We investigated a system identification method for modeling the standing posture without external perturbation. Assuming that postural sway during quiet standing is caused by internal white noise, the method adopts an autocorrelation matrix consisting of body position and velocity parameters. We showed that the method can be applied to center-of-mass fluctuation measurement data. Since we used only a force plate to minimize the burden on the subjects, the velocity could not be measured directly. Therefore, we first demonstrated through simulation that ankle joint stiffness can be estimated by numerically deriving the velocity from the displacement. The velocity was calculated using the central difference approximation, and the ankle stiffness estimation error was obtained. The system identification method was then applied to the center-of-mass measurement data, and the ankle stiffness was estimated. Simulations showed that even when the velocity was calculated through numerical differentiation, the ankle joint stiffness could still be estimated with an accuracy of less than 1% error. The ankle stiffness estimated from the measured center of mass was slightly higher than the gravitational torque coefficient, but lower than that obtained in our previous electrical stimulation study (Uchiyama T: Adv Biomed Eng. 13, 223‒229, 2024). This suggests that the control method for ankle stiffness may depend on the presence or absence of external disturbances.

Urine-related information, such as voided volume and flow rate, is important for the diagnosis of dysuria. Uroflowmeters, which are used for obtaining urine information, measure mainly the weight gain of the collection cup or the rotation of the impeller. These devices pose hygiene-related problems because of the contact between urine and the device. In addition, the patients are forced to urinate in unfamiliar environments and positions, which can lead to unusual results. Therefore, there is a need for a non-contact uroflowmeter that can measure urine information in an environment and a posture to which the user is accustomed. A uroflowmeter that measures the rise of the water level in the toilet bowl is also available; however, it requires extensive installation. To address these problems, we have developed a scale-type uroflowmeter that measures weight loss during standing urination. The purpose of this study was to evaluate the measurement accuracy of our developed scale-type uroflowmeter. For this purpose, human urination was measured simultaneously using the developed uroflowmeter and three different medical uroflowmeters (Freeflow^®, P-Flowdiary^®, and PicoFlow2^®), to compare the voided volume, maximum flow rate, and average flow rate. The results of the developed uroflowmeter showed good agreement with the weight of urine measured by an electronic balance and the voided volume measured by the medical uroflowmeters. The error rates between the true voided volume measured by the electronic balance and the four uroflowmeters ranged from 0.9% to 22.5%. Comparing the maximum and average flow rates of the three medical uroflowmeters with the scale-type uroflowmeter, the error rates ranged from 9.4% to 25.4% and 17.4% to 25.2%, respectively.