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Accuracy of SRM and Power Tap Power Monitoring Systems for Bicycling


Medicine & Science in Sports & Exercise: July 2004 - Volume 36 - Issue 7 - pp 1252-1258
APPLIED SCIENCES: Physical Fitness and Performance

GARDNER, A. S., S. STEPHENS, D. T. MARTIN, E. LAWTON, H. LEE, and D. JENKINS. Accuracy of SRM and Power Tap Power Monitoring Systems for Bicycling. Med. Sci. Sports Exerc., Vol. 36, No. 7, pp. 1252–1258, 2004.

Purpose: Although manufacturers of bicycle power monitoring devices SRM and Power Tap (PT) claim accuracy to within 2.5%, there are limited scientific data available in support. The purpose of this investigation was to assess the accuracy of SRM and PT under different conditions.

Methods: First, 19 SRM were calibrated, raced for 11 months, and retested using a dynamic CALRIG (50–1000 W at 100 rpm). Second, using the same procedure, five PT were repeat tested on alternate days. Third, the most accurate SRM and PT were tested for the influence of cadence (60, 80, 100, 120 rpm), temperature (8 and 21°C) and time (1 h at ~300 W) on accuracy. Finally, the same SRM and PT were downloaded and compared after random cadence and gear surges using the CALRIG and on a training ride.

Results: The mean error scores for SRM and PT factory calibration over a range of 50–1000 W were 2.3 ± 4.9% and −2.5 ± 0.5%, respectively. A second set of trials provided stable results for 15 calibrated SRM after 11 months (−0.8 ± 1.7%), and follow-up testing of all PT units confirmed these findings (−2.7 ± 0.1%). Accuracy for SRM and PT was not largely influenced by time and cadence; however, power output readings were noticeably influenced by temperature (5.2% for SRM and 8.4% for PT). During field trials, SRM average and max power were 4.8% and 7.3% lower, respectively, compared with PT.

Conclusions: When operated according to manufacturers instructions, both SRM and PT offer the coach, athlete, and sport scientist the ability to accurately monitor power output in the lab and the field. Calibration procedures matching performance tests (duration, power, cadence, and temperature) are, however, advised as the error associated with each unit may vary.

1Department of Physiology, Australian Institute of Sport, AUSTRALIA; 2School of Human Movement Studies, The University of Queensland, AUSTRALIA; and 3Triathlon Program, Queensland Academy of Sport, AUSTRALIA

Address for correspondence: Andrew Gardner, Department of Physiology, Australian Institute of Sport, P.O. Box 176, Belconnen, ACT 2616. Australia; E-mail:

Submitted for publication September 2003.

Accepted for publication March 2004.

©2004The American College of Sports Medicine