Stress Evaluation during Low-Speed Mobility Rides Using Ultra-Short-Term Heart Rate Variability Indices

Low-speed mobility devices (LMDs), such as electric wheelchairs, are important transportation options for elderly users. However, environmental factors during outdoor use can cause user stress. Evaluating stress and identifying stress-prone locations is essential for stress reduction. Heart rate variability (HRV) is widely used for stress evaluation, However, conventional short-term HRV analysis typically requires a time window of approximately five minutes, which limits its applicability for identifying stress within short-distance segments. Recently, ultra-short-term HRV (UST-HRV) analysis using windows shorter than five minutes has been proposed and has shown potential for stress evaluation. However, its effectiveness for evaluating stress during LMD rides has not been sufficiently investigated. This study examines whether stress during LMD rides can be evaluated using UST-HRV indices derived from 30 s and 60 s time windows. A field experiment using an electric wheelchair was conducted in an urban environment, collecting HRV, GPS, environmental data, video, and questionnaires. The experimental route was divided into 50 m and 100 m segments to ensure HRV measurement durations of 30 s and 60 s, respectively. Time-domain UST-HRV indices (SDNN, RMSSD, and pNN50) were calculated for each segment and compared between high- and low-stress-potential segments. The results showed that RMSSD was significantly lower in high-stress-potential segments for both segment lengths, whereas SDNN showed no significant differences and pNN50 differed only under limited conditions. These findings suggest that RMSSD derived from 30 s and 60 s time windows can be used as an indicator for evaluating stress during LMD rides over short-distance segments.