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Video equipment for gait analysis

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Rory McFadden, Nov 23, 2006.

  1. Rory McFadden

    Rory McFadden Active Member

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    I have been promised a bit of local health Authority money to invest in developing our paediatric biomechanics service.I would like to invest in some video cameras to aid gait analysis. However I dont think the budget will stretch to system that is compatable with our Tekscan F-scan system.Therefore It is my intention to investigate the cameras sold through electrical retail outlets.
    Can any one out there with experience in these matters give me few pointers as to what features I should look for in a video camera and any web sites that would provide info on the optimum set up.


  2. Jonathan

    Jonathan Active Member

    Hi Roy,

    It is simple: whether you are purchasing a Pal (Europe, China, RoW) or NTSC (North and South America, Japan etc) camcorder you will get no more that 25-30 fps.

    These are important points to note when considering a purchase;

    1) Ensure that the camera is a 3CCD camera - a CCD is the gizmo that colours the pixels (little dots) on your screen (comes from the word 'picture element'). Most use one and then filter the image into Red, Green and Blue - a 3CCD camera on the other hand uses one CCD for each colour - therefore a sharper brighter picture.

    2) Ensure that the camera can be switched from 'Auto' to 'Manual' shutter speed, this is imperative, as you cannot accurately analyse dynamic foot behaviour at speeds below 1/500 of a second.

    Note: There is a difference between shutter speed and frame speed - you will still capture at 25-30 frames per second (frame speed), it is just that you can alter the moment it time that frames shutter (shutter speed) is exposed to the image.

    Therefore 30 fps which frame capturing at 1/100 of a second will be blurry compared to 30 fps captured at 1/1500 of a second.

    3) Obviously as soon as you increase the shutter you will need an increase the amount of light - 150 watts is sufficient.

    4) Adjust the cameras aperture to be as open as far the focal length of the runners/walkers stride length will allow. This will enable the camera to work at its optimum operative capability.

    Finally you will need to run (excuse the pun) a few tests to find the best shutter speed/focal length and aperture opening. You will be surprised at how clear a £350.00 camera can be.

    Hope this helps

    Last edited: Nov 27, 2006
  3. Asher

    Asher Well-Known Member

    Hi Jonathon,

    Thanks for that info but it's all double dutch to the likes of me. Out of interest, could you recommend specific brands / models of video camera??

  4. As Jonathan pointed out, there are a number of factors to look at when purchasing a video camera for gait analysis, many of which share common optical factors with any camera, still or video. Here are a few factors for those without a background in photography:

    1. Shutter speed: Faster shutter speeds (e.g. 1/500, 1/1000 second) will "freeze the action" better than slower shutter speeds (i.e. 1/15, 1/30 second) but will also decrease the amount of light (i.e. number of photons) exposing the film or sensor by the same factor of the decrease in shutter speed. In other words, a 1/500 second exposure will "freeze the action" twice as well as a 1/250 second exposure, but will only let in 1/2 the amount of light for exposuring the film or sensor.

    2. Aperture: The smaller the ""f setting" or aperture setting (e.g. f 2.0) means a greater opening of the iris of the camera lens than does a larger aperture setting (e.g. f 16.0). Therefore, using a smaller aperture setting means more light per unit time is allowed onto the film or sensor than using a larger aperture setting. For example, an aperture setting of f 5.6 lets in twice the light as f 8.0 per unit time. However, the increase in light gathering ability that a larger diameter iris opening affords the camera also produces a decrease in the "depth of field" of the resulting image. A decreased depth of field will be apparent as more of the objects both in front of and in back of the plane of focus of the lens being "out of focus". A larger depth of field is desirable since it will allow the video camera to keep the foot in focus for a longer distance as the foot moves toward and moves away from the camera. This will then prevent the operator or camera needing to continually change the focus of the camera lens as the "camera to subject distance" changes.

    3. Zoom settings: As the zoom of the camera lens is increased to a larger "focal length" setting (e.g. from 50 mm to 200 mm), the image will fill up a larger portion of the film or sensor. This magnifies the image but also will tend to decrease the depth of field of the lens at the same time. So if you are zooming in on a foot to have it look bigger on the screen as it moves toward and away from you, there will also be increased tendency for you to need change the focus setting of the lens (or for the camera to do this automatically) than if there is less zoom (i.e. decreased focal length) of the lens. All video cameras have zoom capability and as one buys a more expensive video camera, you are often buying a higher quality lens with increased zoom capability and with better quality image.

    For video gait analysis, if you are using the video just to show patients their gait pattern, I don't agree that a 3 CCD sensor video camera is necessary, but it certainly does give a clearer image. Even though I can't give you any specific brands of cameras that are best I have always had good results with Sony products.

    I suggest that you should first borrow a friend's video camera, if this is at all possible, bring it into the office, set it on a low tripod and play around with it for a little while to see what type of images you can get from their unit while having someone walk for you. Setting up a relatively evenly and well lit hallway, or with several light sources from different angles with the patient on a treadmill, will allow more depth of field, allow sharper images to be captured, and will allow the autofocus system of the camera to stay focused on the foot more effectively. The quality of lighting setup is everything for both still and video photography.

    If you want to get the most from your video camera purchase, you should really get someone who knows cameras and photographic principles to show you a few things about video cameras in general. In this way, you can make sure you are spending your money wisely for the camera that will allow you to best obtain the image quality that is required for your particular clinical application.

    Hope this helps.
    Last edited: Nov 28, 2006
  5. Asher

    Asher Well-Known Member

    Thank you Jonathon and Kevin, that has been a great help!
  6. Jonathan

    Jonathan Active Member

    Hi Kevin

    Good points and clearly explained -

    Having used several camcorders in my time I have found that a 3CCD 'camcoder'* does improve the quality when 'freezing' the image at 1/1500 to 1/2000th of a second. (*Not a professional 3CCD DV Camera costing £1,000's. :eek: )


    Manual shutter speed/aperture adjustment is key. If you can't change these you will most likely be analysing blur - especailly with runners over 10 km's per hour.

    Don't spend money on features you will never use, most shop assistants have only ever sold camcorders to holiday makers, so visit a couple of good retailers and ask for their DV Camcorder expert – tell them exactly what it is for and that you will need manual shutter speed and aperture adjustment. Take notes and get them to show you how to change the shutter speed. (don't worry if they have to use the manual as every manufacture differs and it is rare to change these.)

    – what country do you live in and I will suggest a few models.



    Tip: If you are playing back directly to a PC/TV screen ensure the camcorder comes with remote control, this allows you to stay close to the TV or PC screen rather than holding the camcorder in one hand and trying to stop start the tiny keys with the other.
    Last edited: Nov 28, 2006
  7. Asher

    Asher Well-Known Member

    I'm in Australia. Thanks so much for the advice ... and the recommendations to come!!

  8. Jonathan

    Jonathan Active Member

    if you leave our cricketters alone - i'll buy it for you!!!

    I will have to confirm if the model numbers are the same down there - it may be a few days - so I'll send it direct to you.

  9. Asher

    Asher Well-Known Member

  10. Rory McFadden

    Rory McFadden Active Member

    Many thanks for your very informative replies to my original query.I will now go armed with a print out of your responses to my local "specialist"photgraphic shop.


  11. Rory McFadden

    Rory McFadden Active Member

    Hi Jonathan

    As an Irishman I have no real interest in the "Ashes", the forthcoming six nations and Rugby World Cup are a different matter though!.I am however interested in any recommendations you can make regarding specific models of camcorders.

    Good luck for the 2nd Test.I fear you may need it


  12. Jonathan

    Jonathan Active Member

    Rory - it will be a pleasure.

    I am trying to find Irish blood in my family so I can support the 2nd best rugby team in the world next year - but the closest is my Great Grandmothers neighbour as she once had a Irish Greyhound run through her flower bed.

    Therefore I am left to put down my Guinness and pick up a VB, as I always prefer to drink with the winners.

    Oh to be ¼ American, ¼ Kiwi, ¼ Swiss and a ¼ Austrian (Golf, Rugby, Tennis and Alpine ski)


    PS. Panasonic NV GS-180 - buy it via the internet, LCBL used them with no problem.

  13. Rory McFadden

    Rory McFadden Active Member

    I appreciate your prompt response.The RFU are currently looking for a man to improve their efficiency, may be you should apply?

    Good luck

  14. Jonathan

    Jonathan Active Member


    Honestly I would rather stick knitting needles in my eyes that work for those guys. :eek:

    I think Martin Johnston would be best employed there, then we would need to find someone to sort out our Rugby Team :D :
  15. LCBL

    LCBL Active Member

    PurelyGadgets were a bit slow so I went withPixmania and was delighted with service etc

  16. Dikoson

    Dikoson Active Member

    Another thing to consider is the number of frames per second captured. I have used 3 50fps (frames per secong) digital camera's (sony). Have just upgraded to 100fps and the difference is staggering especially for use on analysing paediatric patients.

    What system are you using for analysis of the video?

    I have been working with an organisation called Quintic Biomechanics in the uk. They have just sorted out auto digitisation and dual capture (sagittal and coronal). System is easy to use and relatively inexpensive. Free downloads from www.quintic.com.

  17. Asher

    Asher Well-Known Member

    Can anybody offer advice on purchasing a treadmill (Australia). What properties do I need, what do I need to avoid?

  18. David Smith

    David Smith Well-Known Member

    Roy and others

    At the risk of repeating myself, Once you have your Digital Camcorder and have superb reproduction of a subjects gait. Have a think about what information it is that you want to extrapolate from the video.
    I believe reliable qualitive data is impossible even with a synchroised 2D camera system. Even quantitive data must be used with caution especially if evaluating gait that has extreme variation from normal.

    An example; subject standing with knees flexed 20dgs and feet straight ahead, viewed from frontal plane knees look straight but from saggital view knees are flexed.Easy evaluation.
    Now also rotate the hips bilateraly 30dgs. Frontal view = apparent genu valgum (knock knee) and toe in. Saggital view = knees flexed 17dgs. What evaluation might you make if you didn't know the hips were internally rotated.
    There has been no change in any of the unilateral lower limb orientation relative to each other. Only relative to a global reference or the proximal reference of the pelvis. If you then add in that the joint axies are not orthoganal or coronal then you have no hope of defining joint motion by visual analysis alone.

    I would be interested to know how others use their 2D dual camera system with syncronising 'analysis' (I prefer the term evaluation) software (like silicon coach). Can you use an ordered rotation system for joint position evaluation?It seems that this system along with perhaps a vertical pressure analysis system is the system that is being used more often by podiatrists and other related health professionals. This is an understandable compromise given the cost of a 3D force and motion analysis system. But are we in danger of settling for unreliable conclusions based on 2nd rate technology just because its cheap? Is the 2D camera system more reliable than eyeball analysis with the patient present for as long as is needed?

    Yours thoughtfully Dave Smith

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