This study aimed to measure the lower limb joint angles during squats using stretchable strain sensors and examine the results' validity and reproducibility.
The subjects were 20 male students with no orthopedic conditions affecting their lower limbs or trunk. The subjects wore specialized knee and foot supports with stretchable strain sensors and markers for motion analysis attached to their right lower limbs. The subjects performed five sets of 5 squats, and the knee and ankle joint angles during the movements were measured every 10 degrees using 3D motion analysis and stretchable strain sensors. The joint angles obtained from the three-dimensional motion analysis device were used as the reference values to calculate the mean absolute error (MAE), root mean square error (RMSE), and intraclass correlation coefficient (ICC) of the measurements obtained from the strain sensor were calculated to evaluate validity and reproducibility. A statistical significance level of 5% was set for the analysis. This study was conducted after obtaining approval from our institution's medical ethics review board, and informed consent was obtained from all participants after explaining the study's purpose and content both orally and in writing.
The MAE from 60 degrees of knee flexion to extension in squats was 2.87 to 7.63, the RMSE was 2.92 to 8.20, and the ICC was 0.925 to 0.969 (P0.001), and the MAE from extension to 60 degrees of flexion was 3.99 to 8.41, the RMSE was 5.35 to 8.94, and the ICC was 0.801 to 0.986 (P0.001). The MAE from 30° to 10° ankle dorsiflexion was 7.67-12.57, the RMSE was 7.80-12.63, and the ICC was 0.945-0.972 (P0.001). The MAE from 10° to 30° dorsiflexion was 8.11-12.96, the RMSE was 8.19-13.00, and the ICC was 0.884-0.969 (P0.001).
The MAE and RMSE were less than 10° for the knee joint but exceeded 10° for the ankle joint. However, the minimum ICC was 0.801 for the knee and 0.884 for the ankle, confirming high reproducibility. In other words, this shows that the error in the joint angle obtained from the stretchable strain sensor is reproducible for each angle. Therefore, the stretchable strain sensor shows potential for use in joint angle measurement.
The stretch strain sensor may have potential applications as a support tool in physical therapy practice.
squat
joint angle
