Estimation of human skeletal muscles' contraction with circumferential measurements based on smart fabric sensing technology

Abstract

The musculature of the skeletal muscles is of great interests to researchers, since they are the motors of the human motions as well as stabilizers of joint positions. Knowing the properties and characteristics of skeletal muscles is a growing need, driven by health care, medical application and sports training purpose. Hence, this work is aiming to explore the feasibility of an easily accessible anthropometric measurement, i.e., the limb circumferential strain (LCS), as an alternative index for skeletal muscles' contraction. With ultrasound imaging technology has revealed the change of thickness of skeletal muscles during contraction. Limb circumference, which is consequentially affected by muscle thickness, has also been found as correlated with muscle contraction. Automatic anthropometric measuring devices for monitoring muscle contraction in-motion have been designed and reported, among which most were composed of rigid components such as rigid metal wires, which are not capable of seamlessly contacting human skins. For in-situ monitoring, a novel flexible measuring device is required to seamlessly conform to the curved and soft human skins in motion. The fabric sensors are promising and ideal sensing elements for the flexible measuring device, due to excellent bendability, stretchability, long service life as well as stable performance. Using the fabric strain sensors, a limb gauge measurement system (LGMS) based on fabric sensing technology has been designed and fabricated at Hong Kong Polytechnic University. The gradients and fabrication of the 2 versions of sensing belt in LGMS were studied in details. Test results show that, the LGMS can measure the muscular deformation continuously and accurately in real time, and without any discomfort. Elbow flexions have been studied in this project. Linear relationship between muscle's contractile force (indicated by joint torque) and the acquired limb circumferential strain has been derived theoretically in isometric flexions, based on a cylinder model. For experimental verification, 10 subjects participated in elbow isometric tests at 5 elbow positions. Correlations joint torque and the upper-arm circumferential strain have been studied and were observed as linear during loading phases. The circumferential strain was confirmed as alternative index for biceps' contraction in isometric contraction. Meanwhile, ratio of torque to circumferential strain highly depended on joint positions, which has not been reported by others' work. Further extended and exploratory study of upper-arm circumferential strain as an index to monitor biceps' contraction was conducted in elbow isokinetic flexions at 3 angular speeds (90°/s, 120°/s and 60°/s). It's observed that the transfer ratio was significant sensitive to joint positions, but not sensitive to angular velocities. Based on the findings, a modified Hill-based model, which connects the circumferential measurement and joint torque, was specifically proposed. Algorithm is introduced accordingly, through which the joint torque by elbow flexors can be 'predicted' from the circumferential strain during loading phase of isokinetic flexions. The proposed model was preliminary tested as valid in elbow isokinetic flexions at 90°/s with an acceptable error in the estimated torque as 12%. To further verify the proposed Hill-based model with circumferential strain, 16 more subjects in the 3rd trial participated in both the elbow isokinetic and isotonic flexions, which are more common in sports and training. The newly-developed Hill-based model with circumferential strain was determined for each subject. Torque was estimated throughout elbow isotonic flexions and comparisons was made between estimated torque and measured torque by Biodex. Result of error analysis showed that, except for subject No. 3-3, 9, 11, and 14 with singular detected circumferences, the averaged mean relative error was observed as 24.7%. The contributors to the errors in estimation were identified for subject No. 3-3, 9, 11, and 14 through re-test. In general, the new Hill-based model was proved effective. Hence, the upper-arm circumferential measurement by LGMS was confirmed capable of monitoring the contraction of elbow flexors, in all flexion modes.