RC: Re:overweight an issue?

["Jana Crewett" <jfcr@ceh.ac.uk>]

Hi Wolfgang,

thanks for the URL - I've only quickly scanned the webpage so far but
it looks like it will make very interesting reading.


Hi John,

sorry but you didn't get my point. Neither do I know any "truck
drivers" (mainly because we've got lorries, not trucks, over here <g>)
nor do I wish to fit anyone into a stereotype (and I've got nothing
against men whatsoever btw - on the contrary!). All I wanted to point
out was that, while speed appears to be linked very strongly to rider
weight alone, there *might* be more factors that influence speed and
that just happen to be correlated to rider weight.

Common sense tells us that it's harder work for the horse to have to
carry more weight, so it's pretty obvious that rider weight has *some*
effect on performance and speed. However, I was trying to point out
that, in the South African study, weight might have found to have such
a strong influence because other, non-identified factors (like age,
competitiveness, smaller sample size in the heavyweight division, or
whatever) were in fact also contributing to speeds and enhancing the
effect of rider weight, because these factors happen to be correlated
to rider weight to a certain degree.

Let's not risk upsetting any more groups of endurance riders (and I
thought I'd already chosen pretty innocuous examples! ;-P) and take an
example of my own current line of work: Here in the UK, we find that
the average rainfall in a location is very strongly correlated to the
distance between that location and Lyon (France). You can write down a
nice little equation, telling just how much drier it gets for every
50km you get closer to Lyon (without crossing the channel, this is
strictly only valid for the UK). Now, although the equation gives you
very accurate predictions, it is not a "real" causal relationship but
is solely caused by underlying factors. UK geography and climate are
such that the north-west is wettest and the south-east is driest, so
while there seems to be a nice clear cut-and-dried relationship
between the distance from Lyon and average annual rainfall, the real
reasons have nothing to do with Lyon or with distances, but are due to
something completely different.

I was proposing that there might be something similar happening in
the relationship between average speeds and rider weight on endurance
rides. Of course here we *do* have an underlying causal relationship
(at least that's what common sense tells us) between rider weight and
speed, but we also have data (the Tevis study) that apparently
contradicts common sense in this respect. So, I just expressed the
thought that there might be underlying factors which have worked to
*enhance* the influence of rider weight on speeds in the South African
study and that have for some reason worked the other way round and
*decreased* the influence of rider weight on speed in the Tevis study.
Identifying these factors is an interesting task for further research
- so far, there have been lots of suggestions ("difficulty" of the
ride, size and type of the horses ridden by heavyweight riders, age,
competitiveness, ...) and AFAIK some of these factors are already
being looked into - Susan is now gathering data from flatter, faster
rides, Wolfgang is looking a bit more closely at the South African
study to try and extract more information from it and Tom Ivers had
already posted a controlled treadmill experiment which can quantify
the effect of weight on speeds and keep all outside factors constant
as it's a carefully designed experiment, not field data.

(Tom, sorry I deleted that particular post - could you, or someone
else who still has it. please send me the reference again? My email is
jfcr@ceh.ac.uk - thanks!!)

I hope that clears it up a bit?


Jana (who always forgets how easy it is to get misunderstood in this
medium...)