BioRow - Tesing using footplate strain gauges?

[thomas....@googlemail.com]

One for the RSR heavies, I was interested to read the recent newsletter from BioRow where they apparently worked with the Danish national squad (lwt 4- I would imagine so a world leading crew at the moment) to investigate the observed result that the further to the bow you are in a crew, the less power you produce on the water compared to on the erg

http://www.biorow.com/RBN_en_2012_files/2012RowBiomNews04.pdf

I can follow their conclusion where they talk about the different "roles" that rowers have in the boat (i.e a stroke usually gets there head down and pounds out the power where as a bow usually works more on the balance) but I got lost with some of the other conclusions that were made, so wanted to see if anyone can clarify it more for me! For example what I am struggling with are:

1) I understand their example model of two connected ergs with one rower and one box (ive been in boats that felt like that :) ), however I am not sure that really shows or is proof that the strokeman gets the benefit of "free" power as the other rowers are passing their power down the boat, surely if the box was at bow in the example diagram and the rower at stroke it would also slide sternwards and display a force?

2) The conclusion that the "fairest" way to test rowers is by using footplate pressure. Firstly I would have thought the pressure on the footplate would be less important that the power curve taken from the blade/gate since that would be a "more direct" measurement of the force moving the boat. Also surely the "free" power mentioned above would also affect the pressure on the footplate as well?

Hopefully someone can help provide clarity to me on this! :)

[Tinus]

The problem is in the difference between applied power and delivered
power. Many simple models ignore the energy which is in the movement of
the rower and boat system. The energy from the rower is used to move the
boat+rower and to pull the oar. If you only measure the force on the oar
then you won't get to know the energy used to make the boat+rower movements.


In the beginning of the drive the energy in the movement of boat+rower
needs to increase (which requires power from the rower), but at the end
of the drive the energy decreases again (which can be used to propel the
boat and allows the rower to supply less power).

1) These energy fluctuations are transfered from bow to stern and have
the bowards bowards require more input and have the sternwards rowers
gain more output.
2) By only measuring the forces at the handle these energy transfers
mentioned in 1 are unnoticed (you measure the energy supplied to the
handle but not the energy supplied or gained from the rower-boat
movements). Therefore fair testing required footplate pressure
measurements as well.

[Carl]

I'm intrigued by this talk of power transference along the boat. What
is the mechanism? Are we saying that by one rower giving the boat a bit
more of a kick in one direction or the other he transfers energy up the
boat to other rowers, or conversely extracts energy from them?

What one generally finds is that where members of a crew row with
different dynamic characteristics (i.e. different patterns of force
application to the hands & the stretcher) the crew's performance is
inhibited.

Stroke is often the most forceful member of a crew, for rather obvious
selection reasons, so may well be the most powerful (in a four) & is
also going to stamp his mark on how the crew rows. If other crew
members are having to compromise their actions slightly to match
stroke's, one should not be surprised if they experience a greater fall
in their in-boat work rates relative to their erg work rates. And
whoever is steering might suffer a further fall.

We see that, although his handle load is if anything lagging behind the
rest of the crew at the catch (curve a), this stroke's seat moves
significantly sooner (curve b) than the others' do. This means that he
is booting the boat sternwards by relatively more than the rest of the
crew because he is loading his stretcher sooner while loading his blade
later. I don't see that being much help to his colleagues' work
application.

Stroke, with this punchy action, is also ceasing the movement of his
legs & pelvic area much sooner than the other guys (curves b & c). And
his seat accelerations (& therefore those of his lower body) are way
more extreme (curve d) than theirs.

For those reasons I'd have thought the marked inertial disparity of
stroke's movements compared with the rest of the crew were adversely
affecting the performance of the other guys, despite his higher handle
forces. So I don't think power is being transferred but, rather, that
it is being reduced in some of the crew, & that one should first
consider this as more significant than any putative power transference.

Cheers -
Carl





--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
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[Tinus]

On 05/26/2012 08:55 PM, Carl wrote:
> I'm intrigued by this talk of power transference along the boat. What is
> the mechanism?

Technically it is not power transfer. It is only power transfer from the
point of view of the measurements based on handle forces.

What I am wondering about is whether the proposed mechanism isn't
working the other way around. The example of the connected ergs is
misleading. They have the box in the stroke seat and the rower
generating the power in the bow seat. However the principle works the
same the other way around.

I like to use the words sowing and harvesting to explain the two stages
in the drive to rowers. First you put energy into the system. Only in
the end of the drive this energy goes into the speed of the boat. I
believe in this example the stroke advantage is in being the first to
start harvesting. His seat goes to zero speed much earlier.

[Tinus]

On 05/26/2012 11:48 PM, Tinus wrote:
> I believe in this example the stroke advantage is in being the first to
> start harvesting. His seat goes to zero speed much earlier.

Another effect which I missed is the stroke early
deceleration/acceleration at the catch allows him to absorb a lot of the
kinetic energy from the boat.

The person who starts accelerating (relative to the boat) first is
contracting the muscles eccentricly and _absorbs_ energy. The boat
acceleration make the others, who did not push the stretcher, accelerate
(relative to the boat) as well but they are a little bit behind and need
to apply force to the stretcher as well. However pushing at the later
stage is concentric contraction of the muscles and means _delivery_ of
energy.

So the ones who start acceleration early are having a benefit _two_
times in the stroke. They are the ones who absorb the energy before the
catch and before the release, the others are the ones who put the energy
into it.

I may start to rethink the positions of the current crew I am coaching.
The stroke is the strongest but not the best in these plyometric pushes.
Although for the moment I will try using adaptation of technique. I
don't believe the stroke is always the initiator of everything in the crew.

[Carl]

Eccentric contraction only works if there is a force available for the
purpose. However, the popular presumption in rowing that we use
pressure on the stretcher to decelerate our travel along the slide is
largely unfounded - its only consequence would be to severely decelerate
the boat. In reality, we pull less hard with our feet & legs as we
approach front-stops while the continuing fluid drag on the boat does
the rest.

There is no kinetic energy to be absorbed from the boat. Only if the
boat were being kept moving at constant speed by some external force, or
if stroke were coming forward with reckless violence. would eccentric
contraction (as you get it on a fixed erg) be possible.

Those curves do show that that stroke is punching his legs markedly
before his blade is loaded (see the seat acceleration) but not, I think,
that he is in any way in a position to gain from eccentric contraction.

[Tinus]

But there is some energy in the motion of the boat and crew relative to
each other. That, is indisputable.

This energy is fluctuating and the crew is alternatingly putting energy
into it or getting energy out of it. Only the latter part, how much
energy can be recovered is disputed. However, the idea that energy ís
discovered is well proven. Also without the energy recovery part, the
part of supplying energy still works. No matter how you look at it or
how you define the terms, the stroke rower, who starts the motion first,
will have a benefit.

[Tinus]

On 05/28/2012 12:13 AM, Tinus wrote:
> However, the idea that energy ís discovered is well proven.

discovered = recovered

[Tinus]

On 05/28/2012 12:13 AM, Tinus wrote:

>> Eccentric contraction only works if there is a force available for the
>> purpose. However, the popular presumption in rowing that we use pressure
>> on the stretcher to decelerate our travel along the slide is largely
>> unfounded - its only consequence would be to severely decelerate the
>> boat. In reality, we pull less hard with our feet & legs as we approach
>> front-stops while the continuing fluid drag on the boat does the rest.

I believe this is false. All data I have ever seen shows that rowers
push the stretcher in order to get to move bowards relative to the boat.

The fluid drag on the boat is only a part of the equation. If you want
to get the catch as fast as possible, which is beneficial for high
stroke frequency, then you don't want to rely on fluid drag only and you
will push the stretcher. Severely decelerating the boat... it happens
any way but it the rower does it instead of the fluid drag then it
happens faster (not more severely) and the next stroke starts sooner. It
is not a loss of energy since the human body is well capable to reuse
energy from eccentric contractions.

[Carl]

I think you misunderstand the role of fluid drag here? But I do agree
that discussing the stroke process is much complicated by all the
instructional misconceptions which pervade the literature and folk-lore
of rowing

Drag will never get you to front-stops. To the contrary, drag on the
moving boat tries to keep you at backstops by decelerating the boat
beneath you.

To get an eccentric contraction in the rowing action at the catch, there
has to be a substantial relative velocity between you & the boat, & then
your legs must be compressed onto the front-stops by the inertial
reaction between your moving mass & that of the boat. But how can that
happen? Sure, you get it on a fixed erg, because the erg cannot move.
But it can't happen on a dynamic erg. Nor can it happen in a boat -
unless 1 person does things which are completely disruptive to the rest
of the crew. Yes, the crew does have substantial mass, but we've all
known crews which have been vile to sit in because one person's shooting
their slide at the catch.

What we do see in those curves is that stroke moves his seat before the
rest of the crew, but does not load his oar any sooner that they do. So
he is booting the boat (& crew) astern to accelerate his own mass
relative to theirs. That demands positive, premature input by him. It
provides no opportunity for an eccentric contraction, but it does
disadvantage his crew-mates by applying a sudden pull to their feet just
before they want to take the catch.

I still think the explanation of those curves is simple: I see a highly
motivated & strong stroke-man compromising his own technique by his
determination to drive on his crew.

[magnus....@gmail.com]

The way I visualise it, anyone “kicking off” before the others must reduce effectiveness of the crew.
I visualise it thus:
The shell is a more-or-less rigid and lightweight frame, and is the only thing which connects the rowers physically together. The rowers propel the system by simultaneously jumping off this frame while being resisted by force at the end of the lever (and balanced out by oars on the other side).
In practice though, even in the best crews, some of the “kicking off” from the frame occurs before the blades are fully resisted at the water, and some very brief but severe check of the shell takes place.
If any person in the crew, stroke or otherwise, kicks off before everyone else, it has the effect of pushing the jump-platform sharply away from the rest of the crew who are still compressing for it, thereby wasting their compressed position and destabilising the start of their own kick-off.
I liken it to a ski-jump: the timing of the kick is millisecond-critical, and if the lip of the ski-jump platform is suddenly moved away from the skier just at the moment of jump, a very poor jump ensues.
A crew should always strive to compress and kick off exactly together.

Magnus

[Tinus]

On May 28, 1:24 pm, Carl <s...@sss.jjj> wrote:
> To get an eccentric contraction in the rowing action at the catch, there
> has to be a substantial relative velocity between you & the boat, & then
> your legs must be compressed onto the front-stops by the inertial
> reaction between your moving mass & that of the boat.  But how can that
> happen?

If you look at the velocity curve of the boat speed then you see there
is a substantial relative velocity between the crew's CM and the boat
(about -1.5 m/s during recovery). The fluctuation in this relative
velocity is very strong around the catch and the boat deceleration is
much higher than what can be explained from fluid drag. Say a four
with ours weighs 60kg and experiences a 290N force, then the
deceleration would require 0.3 seconds. That's fine for easy rowing
but not for racing (the biorow data shows this acceleration happens in
0.1 second).

> But it can't happen on a dynamic erg.

It can happen especially on a dynamic erg since it doesn't experience
fluid drag and the movement back and forth is solely due to the rower.
It is only that the power eccentricly stored is much smaller compared
to what people know from static ergs. I know for women this power from
eccentric contraction has been measured and is about 10-20 Joules.

> What we do see in those curves is that stroke moves his seat before the
> rest of the crew, but does not load his oar any sooner that they do.  So
> he is booting the boat (& crew) astern to accelerate his own mass
> relative to theirs.  That demands positive, premature input by him.

It does only demand positive input by him if he is accelerating to
increase the speed _away_ from the stretcher.

In the Danish example the stroke is applying force to accelerate while
his speed relative to the boat is decreasing. This does not require
power (and may even gain him power).
The others are applying force to accelerate at a later stage, while
their speed relative to the boat is increasing. That, indeed, requires
positive input by them.

[Tinus]

By the way, I agree that the effect is much smaller than what is
needed to explain the difference in applied power between rowers in a
crew. Also this Danish crew is only a single example. And finally it
is not certain whether the effect is beneficial or whether the action
to induce the effect has other effects as well like for instance
disturbing balance or efficient application of power.