In this study, we propose a system for measuring the rotational angle of a bicycle crank arm. This system consists of a wireless motion sensor with bi-axial acceleration and angular velocity sensors attached to the crank arm. Rotational angles measured by the motion sensor were compared with data from a motion capture system. We found the RMS (root-mean-square) error for the bicycle crank angle was 0.339 ± 0.115 degrees (mean ± SD), and the angular velocity RMS error normalized by the average angular velocity was 0.655 ± 0.217 %. Using the proposed motion sensor system, we were able to measure the rotational angle and angular velocity of the bicycle crank to the same accuracy and high frequency as the motion capture system. Furthermore, we demonstrate it is possible to precisely obtain the rotational crank angle to evaluate fluctuations of crank angular velocity.
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