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The NanoStim project aims to develop a system capable of performing treatment with electrostimulation at the patient\u2019s home, reducing the number of consultations. The knee angle is one of the essential attributes in this context, helping understand the patient\u2019s movement during the treatment session. This article presents a wearable system that recognizes the knee angle through IMU sensors. The hardware chosen for the wearables are low cost, including an ESP32 microcontroller and an MPU-6050 sensor. However, this hardware impairs signal accuracy in the multitasking environment expected in rehabilitation treatment. Three optimization filters with algorithmic complexity O(1) were tested to improve the signal\u2019s noise. The complementary filter obtained the best result, presenting an average error of 0.6 degrees and an improvement of 77% in MSE. Furthermore, an interface in the mobile app was developed to respond immediately to the recognized movement. The systems were tested with volunteers in a real environment and could successfully measure the movement performed. In the future, it is planned to use the recognized angle with the electromyography sensor.<\/jats:p>","DOI":"10.3390\/s22197605","type":"journal-article","created":{"date-parts":[[2022,10,10]],"date-time":"2022-10-10T05:12:21Z","timestamp":1665378741000},"page":"7605","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":28,"title":["Motion Sensors for Knee Angle Recognition in Muscle Rehabilitation Solutions"],"prefix":"10.3390","volume":"22","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-8574-4380","authenticated-orcid":false,"given":"Tiago","family":"Franco","sequence":"first","affiliation":[{"name":"Research Centre in Digitalization and Intelligent Robotics (CeDRI), Polytechnic Institute of Bragan\u00e7a, 5300-253 Bragan\u00e7a, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9344-3075","authenticated-orcid":false,"given":"Leonardo","family":"Sestrem","sequence":"additional","affiliation":[{"name":"Research Centre in Digitalization and Intelligent Robotics (CeDRI), Polytechnic Institute of Bragan\u00e7a, 5300-253 Bragan\u00e7a, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3208-0207","authenticated-orcid":false,"given":"Pedro Rangel","family":"Henriques","sequence":"additional","affiliation":[{"name":"ALGORITMI Centre, University of Minho, 4800-058 Braga, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0100-8691","authenticated-orcid":false,"given":"Paulo","family":"Alves","sequence":"additional","affiliation":[{"name":"Research Centre in Digitalization and Intelligent Robotics (CeDRI), Polytechnic Institute of Bragan\u00e7a, 5300-253 Bragan\u00e7a, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6323-0071","authenticated-orcid":false,"given":"Maria Jo\u00e3o","family":"Varanda Pereira","sequence":"additional","affiliation":[{"name":"Research Centre in Digitalization and Intelligent Robotics (CeDRI), Polytechnic Institute of Bragan\u00e7a, 5300-253 Bragan\u00e7a, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3874-784X","authenticated-orcid":false,"given":"Diego","family":"Brand\u00e3o","sequence":"additional","affiliation":[{"name":"Federal Center of Techonology of Rio de Janeiro (CEFET\/RJ), Rio de Janeiro 20271-204, Brazil"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2151-7944","authenticated-orcid":false,"given":"Paulo","family":"Leit\u00e3o","sequence":"additional","affiliation":[{"name":"Research Centre in Digitalization and Intelligent Robotics (CeDRI), Polytechnic Institute of Bragan\u00e7a, 5300-253 Bragan\u00e7a, Portugal"}]},{"given":"Alfredo","family":"Silva","sequence":"additional","affiliation":[{"name":"INOVA+, 4450-309 Porto, Portugal"}]}],"member":"1968","published-online":{"date-parts":[[2022,10,7]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"2636","DOI":"10.1016\/S0140-6736(19)31138-9","article-title":"Sarcopenia","volume":"393","author":"Sayer","year":"2019","journal-title":"Lancet"},{"key":"ref_2","unstructured":"World Health Organization (2020). 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