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Knee Center of Mass Indicator/Measurements

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Skiing-in-jackson, Sep 15, 2011.

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    What is the little gizmo called that measures knee center of mass???

    And where can I get one?

  2. David Smith

    David Smith Well-Known Member


    I think that this is a vague term incorrectly used in skiing to denote the centre of the knee. see here - http://www.skiervillage.com/showthread.php?82-Center-of-knee-mass. They talk about marking the centre of knee mass with a pen and then wedge the skii boot so the knee centre of mass aligns correctly with the big toe or something :rolleyes:?? You can't mark the centre of mass on the surface of an object with a 3D volume. The Gizmo they talk about is a caliper -
    I could predict by eye, or at least by imagination, the centre of mass of a perfectly spherical object with uniform density and internal structure but seeing as the knee is none of these I would say that it is impossible to do by eye or with any calipers.

    Why did you want to know?

    Dave Smith
    Last edited: Sep 15, 2011
  3. I fit ski boots for a living. Using a common carpenter's square I try to get the center of the patella between the first and second met (saggital plane movement). This makes for a well rounded fit that works well in a variety of conditions.

    I was wanting to see how one worked and see if it changes my fits. I know guys use them for skiing, but I've always thought it may be overkill.
  4. Stephen:

    As David said, the term "center of mass" is incorrect for determining the center of the knee joint by the method you noted. Center of mass is the point location within an object where the mass is considered to be concentrated at. In ski boot fitting, you simply need to get an approximation of the knee joint center, which could easily be done with a caliper or even visually to within a few mm, so that you can align the position of the knee to the position of the foot in a simulated skiing stance.

    As you know, this approximation of using the knee joint center works quite well for boot fitting, but, unfortunately, is more of an empirical method of optimizing boot fit, rather than being scientific, based on experimental research. You could probably "eye-ball" the measurement of knee joint center close enough to arrive at a good boot fit since I know of no scientific evidence that this measurement needs to be that exact in order to optimize skiiing efficiency and comfort.
  5. Indeed.

    Genius....... I'm just guessing, but I reckon his net ground reaction force isn't acting directly through a bisection of his knees relative to his feet at this point of the course.

    When you fit the ski boots do you have them standing on a flat, horizontal surface? At what point on a ski slope is the surface flat and level?

    New game, you know how on the old pools coupons you used to "spot the ball"....spot the centre of mass. Indicate your answer with a X.

    Attached Files:

  6. Starter for ten:

    Attached Files:

  7. efuller

    efuller MVP

    With turns, there has to be some horizontal force from the ground. This will change the direction of the net ground reaction force vector. The sharper the turn (acceleration) the more the lean. If the ground reaction force vector does not go through the center of mass of the body, the body will fall over. So, the interial forces and body weight will go through the (aligned or not) leg and foot of the skier.

  8. Good one Simon.

    Here's a slide I made during my Biomechanics Fellowship in 1985 for a lecture I gave on "Downhill Ski Biomechanics".

    The center of mass of the body, (not the knee), is probably well-aligned with the snow reaction force (SRF) vector acting on the downhill ski edge. Knee positioning is used to more properly align the ski edges to achieve the optimum turning angle and forces for any instant of the skiing turn. In other words, the knee joint center only briefly aligns directly over the 1st or 2nd metatarsal head during the initiation and terminaton of a turn in alpine skiing.

    Attached Files:

  9. What part of his body are you estimating for center of mass, Simon?.....His Vibram One Finger??:rolleyes:

    Attached Files:

  10. You built up that slide the old fashioned way printing out labels on to cellulose and sticking them on.... you tell that to the kids of t'day and they wouldn't believe you :drinks
  11. Yeah.....and I had to walk five miles through snow uphill just to get to the podiatry school to do so....

    Aren't PowerPoint and computer wonderful inventions....now I carry my whole lecture on a little stick...who wudda thought 27 years ago??
  12. I'm just guessing there's a lot of mass in that region. There's got to be. If you're sliding down a mountain at the speeds this guy used to hit without falling over, you're centre of mass has got to be low and you need balls of steel, so I'm just putting 2 + 2 together.;)
  13. I remember doing my first ever powerpoint- amazing. But back then it was a case of printing the powerpoint slides to acetate. When I got access to digital projection, the use of multimedia gave me access to what I'd dreamed of.... the Royal Institution Christmas lectures (started by Michael Faraday in 1825 and broadcast on UK TV each Christmas) were always my motivation. Combining multimedia with live demonstration. Even with several hundred in a lecture room, you've got to engage with the audience and break it up.

    I also remember sitting through physiology lectures as an undergrad- we had a guy that would literally have a number of reels of acetate, sit down, sweat a lot and simply crank the handle to wind it up as he talked at us for hours upon end. We used to judge the lecture by how many reels of acetate he walked in with and decide whether we were going to sit through them all. Lecturers that sit down...:confused: What, you can't even be bothered to muster the enthusiasm to stand while you talk at me and wind that acetate?:bang:
  14. efuller

    efuller MVP

    And before powerpoint there was aldus persuasion. It was so exciting to get the new technology of a laser printer that could print to Kodachrome. Then we would have to walk downhill through the snow to get it developed. A lot of obsolete technology.
  15. Orthican

    Orthican Active Member

    "instantaneous" would be the word I would use to describe the point at which the center of mass is over any one point in the knee at any one time during most activities except quiet standing. And even then there are micro movements occuring that are resultant from the muscular contractions needed to maintain upright standing.

    None of which means anything to what you are trying to do.
  16. Frederick George

    Frederick George Active Member

    Dear Stephen

    It have treated skiers since 1978. US, Canadian, New Zealand and Austrian national team members as well as local teams, instructors, patrolmen, and recreational skiers. I practiced for many years in Squaw Valley, which was especially convenient when the Olympic Training Center was there, before it moved to Boulder. I first came to New Zealand, where I now practice, as a US Ski Team doctor. I guess I should thank the ski team for my discovery of paradise.

    Now, perhaps for some useful boot fitting tips:

    There have been many calipers, levels and lighting systems to calculate boot angle or canting (an old term). They fail for one reason, especially with elite skiers. The skier can position his foot with stability and balance in any position he feels like. This changes the "center of mass," or knee position, or edging angle.

    The skier stance is the "ready position" in other sports, when the foot (subtalar joint) is in neutral to inverted, and the foot is in its "rigid lever" position. This is when the skier can transmit pressure to the inside edge precisely. Now, almost every elite skier has a rigid, high arch or cavus foot. So his foot will stand naturally in this position. Hence his advantage in balance and edge pressure. Recreational skiers, instructors, and some coaches will have the common low arch pronated foot. This foot doesn't transmit pressure easily, and the skier will have to rely more on the cuff of the boot to steer the ski, so his turns are a series of corrections, more octogonal than round.

    This is getting long winded. So: The footbed/orthotic corrects from the ankle down. The cuff corrects for the leg and knee.
    Never overcorrect (too much outward cant) a racer. They won't be able to angulate and hold an edge. They will hook and grab, chatter and skid. They need to be inside the turn at the speeds they are going.
    Downhill racers can use a little more cant, just enough so they can ride a flat ski in the tuck. But better to be knock kneed than bowlegged. You can check this on the videos. A flat ski isn't directionally stable, it tends to windshield wiper, but it's fast.
    Recreational skiers with pronated feet like more cant in the cuff. It gives them easier edging and the feeling of control. They don't have much foot feel anyway. They ski on the inside edges when shussing.

    Well, enough for now. If you have some specific questions maybe I can help. At least I can be more focused. Skiing technique/biomechanics is a fascinating subject.


  17. Boot Dr

    Boot Dr Welcome New Poster

    A knee mass caliper is good point of reference to start with. It is not the end all. In performance skiing and racing it is a must or at least a plus to skiing with less unnesseary stress and forces that compromise clean and more precise and safe fast skiing. Think of a sports car or race car. If alignment is off driving is harder and creates more wear. Get aligned with accurate canting of your ski boots.

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