What's Cool In Road Cycling

SRM and PowerTap: How Accurate are They?

Most of us have been inundated with the beauty of power as the purest and best training variable to monitor. Before jumping on the power bandwagon however, it’s important to understand just how accurate and reliable these units are. Fortunately, the Aussie sports scientists have done exactly that…

Power to the People
It is no secret that the use of power monitors have revolutionized training in cycling, providing quantifiable data that advances and complements heart rate monitoring. The problem however, even for the most well-heeled amongst us, is justifying the huge financial outlay. Being a science geek, I’m not here today to sell you on the merits of power training (see our recent articles on why Santa should bring you a power monitor and also on the power output of T-Mobile pros during a stage race), but suffice to say that power monitors can be a vital cog in maximizing your training.

Accuracy and Reliability Defined
What DOES interest me, however, is whether these tools are accurate and reliable, which is by far the most important parameter in monitoring tools. Accuracy refers to whether the tool is actually measuring what it claims to be measuring. In this case, is the readout of 200 W on the monitor actually 200 W, or is it 210 W in reality? Reliability refers to the repeatability of measurement. If an identical power (e.g., 200 W) is recorded on this interval, will the same power output read 200 W again the next interval? The next day?

In many senses, reliability is the more important of the two parameters when talking about the utility of a tool, whether it be a power monitor or a bathroom scale for measuring body weight. That’s because, the vast majority of the time, the important thing is not to compare your values with anybody else, but to track your individual response over time. To further the bathroom scale analogy, it is more important that my weight reads the same when I weigh myself twice in a row, and not as important whether my “real” weight is 64 or 65 kg.

What the Aussies do for Fun
Thank goodness for us that the sport scientists at the Australian Institute for Sport have a lot of toys on their hands, along with the time to play with them. In a 2004 scientific article in the well-respected journal Medicine and Science in Sport and Exercise, Gardner et al. (1) systematically tested the accuracy and reliability of two of the dominant power monitors on the market – SRM and PowerTap.

The basic experiments were as follows:
1) 19 SRM and 5 PTs were tested for accuracy using a CALRIG standardized device at 100 rpm from a power output range of 50 – 1000 W. The SRMs were “Pro” models (4 strain gauges) < 3 y old at the time. PTs were < 1 y old. 15 SRMs were then retested following a full season of use. 2) To examine the effects of cadence, the most accurate SRM and PT unit from Expt 1 was tested at 60, 80, 100, and 120 rpm using the same setup and test protocol. 3) To examine the effects of temperature, the most accurate SRM and PT unit from Expt 1 was tested during and following exposure to 6oC and 21oC temperatures. 4) A field simulation was performed with random variations in cadence and power outputs using the most accurate SRM and PT unit from Expt 1. Data Summary
This was a very well-designed study with lots of careful thought put into the research plan and ensuring a good setup. What did they find with these four experiments? A lot, but I’ll try to summarize here and then interpret them down below:
• SRM variability in accuracy ranged from -10.4% to +1.0%; PT variability ranged a much tighter -2.0 to -2.9%.

• Once corrected to a range of 0.0 – 2.1% accuracy, 14/15 SRMs retained its calibration within that range over the course of an 11-month racing season. PTs were only tested after 2 days, but accuracy remained similar to the results of the first trial.

• Some variability was observed with cadence for both SRM and PT over the tested range of 60 – 120 rpm. This was also exacerbated by the shifting of different gears.

• Temperature definitely affected accuracy of the readings in both SRM and PT. This was especially true when the units were exposed to and calibrated in cold (6oC) temperatures, then tested again at warm (21oC) conditions. This suggests that the zero offsets be reset over the course of a ride involving large temperature shifts.

• SRM seemed susceptible to sudden shifts in gearing and power outputs, likely caused by hysteresis of the strain gauges. This has implications when recording sudden bursts of power, as compared to a “steady” effort like a time trial.

So Should I Buy One?
Only you, your significant other, coach, and possibly your accountant can determine that. However, if you do spring for one, here are some general considerations from this study that you may want to keep in your mind:
• Most of us will not have access to high-end calibration facilities like the Aussies developed. Therefore, it may be hit and miss concerning the actual accuracy of your SRM or PT. However, at least in the case of the SRM, it seems that long-term reliability is excellent. As I noted above, this is the most important consideration with power monitors and other testing tools.

• Wherever relevant, tests done with power monitors should ideally be done in the same gear and cadences to minimize variability.

• Where there will be large shifts in temperature over the course of a ride, variability may occur in recorded power.

• Technology is fast-moving in the competitive bike industry. This study was published in 2004 and submitted for publication in Sept. 2003. This meant that the study was done with relatively old, 2000-2002 era SRMs (serial numbers ranging from 235 – 1031) and 2002-2003 era PowerTaps. As noted by the authors, many improvements may have been made in that time to the technology in these two systems to minimize the issues reported here.

1. Gardner AS, Stephens S, Martin DT, Lawton E, Lee H, and Jenkins D. Accuracy of SRM and Power Tap power monitoring systems for bicycling. Med Sci Sports Exerc 36: 1252-1258, 2004.

About Stephen:
Stephen Cheung is an Associate Professor of Kinesiology at Dalhousie University, with a research specialty in the effects of thermal stress on human physiology and performance. Stephen’s company, Podium Performance, also provides elite sport science and training support to provincial and national-level athletes in a number of sports. He can be reached for comments or coaching inquiries at [email protected].

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