM)
- Opposing muscle group (Ham/Quad ratio)
Discuss major considerations for positioning an individual within an isokinetic testing apparatus and how it may impact testing results
- Pt body orientation (if not properly aligned will results in mixed data)
- Isolate muscle for joint and directions of interest – align motors axis to joint axis (Prevent compensation from other muscles with stabilization straps)
- Set range of motion angle stops/brakes (Safety mechanism)
- Set concentric and eccentric angular speeds (Machine acceleration/deceleration delay; Possible overshooting)
- Set number of repetitions (Practice vs Test)
- Exercise trials: set desired torque output goals
