Concept and Development of a Real-Time Biofeedback Device for Measuring Quadriceps Lag

This study presents the concept and development of a wearable, real-time biofeedback device designed to measure quadriceps lag (Q-lag) in patients undergoing rehabilitation for knee injuries. Quadriceps lag, commonly observed following anterior cruciate ligament (ACL) reconstruction or in cases of knee osteoarthritis (OA), is characterized by an individual’s inability to actively achieve full knee extension despite having complete passive range of motion. Accurate and objective assessment of Q-lag is critical for monitoring recovery and guiding physiotherapy interventions; however, traditional clinical methods such as goniometry or visual estimation are often subjective, inconsistent, and not suitable for remote use. To address these limitations, a low-cost, lightweight, and portable system was developed that incorporated an MPU 6050 inertial measurement sensor and NodeMCU microcontroller to wirelessly transmit angular kinematic data to an Android application, “QuadEase.” The application provides real-time visual and auditory feedback, enables setting target extension thresholds, and stores session data for tracking progress. The device was designed for both clinic and home-based rehabilitation settings. Initial testing focused on sensor calibration, appropriate anatomical placement, and user interface optimization. Informal usability trials with physiotherapists and volunteers suggested that the system is easy to use, comfortable, and provides meaningful feedback to guide neuromuscular re-education. The system offers potential for integration into post-operative and neuromuscular rehabilitation protocols, supporting patient motivation and therapist decision-making. This paper details the device’s design process, mobile interface, preliminary evaluation, and its potential role in remote physiotherapy and real-time rehabilitation monitoring.