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SUMMARY: Safe momentum to catch

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  • SUMMARY: Safe momentum to catch

    To: Biomech list
    CC: UK trampoline coaches' list

    I would like to thank all those who replied and helped me with this
    question. Here follows a summary of responses and of my subsequent


    Nartha Nichols-Ketchum quoted the NIOSH lifting equation, giving some detail
    on the matter:

    I'm not sure the reason for your question. I agree that practical
    experience shows, as you state, for all but the most trivial situations, the
    spotters cannot prevent a fall. That is why the trampolining community went
    to the use of overhead spotting rigs for advanced students.

    In regard to your question, you might look at the NIOSH lifting equation,
    which sets a recommended limit of 51 pounds for lifting (and states that 150
    would cause injury to the majority of adults). Or manual materials handling
    texts, which state that the upper 10% of adult male industrial workers
    should be able to safely lift 27 kg, and the lower 90% about 22 kg. Pushing
    over short distances is limited to 68-31 kg (male) and 33-18 kg (female).
    Carrying is limited to 27 kg for males and 20 kg females, based on spinal
    compression criteria.

    Ref: A Guide to Manual Materials Handling, 2nd Edition, A. Mital, A.S.
    Nicholson and M.M. Ayoub. 1997. Taylor & Francis.


    Leanne Dwan suggested reference to ergonomics and lifting guidelines, and
    offered the following experience:

    manual handling guidelines eg those used in nursing homes for transferreing
    patients. I used to work in a nursing home and the policy there was that if
    a patient was falling while walking the assistant (spotter) should only
    attempt to guide the patient to the floor when possible; ie slow down the
    motion as much as possible to decrease the possibility of injury. The
    assistant was advised not to try and catch the patient if it may put them at
    risk of injury. Hope this is helpful.


    Pat Parker again suggested the NIOSH lifting equation but also added some
    experience as a Judo coach:

    the first thing that pops into mind is the NIOSH lifting equation. Bear in
    mind that the NIOSH equation is not specifically meant to handle this
    specific situation - but it may give you some guidelines. Basically the
    NIOSH equation says that 51 pounds (23 kg) is the maximum lift that can be
    made a single time under perfect lifting conditions without significantly
    increasing chances of injury. Perfect lifting conditions for the purposes
    of this equation would include slow, symmetric, balanced load close to the
    body with good coupling and no repetition. As conditions deteriorate the
    safe load decreases from 51 pounds toward 0.

    Again, this equation is for lifting and not for catching - but you can
    expect a maximum safe "catch" to be less than this 51 pounds because the
    target is moving, assymetric, unbalanced, and there is poor coupling between
    the spotter and the faller.

    Also I can give you some idea as a judo coach. It is very difficult and
    very dangerous to attempt to catch or slow down a falling person. We try to
    teach the thrower only very specific, very limited ways of slowing the fall
    and we try to teach the falling person not to cling to the thrower's body.
    This resuces injuries due to people trying to catch falling people.

    Just as a point of interest, I'd like to hear more about the practical
    experience that you mention that leads you to believe that spotting a jumper
    does more harm than good.


    Doug McClymont, organiser of a (New Zealand?) national championships
    commented on the subject:

    Hooray, a realist!

    I was involved in trampolining for a few years and acted as
    floor manager for our national champs not so many years ago.
    At that time I questioned the need for spotters, using
    similar agruements to yourself.

    The response I got from the trampoline experts was
    non-scientific, and not very helpful either, but my
    observations over the years have indicated that very rarely
    do trampolinists lose their lateral accuracy, but then the
    major job of the spotter is simply to break the fall of an
    errant flyer.

    Might save the athlete, but your research project I think
    will indicate that damage to the spotter is possibly just as


    Jim Walton, an ex-top class trampolinist carefully examined the question and
    picked up on my loose use of trampolining terminology! His response was
    careful and long, so I will pull out extracts that are directly relevant to
    the issue of spotting from the floor (as opposed to a coach manually
    spotting on the trampoline, or a spotting rig, etc):

    How do you define "harm" and "good"? My own, fairly extensive experience,
    taught me that the presence of spotters did a lot of good ... in schools,
    at home (I have a Nissen "Goliath" model), and in competition. For sure,
    there were times when an inexperienced spotter tried to "catch" someone
    performing a skill that was beyond their level of expertise--and either
    the performer or spotter, or both, suffered the consequences. But the
    problem was not "spotting" per se, but inadequate coaching/supervision.
    In a word, it's inappropriate to tell someone to catch a flying cannon
    ball with flailing appendages. On the other hand, a well-placed coach
    can EASILY manipulate a performer in flight, providing both the performer
    and coach understand who is doing what ... and when. Then again, I
    would not expect a coach to make an attempt to manhandle an adult male
    learning to perform a fliffus--that's why they created spotting rigs!
    It's all a matter of degree and a lot of experience ... and knowledge
    of what is, and what is not, appropriate. If you do stupid things,
    expect painful consequences!

    With advanced somersaulting or twisting skills, spotters a) should be
    knowledgeable and experienced in "manhandling" performers (that's the
    best I could do), and b) should SUPPLEMENT a more substantial support
    mechanism--an overhead rig, for example.

    This is a subject of some interest to this community ... if you turn the
    problem around, it becomes a barrier impact investigation. But again,
    your question needs (significant) qualification. Ask a pitcher, a
    professional wrestler, a football player, or a diver ... ask a car safety
    engineer, a jet pilot, or an astronaut ... I'm sure you'll get quite
    different answers from each and every one. If you try to tackle the broad
    topic you've described in the sentence above, you'll be at work for a long

    He concluded by recommending I make:

    1) A more refined statement of your problem,

    2) A more objective way to evaluate your basic premise, and

    3) A clear statement of what you hope to achieve when your research is

    (Jim subsequently replied with more of his memories and surprise to find out
    the sport is alive and well in the UK!)


    So a little about what I am trying to achieve, and the back-of-an-envelope
    calculation I was then able to make.

    I am attempting to encourage the trampolining community to discuss the issue
    of when and where spotters should be used. Some countries currently
    stipulate number of spotters to be used at any particular time. It is
    interesting to note that on visits to IDHM competitions (Germany), for
    example, no spotters were required.

    The main strand of research is a questionnaire aimed at coaches classifying
    their performers' falls from the sides of trampolines. The results at the
    moment show that such falls are incredibly rare, and resultant injuries are
    rarer still. It also asks about injuries sustained by spotters in assisting
    the falling performers, and whether the spotters mitigated or worsened the
    falling performer's injury. The results from this show the discussion is
    very necessary, but the sample size is currently too small to draw any
    serious conclusions.

    Looking at the problem from a 'safety of the spotter' point of view was
    intended to wake us (trampoline coaches) up. Many of us have never
    considered whether floor spotters are helpful, for no better reason than
    they have been mandatory in safety guidelines, for the last 50 years, and
    taught on coaching courses.

    Using the responses received I was able to specify a hypothetical situation,
    and calculate the required force.

    An adult male spotter is required to assist a falling adult female
    trampolinist. This trampolinist is falling from a CoM 2m above the
    trampoline (ie no bouncing height, as if she has just tripped or stepped

    Assuming the performer is going to land on her head, the spotter is required
    to push the performer around 90 degrees. No slowing is required. In a
    straight position, the performer has a ball park MoI of 20kgm/s2. The
    performer may be assumed to be at a manageable height for 1m of her descent
    (a generous estimate). Assuming the second metre of the descent, she will
    be at a manageable height for .185s (before any support).

    To rotate the performer 90 degrees using a constant force over this time,
    requires an angular acceleration of 91.5 rad/s2 over this time. Applied at
    the shoulder (.5m from CoM), this requires a force of 3660N, assuming
    optimum coupling. It also requires that the spotter get in position during
    the .45s of the first metre of descent, although longer is available for
    falls from height.

    The best case weight an adult male industrial worker could carry was 27kg, a
    lifting force of 270N. Even a performer falling to head from just stepping
    off the side of a trampoline, requires a 3660N force to rotate 90 degrees.
    This 3660N force is of course absolutely huge and so if it was in reality
    applied, directed upwards, (eg. holding the shoulders up), it would 'catch'
    the performer. The calculation then leads onto ifs and buts, but the size
    of the force required is useful information, particularly since many coaches
    say 'don't catch, reach for a shoulder and hold it up' - I don't have
    figures for arm strength but I guess for that kind of force, you need get
    your body directly aligned along the force's line of action, and hope!

    In reality, trampolinists are generally very good at controlling their
    landing, even if leaving the trampoline; this kind of calculation misses the
    point that a spotter's push can prevent a performer using their own control
    to land where they want.

    As Jim Walton pointed out, there are far too many variables, not least the
    difficulty of handling a performer with flying appendages, for this kind of
    calculation to accurately model the capabilities of spotter.

    But it does give an idea of the kind of force required to assist a falling
    performer, and shows that even this best case scenario exceeds lifting
    safety limits by an order of magnitude.

    It is not my opinion that spotters should be removed completely. However,
    particularly given the response times required, I believe they should only
    be placed in danger when a fall is likely, so that they are completely
    concentrating at the time, and only be used if there is an advantageous size
    difference. Where no suitable spotters are available, or the spotters are
    not paying full attention, it is better not to use any spotters at all.
    Floor mats are a helpful measure if available. Otherwise, spotters should
    be kept out of harm's way and all effort put into safe progressive coaching!

    I would like to thank the members of the biomech list for their help, and
    hopefully the discussion among coaches will continue.

    Richard Ollerenshaw

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