The dynamic calibration of bicycle power measuring cranks

The SRM power measuring crankset (PMC) is a novel ergometry system that allows power output to be monitored during field and laboratory cycling. The PMC calculates power from the torque and angular velocity of the cranks. Torque is measured at the crank axle by 4 or 20 strain gauges situated between the crank axle and chain rings and angular velocity is the recorded pedalling rate. The strain gauge output, which is proportional to the effective pedalling torque is inductively transmitted at 500 kHz with pedal rate to a cycle mounted data recorder. The purpose of this study was to assess dynamically the validity of power measurement for this new ergometry system by comparison with a standard laboratory cycle ergometer. The dynamic calibration rig consisted of a modified friction braked Monark ergometer (ME) driven by a motorised treadmill. The PMC was fitted to the ME to allow simultaneous recording of power input and braking power from the respective ergometry systems. Three PMC were assessed, two 20 strain gauge (20sg1 and 20sg2) and one 4 strain gauge (4sg) models. The protocol consisted of 13 braking loads eliciting power outputs ranging from 90 - 625 W at a pedal rate of 1.5 Hz. Braking power was calculated for the ME as: Power (W) = torque (Nm) x angular velocity (rad.s-1)/time (s) and compared with the simultaneously recorded PMC values. Linear regression and 95% limits of agreement (Bland and Altman, 1986) were used to assess the validity of all three sets of PMC. An almost perfect linear relationship was found for all three PMC and ME (r2=1.0). The residuals from the linear regression were used to assess the limits of agreement. The 95% limits of agreement were ± 1.1W, ± 1.8W, ± 2.1W which are equivalent to ± 0.3%, ± 1.0% and ± 1.8% for 20sg1, 20sg2 and 4sg respectively. The results of this study suggest that good agreement exists between the two methods of power measurement. In comparison with the 4sg, the 20sg PMC produces less variability in the differences between ME, probably due to its greater precision and zero position stability. It is concluded that the PMC provides a valid method of assessing power output in the laboratory during scientific research