andrewjcw wrote:No discussion into whether stages' lack of precision was caused by single sidedness as opposed to general inaccuracy? You'd think that would be one of the main practical investigations of such a study.
It was noted as an issue (see quote from study below where they address this point) but assessing
why meters may or may not be accurate wasn't the purpose of the study, rather the study was to determine the level of power meter accuracy under the conditions tested and to use a methodology (negative gradient cycling treadmill and counter weights) which would provide an absolute accuracy benchmark (rather than simple relative tests of one meter v another).
For the rather low bar of steady state aerobic power accuracy, the fact that Stages is a unilateral meter is by its very nature going to result in lower precision. It's just a fact of life for such meters, no matter how well they measure one side, they will be both less accurate and less precise due to the normal and natural human variability in asymmetry of power output.
The low CV of all power meters of 1.2 ± 0.9 % indicates high general precision of most power meters, but manufacturer-dependant differences exist. Power meters from SRM and PowerTap were more precise compared to devices from Stages.
This comparison, however, is not completely legitimate because the power meters from Stages calibrated in this study measured only torque in the left crank arm, with the assumption of right side being equal. Thus, the derived trueness and precision in the current study always depended on the power meter itself and the riding style of the cyclist (left-right balance). A varying left-right balance during the calibration would increase the variability of the measured power outputs and, therefore, lower the precision. Kirkland et al. [16] reported a contribution of 48.9 ± 3.6 % from the left leg in a group of 9 competitive cyclists for similar power outputs. The variability of 3.6 % illustrates how the accuracy of the Stages power meter could be strongly influenced by the cyclist himself, apart from technical measurement error.
I think they are being rather kind. A meter's job is to be accurate and precise. If you design them such that they introduce an unknown and uncontrollable error, well you reap what you sow.
Of course higher bars for power meter performance than this exist, e.g. precision and accuracy of high intensity cycling, sprint efforts and under variable crank velocity scenarios is where we see a much greater variability on the capability of meters, but which is actually quite hard to do relative to an absolute reference.
