In order to solve nonlinear differential equations concerning with the spinal stretch reflexes in pendulum test, mutually synchronized values of angle, angular velocity and angular acceleration in the knee-joint are frequently used as their initial ones. In this study, we propose a method which can detect both angle and angular acceleration in the knee-joint at the same time by simple operations on the outputs of two accelerometers. Here, the principle of knee-joint motion detection, evaluation of amplitude accuracy in measured waveforms, that of synchronization accuracy are described in this order.