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Thread: Trying to physically verify equation for Spring Mass model

  1. #1
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    Trying to physically verify equation for Spring Mass model

    I wanted to experimentally verify the step frequency’s (SF) dependence on Body Mass (BM). It appears in an equation I use to estimate leg-stiffness (kleg) for scenarios that I run through the model.

    The expression is for the frequency of a mass bouncing vertically on a spring. This is similar to a subject running in-place with close to zero aerial phase.

    The equation,

    SF=(1/(2*PI))*SQRT(kleg/BM)

    implies that if the BM increases, the SF should decrease according to 1/SQRT(BM).

    If a subject while running very slowly, adds 20% to their BM using a weight jacket, then their step frequency should decrease by SQRT(1/1.2) or 8.7%. .

    I have tried this myself, but I don’t have access to a weight jacket. I could only hand-hold an additional 10% of my BM while I ran slowly. Then, my SF dropped by 3%; I expected it to decrease by 5%.

    I have looked for research on this issue, but I could not find any. Has anyone ever worked in this area? Or, are there any published articles on this topic?

    Ted

  2. #2
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    Re: Trying to physically verify equation for Spring Mass model

    I would guess it's because the weights are not near the centre of mass, Ted. You are, in effect, using halteres:

    https://www.youtube.com/watch?v=-K-0PU9K-Js

  3. #3
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    Re: Trying to physically verify equation for Spring Mass model

    Ted,

    Could your leg stiffness have increased by 3.5% to go with the increase in mass of 10% to give a resultant 3% increase in SF?

    Cheers
    Allan

  4. #4
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    Re: Trying to physically verify equation for Spring Mass model

    Chris: Thank you for the video link. Marcus Essa is truly an impressive athlete. When I did my initial experiments I held the hand weights tightly against my ilium, so there would only be a small but --- I am sure present -- halters effect.

    Allan: I am not sure that bio-mechanical leg-stiffness could be related to the BM. All the physical systems I have dealt with followed Hooke’s Law. That and Newton’s equations are at the foundation of the spring mass model.

    SFvsBM.png

    I redid this experiment several times. With a weight belt tightly secured around my ilium, I able to get SF measurements (blue diamonds) that agreed with the theoretical values (red squares). I was surprised that the results were so close – experiments have NEVER worked-out that way in the past!

    If anyone attempts do this experiment, they may want to use an 8-pocket weight-belt with 2 lb. (4.4 Kg) weights. They should also add extra supports or blocks under their tread-board to keep it from flexing during the stance phase. I performed my experiment at 3 mph (1.33 m/s, 20 min/mile, 12.4 min/km) just above the walk-run transition speed.

    Thank you for your comments.

    Ted

  5. #5
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    Re: Trying to physically verify equation for Spring Mass model

    Hi Ted
    I wondered if you had looked at this thread on the site and if so what you thought of the ideas -



    Ground reaction forces and gait

    I wonder if shortening the effective arm length during gait would increase step frequency -just a hunch . With regard to your first post ,would holding weights reduce arm swing amplitude compared to a non weight holding set up .

    I don't have the expertise or equipment to look the idea of pelvic acceleration feeding energy into the bodies pendulums (arms legs ) to produce body segment co-ordination but if you did it could be interesting .

    Regards

    Gerry

  6. #6
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    Re: Trying to physically verify equation for Spring Mass model

    Hi Gerry,

    Yes I had followed at that very interesting discussion. Physics and modeling mechanical systems is my area, so part of discussion was beyond me.

    The SM model functions in the sagittal plane, so it does not include rotational considerations about the vertical axis. I could not guess on the coupling between arm length and step frequency. I believe that the hip-hugging weight belt avoided that issue.

    Thanks for your question.

    Ted

  7. #7
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    Re: Trying to physically verify equation for Spring Mass model

    Hi Ted, We did not address this issue directly, but we do present some data in the paper below that may be of help to you. We had a variety of conditions where we varied body weight and body mass independently during treadmill running (the bodyweight increase condition would match closest to what you are doing, I believe). Best of luck on an interesting problem!

    Chang YH, Huang HW, Hamerski CM, Kram R. (2000) The independent effects of gravity and inertia on running mechanics. J Exp Biol 203: 229-238.
    http://jeb.biologists.org/content/203/2/229

    cheers,

    Young-Hui
    *****************************************
    Young-Hui Chang, Ph.D.
    (FYI: it's pronounced "young-hee")
    Professor, Associate Chair
    School of Biological Sciences
    Georgia Institute of Technology
    Atlanta, GA, U.S.A.
    https://biosci.gatech.edu/people/young-chang
    *** "learning how we get from here to there" ***

  8. #8
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    Re: Trying to physically verify equation for Spring Mass model

    Hi Young-Hui,

    Thank you for the link to your publication. It was an interesting read.

    I was fascinated by the way you employed the climbing harness with the lead strips along with the elastic chord and winches to control the trolley. I think I can I can use that idea.

    Ted

  9. #9
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    Re: Trying to physically verify equation for Spring Mass model

    Interestingly, I see the effects on walking are different:

    https://www.researchgate.net/profile...ication_detail

  10. #10
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    Re: Trying to physically verify equation for Spring Mass model

    Hi Chris,

    The first line of the abstract you listed led me on an enjoyable excursion to the ISS and an examination of the COLBERT or T2 in Node 3 (near the head).

    Karen Nyberg does a great presentation at:

    https://www.youtube.com/watch?v=_ikouWcXhd0

    I knew there was a problem with the harness when “Suni” Williams ran Boston on the earlier TM in ’07.

    https://en.wikipedia.org/wiki/Sunita_Williams

    She gives a great tour of the station at:

    https://www.youtube.com/watch?v=FXv9AZl3fw4

    Doug Wheelock had some interesting comments on the nerve ending on the bottom of a crewmember’s feet:

    https://www.youtube.com/watch?v=8scFoH36EmY

    I worked on reentry analysis, so I was interested in the video on the ride home on the Soyuz.

    https://www.youtube.com/watch?v=-l7MM9yoxII

    I hope some may enjoy the above links.

    Ted

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