Power And Body Weight

[old] Jez · Post by **[old] Jez** » January 16th, 2005, 10:26 am

<table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td class='genmed'>QUOTE </td></tr><tr><td class='quote'>The moving mass of the rower is reduced to the seat. Head shoulders and hands are moving over a greater distance than the seat, thighs and knees over a smaller distance and feet are not moving at all. Rough estimate: considering a rower of 80kg, the moving mass is m = 50 kg. The kinetic energy stored in the rower when moving at maximum seat speed is: E=0.5mu^2=0.5*50*2^2=100 J During the recover: as the rower slows down his seat speed this amount of energy is completely dissipated (in the rower’s muscles and articulations), as there is no other possibility of energy transfer. During the drive: At least a part of body’s kinetic energy is transferred to the flywheel but it is very difficult to estimate what part."  </td></tr></table> First of all, this section strikes me as highly dubious, since it completely neglects the square dependence on velocity of k.e. - a 2kg mass travelling at 1m/s has only half the kinetic energy of 2 one kg masses, one of which is stationary and one travelling at 2 m/s. However, this attempt to estimate the magnitude of the effect under discussion only really affects how big the effect is, not whether it exists, so I'll leave it for now... It is clearly true that the kinetic energy of the rower during the drive must dissipated, either 'wasted' as heat etc. or be transferred to the flywheel (indeed this is the essence of how to row strapless, stopping the body by maintaining tension in the chain). However, this is not an advantage to having a heavier rower, since that kinetic energy had to come from somewhere - and guess where that is, yes, the rower... So if the heavier rower has greater kinetic energy during the drive, that is because he has done more work - hardly an 'advantage'. Secondly, if the kinetic energy of the heavier rower is greater during the drive, presumably it also is during the recovery, and definitely none of that kinetic energy is transferred to the flywheel. My impression of the above reference is that it is simply a discussion of the best recovery profile to minimise this wasted energy - nothing to do with the effect of varying rower mass. Incidentally, the section on an athlete in a boat is complete garbage - it says 'we assume no external forces' i.e. completely neglecting fluid drag, which some might argue is somewhat crucial to any analysis of optimising recovery technique in a boat

[old] Physicist · Post by **[old] Physicist** » January 16th, 2005, 10:32 am

Remador, all seems perfectly reasonable. This piece only allows for a 'part' of the body's kinetic energy on the drive to be transmitted to the flywheel: I assumed perfect technique and put it all in Only through technical faults can this energy be lost (most obviously by catching your feet in the shoes.) At race pace though, nobody has perfect technique. The piece neglects the tension in the bungee and the gradient on the slide as well - both sources of energy at backstops that are lost during the recovery (the bungee energy is not mass-dependent actually, but the potential energy of the height difference is!) I must apologise but I've slightly lost track of the ultimate point of this discussion? If I recall correctly I originally posted here to refute the suggestion that extra body mass could be used to increase erg scores, which is a scientifically impossible suggestion that has reared its head on this forum before. We now seem to be discussing the opposite - that mass in itself has an extra energy requirement on the erg. I accept this fully! The piece you posted seems to just be a simplistic analysis of this effect.

[old] Ralph Earle · Post by **[old] Ralph Earle** » January 16th, 2005, 4:19 pm

<table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td class='genmed'>QUOTE </td></tr><tr><td class='quote'>What I [Physicist] was insisting was that this better performance is not a direct consequence of their weight. It is a result of the associated factors - larger muscle mass, taller, larger heart+lungs etc. </td></tr></table> The regressions controlled for height, and therefore also for any other advantage which results from being taller but not heavier. To the extent that larger hearts and/or lungs weigh more, that greater mass is subsumed by the coefficient for weight, independent of height. I assume that no one reading this thread thinks that in this context, "heavier" means "carries ten kilos of lard." Given that, does anyone not expect that crews of 6'1" elite oarsmen will usually be beaten in head races by crews of 6'5" elite oarsmen?

[old] John Rupp · Post by **[old] John Rupp** » January 16th, 2005, 10:36 pm

peek a boo

[old] John Rupp · Post by **[old] John Rupp** » January 16th, 2005, 10:38 pm

</div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td class='genmed'>QUOTE (jamesg @ Jan 7 2005, 10:17 PM)</td></tr><tr><td class='quote'>Remador, All you have to do is use the W/kg ratio, for a specified distance. Unfortunately the logs and race results don't require a specification of weight, so a vast amount of data is getting lost. The only race I took part in where results were on a W/kg basis was a 500m. There was no correlation between this ratio and weight, and little with age between 20 and 50. We saw values between 7 and 3. It seems to be an excellent indicator (maybe too good for comfort) of fitness and athletic ablity. This mostly because it removes the cube root inherent in pace calculations that try to make everyone look the same. </td></tr></table> James, Right on.

[old] PaulS · Post by **[old] PaulS** » January 17th, 2005, 12:16 pm

</div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td class='genmed'>QUOTE (Ralph Earle @ Jan 16 2005, 12:19 PM)</td></tr><tr><td class='quote'> Given that, does anyone not expect that crews of 6'1" elite oarsmen will usually be beaten in head races by crews of 6'5" elite oarsmen?  </td></tr></table> Cambridge seems to "expect" this exact thing each year for The Boat Race, it's worked out for them about half the time.

[old] Ralph Earle · Post by **[old] Ralph Earle** » January 18th, 2005, 1:10 am

Good point, Paul -- I should have said lightweight crews. The effect for heavyweights is small -- 3s/inch over 6K, and the Rsq was practically zero, which means almost anything and everything EXCEPT height and weight matters among heavyweights!. Does anyone know if Cambridge's recent crews were as heavy or heavier than Oxford's? I estimated that from 1993 - 2003, Cambridge's average margin was about 7s, or about 2" worth of height. But I infer from your comment that Cambridge's crews are noticeably shorter, not taller, than Oxford's.

[old] PaulS · Post by **[old] PaulS** » January 18th, 2005, 10:20 am

</div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td class='genmed'>QUOTE (Ralph Earle @ Jan 17 2005, 09:10 PM)</td></tr><tr><td class='quote'> But I infer from your comment that Cambridge's crews are noticeably shorter, not taller, than Oxford's.  </td></tr></table> Cambridge crews tend to be taller and heavier than the Oxford Crews, at least for the last few years where it was made a big deal of. The won by quite a margin last year, but hard to project anything when "demolition derby" rules are in play.

[old] remador · Post by **[old] remador** » January 20th, 2005, 4:30 pm

Physicist, I was just trying to stand my point that body mass is related with the effect of the stroke in the flywheel. You say that the kinetic energy associated to mass acceleration comes from the rower. Yes, it surely does; but, maybe, a part doesn't. The rower's trunk makes a circular movement during the drive. Beyond 90º in relation to the horizontal, the trunk mass is accelerated by gravity, which can be represented as an acceleration, a tangencial force in the radial trajectory. The rower's body will produce a centripetal force, roughly pointed to his hips, and total acceleration will not be square to the horizontal plan. AM

[old] remador · Post by **[old] remador** » January 21st, 2005, 4:58 am

Certainly, potential energy will vary with rower's height (as with mass).

[old] Physicist · Post by **[old] Physicist** » January 21st, 2005, 5:25 am

Theoretically, rotating the body about the hips could utilise weight to accelerate the flywheel, by dropping the centre of mass. This would add a component of energy output which would increase with the rower's mass. But this energy would then need to be replaced as the rower sat up again on the recovery. There would be no net gain. You might argue that this effect could increase the physiological efficiency of the stroke, by allowing a heavier rower to move some of the energy requirement from the drive to the recovery and therefore creating a smoother power profile. I would be inclined to disagree and say that the effect will be negligible: smaller than the increased energy cost of accelerating/deccerating up the slide. I'll try putting some numbers to it later: I've got work to do at the moment!

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