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Ergometer Power Bias

© 2002 Atknsopht (07/26/12)

Note: This file, created in 2002, has been inadvertently omitted from appearance on this website until now (11/07/07).

The computer displays for power output on rowing or exercise fanwheel resistance ergometers underestimate the total effort exerted by the large "rower" and offer a hidden bonus to the lightweight.

Ergometers displaying only the power consumed by their dissipation devices (usually flywheel and fan combinations) do not record the internal momentum work done by the rower on the slide as he repeatedly establishes and cancels horizontal momentum with his legs against the fixed footboard and his back and his stomach muscles "against" the momentum of his head and torso [1]. (Neither do they record the work done on the chain sprocket return spring--as much as 10% of the total rower effort.)

The reason for this is that the footboard is fixed so that the work done on it is necessarily zero.

I notice, in a recent Concept-2 brochure (Fall 2007), that a Concept2 "Slide" is offered. While it may enhance the "feel" of the rowing effort it will not alter the rower's work output unrecorded by the computer- namely the work done, now partly on the footboard (no longer fixed), to accelerate and decelerate the mass of the machine (or the rower). It could be added in to the recorder by summing the instantaneous product of the footboard displacements and forces thus making the ergometer work more like the work of real rowing.

Distances--slide travel and torso excursion--"map" more or less directly to the size of a rower. And, assuming that the motion frequency (stroke rating) remains constant, the momentum forces directly reflect his mass. Thus a big rower moves the slide farther generating greater force in so doing--in the same time--than would a smaller one. He does more work and consumes more power; power which goes unrecorded.

The work required to accomplish this motion stands in rough proportion both to the slide travel and to the forces required to accelerate the body masses riding it. Figure 1 shows how the total work and power of rowing the ergometer is resolved into "useful" fan work and "lost" momentum work at the seat slide. These figures emerge as output from a comprehensive computer model ERGMOM Ver. 3.00.

Figure 1
Figure 1

Compare a 90 kg man 1.95 m tall with a 60 kg woman 1.6 m tall. And assume that the various motion excursions of each are in rough proportion to their heights (sizes) and that the momentum forces generated are, as Newton tells us, in direct proportion to their masses (weights). Each works the ergometer in the same cadence and each, let's say, exerts force on the erg handle in rough proportion to his weight (strength).

The directly measured (displayed) pull-handle works will stand in the ratio 90/60 times 1.95/1.60 = 1.83 (Work = Fs). The slide momentum works will stand in the ratio (90/60) times (1.95/1.60)^2 = 2.22 (the size is squared because the body mass point travel distances dictate the acceleration forces and the distances, applied yet again, determine the works done).

Assume then that the man shows 314 W at the pull-handle and 120 W at the slide, Figure 1) and that the woman shows 314 /1.83 = 172 W at the handle and 120 /2.22 = 54 W at the slide. Both of these power totals, (314 +120 =434) and (172 +54 =226), are under-reported at the dynamometer display by the amount of the slide work (plus the neglected return spring work).

In an effort better to make the recorded power reflect the actual output of the rower one ergometer manufacturer adds to the dynamometer output a fixed figure for the slide momentum work [3].

If now, for example, a fixed "correction" figure for the slide power has been independently established as, say, 120 W (103 kCal/hr) for a 90 kg man then his total output "corrects" rightly to 314 plus 120 = 434 W.

On this fixed basis, though, the woman's total output "corrects" to 172 plus 120 = 292 W, a gross overestimate since her true total power equals only 226 W. To reduce this discrepancy one should apply weight-size ratios to fixed correction estimates for the slide work: here 1.00 in the case of the man (120 /1.00 = 120 W) and 2.22 (in the case of the woman (120 /2.22 = 54 W (45.5 kCal/hr)). Of two rowers of comparable strength but of differing weights and sizes--and showing equal fanwheel plus fixed "correction" power-- the lighter and smaller rower is actually exerting less total effort than the larger one. The one showing the larger power "wins the race" but it was easier for the smaller rower.

As estimates for the size of the fixed correction increase the bias against the lightweight rower increases correspondingly. If the estimated fixed correction to a dynamometer display for a specified rower weight and size is in serious error then no amount of the use of corrective ratios can eliminate the bias. Hence the need to be sure that independent fixed estimates for slide work are carefully determined and that, if used at all, the size of ergometer competitors is taken into account.

CRASH organizers take note: competitors in ergometer "races" or "CRASH" competitions should therefore inform themselves of manufacturer's dynamometer output allowances for slide work especially if the amounts are fixed for all sizes and weights of rower--in which case a heavyweight will row the "distance" under a penalty not suffered by the lightweight.

Note that in one sense the work reported by a fanwheel dynamometer is size-biased even when fixed external additions are not applied. Thus, of two "rowers" producing the same recorded fanwheel power the larger rower expends more total energy than the smaller one. However this "bias" is not artificially introduced--it's not really a bias at all; it "comes with the territory" so to speak. Big people must expend more energy than smaller ones simply in moving themselves about.

In order completely to eliminate ergometer display bias consideration should be given to abandoning fixed factors attempting to account for unmeasured slide work. It is when efforts are made to "improve" the reporting by means of fixed allowances that the output display acquires a hidden bias which skews the estimates of work done by ergometer competitiors. Factors based on size and weight should be used instead.


1. Atkinson, W. C., "Modeling the Dynamics of Rowing Ergometers", abstract documentation of the computer model ERGMOM (unpublished).

2. Bassett, D.R., Jr., P.A. Smith, L.H. Getchell. "Energy Cost of Simulated Rowing Using a Wind-Resistance Device". The Physician and Sportsmedicine, Vol.12, No.8, pp 113-18, August, 1984.

3. Wilhide, David, S. LaRose. "Calculation of Power, Pace, and Calories on the PM2", Concept II Ergometer, Concept II, Morrisville, VT, 1996.

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