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Objective: Accurate monitoring of joint kinematics in individuals with neuromuscular and musculoskeletal disorders within ambulatory settings could provide important information about changes in disease status and the effectiveness of rehabilitation programs and/or pharmacological treatments. This paper introduces a reliable, power efficient, and low-cost wearable system designed for the long-term monitoring of joint kinematics in ambulatory settings. Methods: Seventeen healthy subjects wore a retractable string sensor, fixed to two anchor points on the opposing segments of the knee joint, while walking at three different self-selected speeds. Joint angles were estimated from calibrated sensor values and their derivatives in a leave-one-subject-out cross validation manner using a random forest algorithm. Results: The proposed system estimated knee flexion/extension angles with a root mean square error (RMSE) of $5.0^{\circ }\pm 1.0^{\circ }$ across the study subjects upon removal of a single outlier subject. The outlier was likely a result of sensor miscalibration. Conclusion: The proposed wearable device can accurately estimate knee flexion/extension angles during locomotion at various walking speeds. Significance: We believe that our novel wearable technology has great potential to enable joint kinematic monitoring in ambulatory settings and thus provide clinicians with an opportunity to closely monitor joint recovery, develop optimal, personalized rehabilitation programs, and ultimately maximize therapeutic outcomes.