Biomechanics Flashcards
What are kinetic forces?
Forces acting on an object/ the body responsible for casing or maintaining stationary position
What do force platforms do
- Ground reaction force is the most common force measured by force plate
- Measured force acting on performer has the same magnitude as the reaction force exerted on the performer by force plate by in the opposite direction
- Force platforms incorporate a force transducer (strain gauge of pizoelectic) which converts the force into an electric signal
- Signals from transducers are amplified and sometimes modified
- Converted to digital signals
Name the 2 types of force plate
Piezo electric (Kistler) and Strain Guage
Explain how a piezo electric force plate works
Contains crystals which change electrical charge depending on the force applied
What are the advantages and disadvantages of a pizo electric force plate
:) Sensitive to rapid changes in force
:( Temperature sensitive
Explain how a strain gauge force platform works
Contains beams which bend depending on the force applied
What are the advantages and disadvantages of a strain gauge force platform
:) Stable over long times, cost
:( Slower
Name the main system requirements for a force platform
Linearity Rigitidy Range Cross talk Sensitivity Hysteresis
What are the force platform outputs
Forces in 3 dimensions: vertical, anterior-posterior, medio lateral
- Rate of force development –> slope of force time trace
- Impulse
What are the uses of force platforms
Gait analysis Shoe research Training Rehab Impulse in take off
When does hysteresis exist
When the input output relationship depends on whether the input force in increasing or decreasing
What should the range be for a force platform
Range of forces that can be measured must be adequate for the application and the range should be adjustable
If the range is too small for the forces being measured, the output voltage will strata
Ranges usually -10 to 10kN for the 2 horizontal aces and -10 to +20 kN for the vertical axis
Explain sensitivity in terms of force plates
=the change in the recorded signal for a unit change in the force input of the slop of the idealised linear voltage force relationship
Sensitivity decreases with increasing range
What is cross talk
Force plates normally measure force components in more than one direction
The possibility then exists of forces in one component direction affecting the forces recorded by the transducers used for by the other components
Cross talk= the interference between the recording channels for the various force components
-Cross talk must be small, preferably less than 3% of full scale deflection[;;
Name the stages in qualitative analysis
Preparation
Observation
Evaluation
Intervention
What does preparation involve in qualitative analysis
Development of theoretic model of the most effective technique
Indentify goal and factors which affect the outcome
Knowledge of the performers
Biomechanics factors that affect performance
Split the activity down to several phases
What does observation involve in qualitative analysis
- Selection of critical features to observe
- Selection of appropriate number of trials/observers
- Consider number and experience of observers
- Selection of appropriate viewing distance
What does evaluation involve in qualitative analysis
-Errors corrected in the order that generates the most improvement
What does intervention involve in qualitative analysis
Feedback and instruction to the performer
What is semi quantitative analysis
Same as qualitative but with some simple measurements such as simple joint angles
Can give more detailed comparisons
Name quantitative kinematic methods
video high speed cinematography ultrasound gyroscopic sensors Electrogoniometry
Name quantiative kinetic analysis methods
Force plates
Pressure transducers
Electromyogrpahy
What are the 2 requirements to measure kinematic parameters
Accurate measurement of the position the centre of rotation of each of the moving body segments
An accurate time measurement for each position measurement
What are the problems with 3D and 2D
Need to see all joints from all angles simultaneously - cameras need to be synchronised which is expensive and time consuming g
Optical errors
Time measurement
How to get real life size co ordinates
Advantages and disadvantages of video
:) Immediate feedback (but not with computer digitizing)
Standardization
High Shutter Speeds (up to 1/8000 sec)
Digital Technology (CCD/CMOS chips and DV/DVD or memory card)
Quality - HD
Cost
:(
Genlocking
‘Rolling’ shutters now on most consumer HD cameras
Speed limited to 25 or 50Hz (pictures/second in UK) [30/60 Hz in
USA] on most cheap systems (some up to 1000 Hz with reduced
resolution). Full resolution >50/60 Hz expensive
Errors involved in manual digitization can be high (without markers
and/or in competition)
Advantages and disadvantages of opto electronic
:) Fast collection
Immediate feedback (<30 secs)
Software already written
Fast (up to 1000Hz (pictures/sec) with shutters up to 1/8000s)
Tracking markers can be very accurate (small errors)
no need for digitising
once set up, can use for multiple participants
:(
Markers required (not competition) - not on joint centres but on
skin.
Cost (usually need at least 6 cameras = >E50k)
Calibration?
Markers crossing cause confusion (depends on software)
Low levels of light needed
Set-up time to position cameras to see all markers all the time
Advtanges and disadvantages of 16mm cinematography
:)
Fast (up to 10000 Hz pictures/sec) - most cameras up to 500Hz
Very high quality image
Timing lights built-in (you know camera speed exactly)
High speed shutters
Easier to synchronize - link cameras directly
:(
Delayed feedback
Cost
Filming ‘blind’ - limited viewfinder.
High light levels needed (Daylight)
Availability of film and processing - VERY rare?
Not mains powered - need a large battery and charger
Errors involved in manual digitization can be high
Advantages and disadvantages of inertial sensors
:)
Fast Collection & Easy Setup
Immediate feedback
Software already written — ‘plug and play’
No cameras needed so no problems with field of view
Wireless (advanced system) so no leads
Basic electronic sensors cheap
:(
Cost - Systems expensive z E30000-E40000 (Xsens)
‘Drift’ -
Speed of sampling (how many data/points per second) depends on
the number of sensors used.
Sensor are z size of matchbox and have to be strapped on with belts
or suits.
Advantages and disadvantages of electrogoniometry
:)Gives joint angles directly
Cost (cheap)
Simple
Easy to interface to computer - fast data collection and analysis
\:( Difficult to get limb exact position in space - no fixed reference frame, because only angles are measured Restrict movement Usually used only on up to 3 joints.
What is filming speed?
How many pictures per second camera records(frames)
Each video picture is made up of 2 fields interlaced
Film speed is usually 25 frames per second
What is shutter speed?
length of time when the film or digital sensor inside the camera is exposed to light,
What does it mean if the shutter speed is set at 1/1000
the camera will not ‘see’ anything for 0.019s out of 0.020s in each field
This will freeze the action for each field
What is the best shutter speed for fast sporting actions
very short - maybe 1/4000
What is the problem with light for short shutter times
Only exposes the film to light for a very short time in each picture which means not much light can get onto the film –> dark
What are the procedures for 2D kinematic data collection
Procedures for 2D kinematic data collection
Mount camera stationary on tripod (no pan/tilt)
Action fits in field of view (including clubs/rackets/jumps etc)
Unhindered subject performance
Zoom lenses best (reduce perspective error)
Performer moves at 90 degrees to optical axis of camera
Calibration lengths and vertical reference needed
Contrasting joint centre markers (Careful)
Suitable frame rate (8-10 times fastest action frequency - week 4)
Good lighting (Daylight best). Use floodlights if inside or dark
Trial/subject information should be recorded
What is accuracy
the difference between an observed value and the true value
What is precision
The variation in measurements that you would expect to be the same
What is resolution
finest with which a measurement can be made
What are systematic errors
Errors that stay the same from video field to field
E.g camera not running at the correct speed
What are random erros
Errors that vary from field to field
Spiky on a graph
E.g joint markers moving out of a plane
What is a fourier transform used for
to transform position time information to frequency data
split analysis into 2 sections (less or more than 12Hz)
identify section of noise and remove it
truncated = all info above a particular frequency can be removed
What is frequency spectrum
When a tourer transform has been performed, the amount of information present at each frequency is available which is called frequency spectrum
What is filtering
One way of smoothing
reduces the data above a certain frequency but doesn’t completely remove them
filters can be non recursive (simple) or recursive( some of the already filtered data used to make each point more accurate)
E.g butterworts filter
What is spline smoothing
most efficient way of smoothing
a curve
equation is fitted between all points or even each point. This can be a
Cubic or Quintic spline - the curve fitted used x3 or x5 to ‘best fit’ the line
between points
generate pathway which best describes all points
What is inverse dynamics
We can use the displacement, velocity , acceleration to get the forces, moments, energy, power etc
What are kinetograms
stick figures
movement patters
When would be most appropriate to use a short shutter speed?
fast sporting actions
need lots of lights
What should the frame rate be for slow motion
high
If the shutter speed of a camera is set at 1/1000, how long won’t the camera ‘see’ anything for out of 0.020 seconds
0.019s
What would you do if the cameras can’t be genlocked?
the two dimensional co ordinates obtained from each of the camera views must be synchronised by interpolating the data and then shifting one data set by the time lag between the 2 shutters
What are CCDs and CMOSs
Sensors in cameras controlled by the shutter
CCD is the most common type
CMOS has less power
How does gen locking take place?
Physically linked the cameras with a gen lock cable
What is a co ordinate digitiser used for?
To undertake quantitative analysis
Device enables 2D co ordinate of specific points on the video image such as anatomical landmarks to be recorded
What does resolution refer to?
the minimum separation between two points on the screen that the system is able to detect
What is an increased frame rate important for?
to improve the precision and therefore the accuracy of temporal measurements
this is particularly important where the phases are of short duration such as the hitting phase of a tennis serve
suggested frame rates for walking and swimming
25-50Hz
Suggested frame rates for running and high jump
50-100 Hz
suggested frame rates of sprinting and a football kick
100-200 Hz
suggested frame rates for tennis serve or golf swing
200-500Hz
What is simi used for?
Digitising
the process of obtaining 2 dimensional co ordinates of specified landmarks on the performer from a video
Why is smoothing used
to remove the high frequency noise introduced by the digitising process - split fitting and fourier series truncation
What happens when there is a failure to smooth co ordinates sufficiently
lead to high levels of noise
What is the linear displacement of a body landmark
the change in the relevant scaled co ordinate of that landmark during a specified time period
what is perspective error
brains expect closer objects to appear larger than those far away
When does hysteresis exist
when the input output relationship depends on whether the input force is increasing or decreasing
E.g an be caused by the presence of deforming mechanical elements in the force transducers
What should the range be for a force platform
must be adequate for the application
range too small for the forces being measured —> output voltage will saturate
Suitable maximum ranges for many sports biomechanics applications would be -10 to +10kn
what is sensitivity
the change in the recorded signal for a unit change in the force input or the slope of the idealised linear voltage force relationship
sensitivity decreases with increasing range
what is cross talk
possibility of forces in one component direction affecting the forces recorded b the transducers for the other components
cross talk expresses this interferes
cross talk should be less that 3 % of full scale defection
why is it important that force plate systems are calibrated
to minimise systematic errors
what are the limitations of smoothing
if there are mixed frequencies, smoothing routines have to be available for each point separately
What should be done to overcome the issue of errors usually being at the ends of the data
take more data than you need
e.g 10 frames before an 10 frames after action of interest
what are the implications of force plate analysis
measurements of forces applied by an athlete can aid a coach to quantitatively evaluate an athletes execution of a skill
E.g data of the forces applied to the ground in a vertical jump provide a picture of the explosive abilities of the athlete
name the different plates force platforms produce data for
vertical
anterior-posterior
medial lateral
