{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,1,21]],"date-time":"2026-01-21T15:51:44Z","timestamp":1769010704552,"version":"3.49.0"},"reference-count":28,"publisher":"MDPI AG","issue":"10","license":[{"start":{"date-parts":[[2018,9,26]],"date-time":"2018-09-26T00:00:00Z","timestamp":1537920000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100003621","name":"Ministry of Science, ICT and Future Planning","doi-asserted-by":"publisher","award":["NRF-2017R1A5A1015596"],"award-info":[{"award-number":["NRF-2017R1A5A1015596"]}],"id":[{"id":"10.13039\/501100003621","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Photoplethysmography (PPG) of the foot sole could provide additional health-related information compared with traditional PPG of the finger or wrist. Previously, foot PPG required the procedural binding of a light-emitting diode (LED)-photodetector (PD) pair. We achieved PPG of the foot sole without binding any sensors to the foot while the participant stood in a natural standing position on the testing device. Foot PPG was performed using multiple LED-PD pairs to overcome motion artefacts caused by stabilization. We identified regions of the sole suitable for reliable sensor positioning with optimal LED-PD pairs on the basis of the estimated heart rate (HR) and signal quality index derived by dynamic time warping (wSQI). The first experiment included four participants with direct skin-to-sensor contact, and the results showed a mean HR estimation error of 0.01 beats\/min and a wSQI of 0.909. The extended experiment with 53 participants, which involved including a gap between the skin and sensors to consider real-life applications, yielded a mean HR estimation error of 0.638 beats\/min and a wSQI of 0.751. Based on the selection ratio of optimal LED-PD pairs, the best region of the sole for PPG was the midfoot, except the medial longitudinal arch. In conclusion, we confirmed that foot PPG using multiple LED-PD pairs is appropriate for HR evaluation and further applications.<\/jats:p>","DOI":"10.3390\/s18103239","type":"journal-article","created":{"date-parts":[[2018,9,26]],"date-time":"2018-09-26T10:39:58Z","timestamp":1537958398000},"page":"3239","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":16,"title":["Unobtrusive Photoplethysmographic Monitoring Under the Foot Sole while in a Standing Posture"],"prefix":"10.3390","volume":"18","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-4242-7638","authenticated-orcid":false,"given":"Seunghyeok","family":"Hong","sequence":"first","affiliation":[{"name":"Interdisciplinary Program in Bioengineering, Seoul National University, Seoul 110-799, Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Kwang Suk","family":"Park","sequence":"additional","affiliation":[{"name":"Department of Biomedical Engineering, Seoul National University College of Medicine, Seoul 110-799, Korea"},{"name":"Institute of Medical & Biological Engineering, Medical Research Center, Seoul National University, Seoul 110-799, Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2018,9,26]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"161","DOI":"10.1177\/153473460200100303","article-title":"Noninvasive measurement techniques for monitoring of microvascular function in the diabetic foot","volume":"1","author":"Cobb","year":"2002","journal-title":"Int. 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