Abstract
This paper presents an unrestrained measurement system based on a wearable wireless ultrasonic sensor network to track the lower extremity joint and trunk kinematics during a squat exercise with only one ultrasonic sensor attached to the trunk. The system consists of an ultrasound transmitter (mobile) and multiple receivers (anchors) whose positions are known. The proposed system measures the horizontal and vertical displacement, together with known joint constraints, to estimate joint flexion/extension angles using an inverse kinematic model based on the damped least-squares technique. The performance of the proposed ultrasonic measurement system was validated against a camera-based tracking system on eight healthy subjects performing a planar squat exercise. Joint angles estimated from the ultrasonic system showed a root mean square error (RMSE) of 2.85° ± 0.57° with the reference system. Statistical analysis indicated great agreements between these two systems with a Pearson's correlation coefficient (PCC) value larger than 0.99 for all joint angles' estimation. These results show that the proposed ultrasonic measurement system is useful for applications, such as rehabilitation and sports.
Highlights
Squat exercise is an effective training exercise for maintaining mobility and improving lower-limb muscle function [1]
This study has demonstrated that one ultrasound transmitter and four receivers can be used to obtain high accuracy ankle, knee, hip joint flexion/extension angles estimates
The extended Kalman filter is applied to estimate the displacements in the vertical and horizontal direction of the ultrasonic sensor, and the recorded displacements together with known joint constraints are used to estimate the joint angles of the trunk using the damped least-squares-based technique for the singularity avoidance problem of redundant systems
Summary
Squat exercise is an effective training exercise for maintaining mobility and improving lower-limb muscle function [1]. It has been included as part of athletic training and rehabilitation. Knee and hip joint kinematics have been used in [3] to quantitatively evaluate the motion of squat. In many of these applications, it is essential to monitor the squat exercise under a natural environment without hindrance. This entails the need for an unrestrained low-cost motion tracking system
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