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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
calculation.
-*--
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
great!
-*--
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
time.
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
complete.
(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
off).
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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