Power And Body Weight

[old] remador · Post by **[old] remador** » January 13th, 2005, 4:59 pm

I think we should go back to basics (oh, yes!). For instance, body weight has always been a basic variable on rowing (and I do not think someone has proved it isn't in a C2, yet); the use of body weight is, namely, one of the major points of the art of rowing - not just raw strength. Having said this, I would invite people to read an article in the FISA site about rowing technique (the link is below) and, afterwards, to think about this: if force = movement + mass (weight), shouldn't we think about body weight as a factor affecting performance? Back to basics, as I said... <a href='http://www.fisa.org/download/chapter3.pdf' target='_blank'>http://www.fisa.org/download/chapter3.pdf</a>

[old] Physicist · Post by **[old] Physicist** » January 13th, 2005, 5:51 pm

<table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td class='genmed'>QUOTE </td></tr><tr><td class='quote'>body weight has always been a basic variable on rowing </td></tr></table> Weight is a force vertically downwards. The handle on the erg moves horizontally. Vertical and horizontal are at right-angles. No component of the one resolves onto the other. Weight has NO direct effect on erg scores. <table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td class='genmed'>QUOTE </td></tr><tr><td class='quote'>force = movement + mass (weight) </td></tr></table> I'm going to kill myself.

[old] remador · Post by **[old] remador** » January 13th, 2005, 5:54 pm

God, you are angry, or what? Is this an argument?

[old] Physicist · Post by **[old] Physicist** » January 13th, 2005, 6:01 pm

Apologies. Been a long day:) But I am right...

[old] remador · Post by **[old] remador** » January 13th, 2005, 6:04 pm

I am not convinced (although I might be, in the future, I am quite an open guy, altough it may seem the contrary). In rowing, trunk force is quite relevant. If you have the same acceleration and two different values for the body mass, the "heavier" will produce more force, right? AM

[old] Physicist · Post by **[old] Physicist** » January 14th, 2005, 7:54 am

Correct. But the erg measures power, not force. From a purely mechanical point of view a heavier person is at a disadvantage on the erg, as he needs to accelerate his own body mass as well as the flywheel on each stroke cycle. There is no advantage to his weight in generating useful power (ie power that goes into the flywheel.)

[old] remador · Post by **[old] remador** » January 14th, 2005, 9:42 am

Yes. But force has an impact in power (through work, I think). And, if in the rowing stroke (cfr. FISA article), the acceleration of body mass is important, in an erg it must be the same. But you made the right question: the heavier erger must accelerate his body mass, and it will demand more effort than if he was lighter. However, I think - from a theoretical point of view - that with less acceleration, he might be able to generate the same force as a lighter guy. So, I suppose the answer to all this is somewhere around here: is it possible that an x body mass difference between rowers allows the heavier to create the same, or more, power than the lighter rower, using less, or the same, objective effort? To answer this, I would have to know a lot more about biomechanics, I guess. From an intuitive standard (based on personal experience), I would answer "yes", though. AM

[old] Physicist · Post by **[old] Physicist** » January 14th, 2005, 12:27 pm

<table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td class='genmed'>QUOTE </td></tr><tr><td class='quote'> is it possible that an x body mass difference between rowers allows the heavier to create the same, or more, power than the lighter rower, using less, or the same, objective effort? </td></tr></table> No. And here's my explanation: forgive its length. Define a rower as a body of mass M. Let his stroke length begin at x=0 and finish at x=L. Let the force he applies be F and the tension in the chain be T. Let the stroke begin at time t=0 and finish at time t=S. Let the speed of the chain be characterised by v. T is clearly a function of v and v alone - the resistive force on the handle varies only as the speed of the handle (seen from the dynamics of the flywheel.) F will vary as a function of both x and v in a way unique to each rower. I take the same function F(x,v) to be the only measure of 'objective effort'. What I am refuting is that a rower of greater mass M can generate more power in the flywheel than a rower of smaller mass M that has the same profile F(x,v). Newton's 2nd law: F(x,v) - T(v) = M.dv/dt Work input to the flywheel on a stroke, W, given by: W = INT[x=0,x=L]T.dx = INT[x=0,x=L]F.dx - M.INT[x=0,x=L](dv/dt).dx Only the second term here contains mass M. But this term must go to zero! (dv/dt).dx = (dx/dt).dv = vdv. And the b.c.s on v(x) are clearly that v(x=0)=v(x=L)=0. So the mass M can not appear in the expression for the work, W, which is done on the flywheel per stroke. This has but one caveat that I can see, and that is it is not yet a proof that S is independent of M - if S varies with M then the overall stroke time would change with rower mass, and this would enable the rower with the shorter stroke time to rate higher and generate a higher power. But that S is independent of M can be seen by reductio ad absurdum. Were rower 1 of mass M1 to complete the stroke in a time S1 shorter than time S2 taken by rower 2, then at some instantaneous point in the stroke rower 1 must have had a higher speed v2 than v1. But F(x,v) could theoretically be such as to be ill defined or strongly negative at this coordinate, which would make the stroke impossible. I can conclude that if two rowers exert the same force on the chain as a function of their position on the slide and the speed they are moving, and they row with the same stroke length, then they will necessarily take identical strokes which take the same time and generate the same power as each other REGARDLESS of their mass. The only correction to the calculation comes from the energy needed to accelerate and deccelerate the rower on the recovery - which can trivially be seen to be a greater cost (for no output reward) for the more massive rower. Mass is a hindrance to erging, not a help, though only through the effort of the recovery.

[old] PaulS · Post by **[old] PaulS** » January 14th, 2005, 1:28 pm

I'm glad to see that cleared up! But you missed the point of the original post, you are supposed to prove that lighter Ergers are faster than heavier Ergers even though they take longer to go the same distance. Newtons Laws (and reasoning in general) can be ignored as "too inconvenient". Now get to work!

[old] remador · Post by **[old] remador** » January 14th, 2005, 4:52 pm

Paul S, I like your assumptions. Mostly, on what it concerns about others' intentions when writing a post, or about their "reasoning". AM

[old] remador · Post by **[old] remador** » January 15th, 2005, 5:23 am

Physicist, All the information I had until today - although on-water based - took rower's body mass into account in an horizontal axis. On the other hand, guys like Thor Nielsen (check the last link) keep saying that the use of body mass by oarsmen is fundamental. <a href='http://home.hccnet.nl/m.holst/report.html' target='_blank'>http://home.hccnet.nl/m.holst/report.html</a> <a href='http://www.atm.ox.ac.uk/rowing/physics/ ... ml#newton1' target='_blank'>http://www.atm.ox.ac.uk/rowing/physics/ ... <a href='http://www.cyberiad.net/library/rowing/ ... smodel.htm' target='_blank'>http://www.cyberiad.net/library/rowing/ ... <a href='http://dps.twiihosting.net/fisa/doc/con ... _7_660.pdf' target='_blank'>http://dps.twiihosting.net/fisa/doc/con ... br> I'm not going to be around for the next days, so... "see" you soon! AM

[old] PaulS · Post by **[old] PaulS** » January 15th, 2005, 10:54 am

</div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td class='genmed'>QUOTE (remador @ Jan 14 2005, 12:52 PM)</td></tr><tr><td class='quote'> Paul S, I like your assumptions. Mostly, on what it concerns about others' intentions when writing a post, or about their "reasoning". AM  </td></tr></table> Thanks, I'm glad you didn't cut your post short, people might start to talk. No offense intended, it's just that this discussion has been done so much that I'm a bit jaded on it. It seems the basic premise is that if a hwt gets a faster time (water/erg) it's because they were "heavy", and even though the lwts were slower that somehow their performance was "better" through some magical mathematical manipulation. OTOH, when a lwt gets the faster time, it's because they have exceptional technique, rather than they just had less work to do to get down the course (water); this shows up on the Erg since avg power is directly translated to "virtual boat speed". Does that mean there is a "disadvantage" for a lwt on the Erg, no, the PM, chain and flywheel couldn't care less, the lwt simply produces less power, fortunately for them they need less power to move a boat across water. Back to equal "ground" as the case may be. While there was a moment when a "worlds best time" for a lwt boat class was faster than the hwt class, it's not the case at this time. And until this years Olympics the Worlds best time for a M8+ was held by a Junior crew, illustrating how conditions can effect rowing times dramatically (The heat where the USA and CAN M8+ both beat the previous WBT were not what one might have called "ideal", but they were fast). All that said, if lwts can hold their own on the water so well, why was the lwt classification created in the first place? Wouldn't it be more prestigious to be the 1x, 2x, 4x, 2-, etc... champion, rather than to have to qualify it? Don't get me wrong, I'm happy for the lwts to have a place in our sport, regattas wouldn't be the same without them running about underfoot, and an "8" would be downright dangerous without the "+".

[old] Ralph Earle · Post by **[old] Ralph Earle** » January 16th, 2005, 7:24 am

Physicist's Beautiful Theory: <table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td class='genmed'>QUOTE </td></tr><tr><td class='quote'>Weight is a force vertically downwards. The handle on the erg moves horizontally. Vertical and horizontal are at right-angles. No component of the one resolves onto the other. Weight has NO direct effect on erg scores. ... From a purely mechanical point of view a heavier person is at a disadvantage on the erg, as he needs to accelerate his own body mass as well as the flywheel on each stroke cycle. There is no advantage to his weight in generating useful power (ie power that goes into the flywheel.) ... And here's my explanation: forgive its length. Define a rower as a body of mass M. Let his stroke length begin at x=0 and finish at x=L. Let the force he applies be F and the tension in the chain be T. Let the stroke begin at time t=0 and finish at time t=S. Let the speed of the chain be characterised by v. T is clearly a function of v and v alone - the resistive force on the handle varies only as the speed of the handle (seen from the dynamics of the flywheel.) F will vary as a function of both x and v in a way unique to each rower. I take the same function F(x,v) to be the only measure of 'objective effort'. What I am refuting is that a rower of greater mass M can generate more power in the flywheel than a rower of smaller mass M that has the same profile F(x,v). Newton's 2nd law: F(x,v) - T(v) = M.dv/dt Work input to the flywheel on a stroke, W, given by: W = INT[x=0,x=L]T.dx = INT[x=0,x=L]F.dx - M.INT[x=0,x=L](dv/dt).dx Only the second term here contains mass M. But this term must go to zero! (dv/dt).dx = (dx/dt).dv = vdv. And the b.c.s on v(x) are clearly that v(x=0)=v(x=L)=0. So the mass M can not appear in the expression for the work, W, which is done on the flywheel per stroke. This has but one caveat that I can see, and that is it is not yet a proof that S is independent of M - if S varies with M then the overall stroke time would change with rower mass, and this would enable the rower with the shorter stroke time to rate higher and generate a higher power. But that S is independent of M can be seen by reductio ad absurdum. Were rower 1 of mass M1 to complete the stroke in a time S1 shorter than time S2 taken by rower 2, then at some instantaneous point in the stroke rower 1 must have had a higher speed v2 than v1. But F(x,v) could theoretically be such as to be ill defined or strongly negative at this coordinate, which would make the stroke impossible.  </td></tr></table> Statistician's Ugly Facts: Data = US 6K team trials from 1999, 2001 and 2002. Analysis = robust regression of 6K time on height and weight: Group n s/inch signif? s/pound signif? Rsq signif? HM 152 -3.4 No -0.4 No 0.07 No LM 107 -5.8 Yes -2.3 Yes 0.31 Yes HW 122 -11.9 Yes 0.3 No 0.22 Yes LW 63 -8.5 Yes -2.0 Yes 0.21 Yes For 152 heavyweight men (avg 6'4", 205lbs, 19:41 6K) one inch of height was worth about three seconds and one pound was worth about a half a second; neither amount was a statistically significant reduction. For 107 lightweight men (6'0", 162, 20:43) one inch was worth about six seconds and one pound was worth about two seconds; both reductions were statistically significant. For 122 heavyweight women (5'10", 168, 22:56) one inch was worth about twelve seconds but one pound was worth almost nothing; the reduction due to height was statistically significant. For 63 lightweight women (5'7", 134, 23:57) one inch was worth about eight seconds and one pound was worth two seconds; both were statistically significant. So Physicist is absolutely correct (well, almost ...): weight makes no appreciable difference among real rowers = elite heavyweight men and women. But the poor benighted lightweights, not knowing this, go faster if they are heavier. PS: <table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td class='genmed'>QUOTE </td></tr><tr><td class='quote'>I can conclude that if two rowers exert the same force on the chain as a function of their position on the slide and the speed they are moving, and they row with the same stroke length, then they will necessarily take identical strokes which take the same time and generate the same power as each other REGARDLESS of their mass. </td></tr></table> No debate on this one, P: even if I weighed a microgram, if I were as fast as you, you would never beat me.

[old] Physicist · Post by **[old] Physicist** » January 16th, 2005, 7:51 am

Ralph, I think you badly misconstrued what I wrote. I have never, and would never, suggest that heavier rowers do not perform better on average, both on the erg and on the water. Of course they do (up to a point.) What I 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. This is not a postulate that you can prove or disprove epidemiologically as there is no way of measuring or compensating for all these variables. Luckily it is, however, completely obvious.

[old] remador · Post by **[old] remador** » January 16th, 2005, 9:20 am

Paul S, I am not a lightweight myself (my average competitive weight is 78-80 kg, although now I am about 74 kg - anyway, a lighweight must be 70 or 72,5, depending of the boat); I am just a former competitive rower and director of a rowing club who wants to deal with differences between on water and off-water rowing. Having said this, I think I wasn't quite alerted to your irony; apologies if I seemed to be a bit angry (I was not, though). Physicist, What is obvious for you might not be obvious for others. Marinus Vas Holst, rowing physics studious, states this (which I would label as "obvious", also): "Kinetic energy dissipated by the moving rower. Revised May 2004 Introduction The boat is moving through the water and the rower is moving on the seat with respect to the boat. Discussed is the question how much energy is dissipated as a result of the repeating acceleration and deceleration of the rower. The subject is also covered by Bill Atkinson. Despite the different approach, the conclusion is the same: energy loss as a result of the motion of the rower on the sliding is minimized by keeping the maximum seat speed as close as possible to the mean seat speed. On the Concept 2 ergometer On the Concept 2 ergometer the situation is somewhat simpler as there is no relative motion of the seat with respect to the boat but motion with respect to the earth only. First we make an estimate of the seat speed. The average speed is about 1 m/s because the distance covered by the seat is somewhat less than 1m, and this distance is covered in somewhat less than 1s. (stroke rate somewhat above 30 s-1). The maximum speed will be about twice the average speed (probably somewhat less) u = 2 m/s. 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." In a sport where you fight for a tenth of a second... The link is the following: <a href='http://home.hccnet.nl/m.holst/KinEn.html' target='_blank'>http://home.hccnet.nl/m.holst/KinEn.htm ... > Lucky to have a computer nearby! Stimulating discussion... AM

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