Detection of knee unlock during stance by accelerometry

Detection of knee unlock is a crucial part of finite state artificial reflex control of paraplegic standing supported by functional neuromuscular stimulation (FNS). This paper investigates knee unlock detection schemes using small uniaxial accelerometers mounted on the thigh and shank. Four single and two differential accelerometer configurations were evaluated with respect to knee unlock detection. Their performances were compared to goniometer-based knee unlock detection. Two accelerometers were mounted on the thigh, one just distal to the hip and one just proximal to the knee. The other two accelerometers were mounted on the shank, one just distal to the knee and the other just proximal to the ankle. The sensitive axes of all four accelerometers were directed in the sagittal plane, perpendicular to the long axes of the body segments. The first differential configuration consisted of the two accelerometers on the thigh, the second of the accelerometer on the thigh just proximal to the knee and on the shank just distal to the knee. Detection thresholds were see such that anterior-posterior body sways with locked knees would not yield false knee unlock detections. Experiments were performed in five healthy adult subjects, performing fast and slow knee flexions and anterior-posterior body sways. The first differential accelerometer configuration and the single accelerometer just proximal to the knee gave the earliest detection of fast knee unlocks, earlier than the goniometer-based detection. The second differential accelerometer configuration gave the earliest detection of slow knee unlocks, comparable to the goniometer-based detection. The first differential accelerometer configuration did not provide a reliable detection of slow knee unlocks. The single accelerometer configurations could only detect slow knee unlocks, while rejecting whole body movements, at relatively large knee flexion.