Low-Cost Stretchable Sensor: Range of Motion Evaluation with Open-Source Software Validation

Range of motion (ROM) analysis allows defining the functional state of a joint and is crucial in healthcare field. This paper proposes the use of a low-cost elastomeric strain sensor, employed to determine, as an example of application, the ROM of the knee during a squat exercise. Its validation was performed by using smartphone cameras to record video and open-source portable software tools (OpenCap and Pose2Sim). The results show that the sensor is capable of providing the joint angle with appreciable accuracy, with a maximum error of about 4°, falling within the mean error of the validation tools, and a larger error only for angles at the extremes of the analyzed range of motion. The sensor thus represents a reliable alternative to costly video-based systems. Moreover, its adaptability to non-laboratory environments makes it suitable for various applications, including event detection and human activity recognition.