Force Measurement

Question 1

What is a force transducer?

Answer 1

• Can measure magnitude, duration and direction of force
• Force transducers give an electrical signal proportional to an applied force
• They detect the force to be measured and convert (transduce) it into an electrical signal proportional to the force applied

Question 2

Explain the difference between Piezoelectric and Piezoresistive transducers and outline the
advantages and disadvantages of each

Answer 2

The main difference is the material used

• Piezoelectric uses crystal
• piezoresistive uses metal

Piezoelectric
Advantages
- Drift is present for static or quasi-static applications (alter the rate of electrical flow from time point to time point)
* Reset to zero before measuring
- Well suited for sport movements (rapidly changing forces)
- Less affected by temperature than piezoresistive systems
- Possess a widely adjustable range

Disadvantages

• Generally more difficult to install
• More expensive

Piezoresistive
Advantages
- Easier to install and less expensive than piezoelectric force platforms

Disadvantages

• Temperature compensation effect
  
  • Frequent calibration checks needed

Question 3

In what form is the signal collected using force transducers and what must be done to it to allow a
numerical value of force to be calculated?

Answer 3

• To obtain a signal that can be interpreted and analysed via computer hardware and software we must convert the physiological (analogue- voltage) signal to a digital signal
  
  • Allows a numerical value of force to be calculated

Question 4

Should the sampling theorem be considered when collecting force data? Explain your answer

Answer 4

Sampling rate must be at least twice the frequency of the highest frequency of oscillation (Sampling Theorem)
- In practical settings it is often at least four times the highest frequency
(so that important information is captured)

Question 5

Several operational characteristics of force transducers can influence the accuracy and reliability of
the system. Outline and explain several points of consideration regarding these characteristics.

Answer 5

Linearity

• Relationship between input force and output signal
• Input (force) to output (voltage) - Maximum deviation from linearity. Percentage of full scale deflection - should be less than 0.5% FSD
• Non-linear systems can be calibrated but more difficult

Hysteresis

• This exists when different input-output relationships occur
• Slightly different outputs may be recorded for the same input if increasing/decreasing. Maximum difference as a percentage of FSD - again less than 0.5%

Range

• The range of forces that can be measured must be adequate for the application and adjustable
• If range too low – saturation occurs (reach constant, peak clipped)
• If range too high - reduction in sensitivity

Sensitivity

• Change in recorded signal per unit change in force.
• Sensitivity decreases with increasing range
• A-D converter is usually the limitation

Cross talk
- Forces in one direction affecting forces recorded in another (X Y Z). Less than 3% necessary

Dynamic response

• Forces change rapidly - measuring system should also change rapidly
• Crucial to accuracy of measurement

Question 6

Explain the term “ecological validity” and provide examples in relation to measuring ground
reaction forces.

Answer 6

Ecological validity examines whether the results of a study can be generalized to real-life settings

Ecological validity - changes in movement pattern due to being measured, targeting platform, different surfaces etc

Question 7

List several biomechanical measures that can be determined using force platforms.

Answer 7

• Pressure
• Acceleration
• Velocity
• Momentum

Question 8

Injury, or the susceptibility to injury is closely linked with which biomechanical variable that can be
determined using a force measurement system? Can you explain why?

Answer 8

Load rate or rate of force loading

- Having higher amounts of force in a small amount of time and increase an individuals susceptibility to injury

Question 9

Other than force platforms, what force measuring devices are often used in human movement
analysis?

Answer 9

Pressure mat
- Pressure mat is simply a force platform that has the appropriate software to calculate the previous equations and thus calculate the COP at a given point in time

Accelerometry

• A device that measures acceleration
• Force is measured by a strain gauge that produces a signal voltage V which is proportional to the force

Inverse dynamics

In-shoe force measurement

Question 10

Describe (with some detail) how you might use a force measuring system to analyse 4 different
sporting movements / techniques and list what information could be obtained and why? For
example: A device designed to measure punching force might be used to assess whether
technique changes exhibit improvements force and power of a punch.

Answer 10

Running - A device which measure force with different walking patterns

Swimming -