Simplified appraisal method for fatigue on sitting for extended periods by using finger plethysmogram

For the purpose of quantifying and indicating the degree of fatigue as a result of sitting in an automobile seat for long periods of time, a method was developed by using a baseline oscillation and the amplitude of a finger plethysmogram. Between a finger plethysmogram and the degree of fatigue, there is about one hundred times difference in a fluctuation cycle. So, an idea was proposed to convert a finger plethysmogram into a gradient of comprehensive change to make fluctuation cycle scales easily comparable. The gradients of the square of the finger plethysmogram amplitude were converted into absolute values. Then, we drew a fatigue curve based on the results calculated at 18-second integrated values of the time series signals. This curve and the muscle fatigue curve from subjective judgment were in general accord. By using a finger plethysmogram featuring high sensitivity to the fatigue progress, differences in various subjects and seat performances could be found on this fatigue curve using only a 30-minute fatigue test. This new method has made it possible to greatly shorten the measurement time necessary to predict fatigue level by sensory evaluation.