A novel method based on first principles to determine the accuracy and reliability of force measurements reported by bicycle power meters.

Abstract

The accuracy and reliability of instrumented bicycle crank systems (i.e. power meters) is an important consideration for sport scientists who evaluate cycling performance and pedalling biomechanics. Many crank systems report power and or force/s on the left and right crank arms separately, or indexes of pedalling effectiveness, although crank systems that have genuinely independent force transducers on the left and right crank arms are rare. There is a need to be able to evaluate the accuracy and reliability of the measurements of power meters without the requirement for expensive and or complex instrumentation. The present study describes a relatively simple and novel method of assessing the accuracy and reliability of measures of crank angle, radial force and tangential force. The method is demonstrated in its application to an instrumented crank system (Axis Cranks ™). Reported crank forces were compared with actual applied forces to determine accuracy and some procedures used to assess the measurement of force were duplicated to determine reliability. The crank system measured crank angle with an average RMS error of 1.84 degrees across pedalling rates of 30-150 r/min. The absolute error of radial and tangential force measurements were 6% and 3.2% respectively (RMSE) and the relative error (accuracy of change in force) of radial and tangential force were 1.48% and 0.25% respectively (RMSE). Repeated measurements of force were found to be highly reliable (intra-class coefficient > 0.998). The method presented in this report could be used to evaluate the accuracy and reliability of other power meters and instrumented crank systems.