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YEAR 3 > Biomechanics > Flashcards

Biomechanics Flashcards

1
Q

What are kinetic forces?

A

Forces acting on an object/ the body responsible for casing or maintaining stationary position

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2
Q

What do force platforms do

A
  • 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
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3
Q

Name the 2 types of force plate

A

Piezo electric (Kistler) and Strain Guage

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4
Q

Explain how a piezo electric force plate works

A

Contains crystals which change electrical charge depending on the force applied

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5
Q

What are the advantages and disadvantages of a pizo electric force plate

A

:) Sensitive to rapid changes in force

:( Temperature sensitive

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6
Q

Explain how a strain gauge force platform works

A

Contains beams which bend depending on the force applied

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7
Q

What are the advantages and disadvantages of a strain gauge force platform

A

:) Stable over long times, cost

:( Slower

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8
Q

Name the main system requirements for a force platform

A 
Linearity 
Rigitidy 
Range
Cross talk 
Sensitivity 
Hysteresis
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9
Q

What are the force platform outputs

A

Forces in 3 dimensions: vertical, anterior-posterior, medio lateral

  • Rate of force development –> slope of force time trace
  • Impulse
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10
Q

What are the uses of force platforms

A 
Gait analysis 
Shoe research 
Training 
Rehab 
Impulse in take off
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11
Q

When does hysteresis exist

A

When the input output relationship depends on whether the input force in increasing or decreasing

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12
Q

What should the range be for a force platform

A

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

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13
Q

Explain sensitivity in terms of force plates

A

=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

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14
Q

What is cross talk

A

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[;;

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15
Q

Name the stages in qualitative analysis

A

Preparation
Observation
Evaluation
Intervention

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16
Q

What does preparation involve in qualitative analysis

A

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

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17
Q

What does observation involve in qualitative analysis

A
  • Selection of critical features to observe
  • Selection of appropriate number of trials/observers
  • Consider number and experience of observers
  • Selection of appropriate viewing distance
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18
Q

What does evaluation involve in qualitative analysis

A

-Errors corrected in the order that generates the most improvement

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19
Q

What does intervention involve in qualitative analysis

A

Feedback and instruction to the performer

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20
Q

What is semi quantitative analysis

A

Same as qualitative but with some simple measurements such as simple joint angles

Can give more detailed comparisons

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21
Q

Name quantitative kinematic methods

A 
video
high speed cinematography
ultrasound
gyroscopic sensors
Electrogoniometry
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22
Q

Name quantiative kinetic analysis methods

A

Force plates
Pressure transducers
Electromyogrpahy

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23
Q

What are the 2 requirements to measure kinematic parameters

A

Accurate measurement of the position the centre of rotation of each of the moving body segments

An accurate time measurement for each position measurement

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24
Q

What are the problems with 3D and 2D

A

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

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25
Q

Advantages and disadvantages of video

A

:) 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)

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26
Q

Advantages and disadvantages of opto electronic

A

:) 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

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27
Q

Advtanges and disadvantages of 16mm cinematography

A

:)
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

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28
Q

Advantages and disadvantages of inertial sensors

A

:)
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.

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29
Q

Advantages and disadvantages of electrogoniometry

A

:)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.
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30
Q

What is filming speed?

A

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

31
Q

What is shutter speed?

A

length of time when the film or digital sensor inside the camera is exposed to light,

32
Q

What does it mean if the shutter speed is set at 1/1000

A

the camera will not ‘see’ anything for 0.019s out of 0.020s in each field

This will freeze the action for each field

33
Q

What is the best shutter speed for fast sporting actions

A

very short - maybe 1/4000

34
Q

What is the problem with light for short shutter times

A

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

35
Q

What are the procedures for 2D kinematic data collection

A

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

36
Q

What is accuracy

A

the difference between an observed value and the true value

37
Q

What is precision

A

The variation in measurements that you would expect to be the same

38
Q

What is resolution

A

finest with which a measurement can be made

39
Q

What are systematic errors

A

Errors that stay the same from video field to field

E.g camera not running at the correct speed

40
Q

What are random erros

A

Errors that vary from field to field

Spiky on a graph

E.g joint markers moving out of a plane

41
Q

What is a fourier transform used for

A

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

42
Q

What is frequency spectrum

A

When a tourer transform has been performed, the amount of information present at each frequency is available which is called frequency spectrum

43
Q

What is filtering

A

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

44
Q

What is spline smoothing

A

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

45
Q

What is inverse dynamics

A

We can use the displacement, velocity , acceleration to get the forces, moments, energy, power etc

46
Q

What are kinetograms

A

stick figures

movement patters

47
Q

When would be most appropriate to use a short shutter speed?

A

fast sporting actions

need lots of lights

48
Q

What should the frame rate be for slow motion

A

high

49
Q

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

A

0.019s

50
Q

What would you do if the cameras can’t be genlocked?

A

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

51
Q

What are CCDs and CMOSs

A

Sensors in cameras controlled by the shutter

CCD is the most common type

CMOS has less power

52
Q

How does gen locking take place?

A

Physically linked the cameras with a gen lock cable

53
Q

What is a co ordinate digitiser used for?

A

To undertake quantitative analysis

Device enables 2D co ordinate of specific points on the video image such as anatomical landmarks to be recorded

54
Q

What does resolution refer to?

A

the minimum separation between two points on the screen that the system is able to detect

55
Q

What is an increased frame rate important for?

A

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

56
Q

suggested frame rates for walking and swimming

A

25-50Hz

57
Q

Suggested frame rates for running and high jump

A

50-100 Hz

58
Q

suggested frame rates of sprinting and a football kick

A

100-200 Hz

59
Q

suggested frame rates for tennis serve or golf swing

A

200-500Hz

60
Q

What is simi used for?

A

Digitising

the process of obtaining 2 dimensional co ordinates of specified landmarks on the performer from a video

61
Q

Why is smoothing used

A

to remove the high frequency noise introduced by the digitising process - split fitting and fourier series truncation

62
Q

What happens when there is a failure to smooth co ordinates sufficiently

A

lead to high levels of noise

63
Q

What is the linear displacement of a body landmark

A

the change in the relevant scaled co ordinate of that landmark during a specified time period

64
Q

what is perspective error

A

brains expect closer objects to appear larger than those far away

65
Q

When does hysteresis exist

A

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

66
Q

What should the range be for a force platform

A

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

67
Q

what is sensitivity

A

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

68
Q

what is cross talk

A

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

69
Q

why is it important that force plate systems are calibrated

A

to minimise systematic errors

70
Q

what are the limitations of smoothing

A

if there are mixed frequencies, smoothing routines have to be available for each point separately

71
Q

What should be done to overcome the issue of errors usually being at the ends of the data

A

take more data than you need

e.g 10 frames before an 10 frames after action of interest

72
Q

what are the implications of force plate analysis

A

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

73
Q

name the different plates force platforms produce data for

A

vertical
anterior-posterior
medial lateral

YEAR 3 (13 decks)

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