AN INDIRECT METHOD FOR MEASURING THE TORQUE APPLIED TO FRICTION-LOADED CYCLE ERGOMETERS

The aim of the study was to measure directly the applied torque whilst pedalling on a friction-loaded cycle ergometer and to compare the profiles obtained with those calculated indirectly using a computer to analyse the flywheel accelerations resulting from the torque. Twenty subjects performed a series of 6 s trials with resistive loads of 9.8, 19.6, and 29.4 N whilst pedalling on a Monark 864 cycle ergometer. Each test was performed at a constant pedalling speed of either 5.2 or 7.3 rad.s-1. The ergometer was modified by having the chain from the pedal cog drive a rotary torque transducer. A second chain coupled the output drive of the transducer to the flywheel. The signal from the transducer and the speed of the flywheel was constantly monitored by a computer at the maximum analogue sampling rate of the analogue port of 40Hz. At the end of each trial the computer calculated the torque which must have been applied to the flywheel to cause it to accelerate (indirect method) and was compared it with that obtained from the transducer (direct method). The two profiles were very similar in shape and magnitude with the directly measured profile having the slightly larger magnitude (range 0.48 to 0.79 N.m; p<0.05). The difference was assumed to be due to the losses in the drive system and bearings. A mean phase shift of 0.022 s in the torque profiles was found between the two methods, with the direct method always preceding the indirect method. This value was close to the anticipated 0.025 s delay, which would result from the sampling frequency of the analogue port. The findings of the study suggest that the net torque applied to the pedals can be calculated from the simple measurement flywheel acceleration, without the necessity of employing the difficult method of crank instrumention. In addition the indirect method was found to be sensitive to small variations in the applied torque.