Background

The objective and non-invasive monitoring of knee joint movements is crucial to treat musculoskeletal disorders, supervise the rehabilitation process, and assess motor functions after surgery. Conventionally, knee movements analysis relies on rigid instrumentations (e.g., goniometers) and clinical assessments. A breakthrough solution is offered by unobtrusive and comfortable wearable systems embedding flexible strain sensors, adaptable to different anthropometrics. The knee joint is subject to repetitive movements and continuous stress during activities of daily living. New instrumentations and methodologies must track knee motor functions in patients during everyday life and clinical assessment.

AIMS
To develop a wearable system embedding flexible textile-based strain sensors for monitoring flexion-extension movements of the knee joint. Then, to evaluate the performance of the wearable system during knee movements.

Methods

The wearable system consists of a knee sleeve and integrates a textile-based strain sensor (EeonTex™ LG-SLPA) encapsulated into two silicone rubber layers (Ecoflex 00-30). The silicone rubber enhances the sensor’s flexibility, robustness, and biocompatibility. The sensing element was positioned vertically across the knee joint and attached to the knee sleeve through automatic buttons. Sensor positioning resulted from a previous analysis using a motion capture system (Qualisys Inc., Gothenburg, Sweden). During this experimental test, a grid of 35 retroreflective markers was positioned on the knee joint to identify local areas with maximum deformation during knee flexion-extension. Experimental tests were performed to evaluate the performance of the wearable system during repetitive knee flexion-extension movements. Volunteers were asked to execute ten consecutive knee flexion-extension in each of the following trials: i) starting with the knee flexed at 90°, reach 0° of extension and then return to the initial position (seated position); ii) beginning with the knee flexed at 90°, reach 45° of extension and then return to the initial position (seated position); iii) starting upright, execute hip flexion with the knee flexed at 90°; iv) starting upright, execute mini-squat flexing both knees about 50°. Ten markers (8 mm diameter) were positioned around the sensing element, to calculate the geometrical knee flexion-extension angles.

Results

Experimental results showed that the knee sleeve integrating a flexible strain sensor was successful during knee flexion-extension movements. In particular, the sensing element was able to track all the knee flexion-extension repetitions in each trial. Sensor’s output showed an increase of the resistance during knee extension, corresponding to the minimum applied strain and decreased resistance during knee flexion, corresponding to the maximum applied strain.

Discussion

This research proved the potential application of a wearable system embedding a flexible strain sensor for knee joint monitoring. The main advantages of the developed wearable system are wearability, excellent measurements performances, and the ability of monitoring knee flexion-extension without any discomfort for the users.
The promising results foster new investigation scenarios, from rehabilitation to medicine sports and clinical practice. Such systems would provide relevant information for both the clinicians and patients, as additional support to increase motor recovery successes.