Validation of Novel Relative Orientation and Inertial Sensor-to-Segment Alignment Algorithms for Estimating 3D Hip Joint Angles

Abstract

Wearable sensor-based algorithms for estimating joint angles have seen great improvements in recent years. While the knee joint has garnered most of the attention in this area, algorithms for estimating hip joint angles are less available. Herein, we propose and validate a novel algorithm for this purpose with innovations in sensor-to-sensor orientation and sensor-to-segment alignment. The proposed approach is robust to sensor placement and does not require specific calibration motions. The accuracy of the proposed approach is established relative to optical motion capture and compared to existing methods for estimating relative orientation, hip joint angles, and range of motion (ROM) during a task designed to exercise the full hip range of motion (ROM) and fast walking using root mean square error (RMSE) and regression analysis. The RMSE of the proposed approach was less than that for existing methods when estimating sensor orientation ( 12 . 32 ∘ and 11 . 82 ∘ vs. 24 . 61 ∘ and 23 . 76 ∘ ) and flexion/extension joint angles ( 7 . 88 ∘ and 8 . 62 ∘ vs. 14 . 14 ∘ and 15 . 64 ∘ ). Also, ROM estimation error was less than 2 . 2 ∘ during the walking trial using the proposed method. These results suggest the proposed approach presents an improvement to existing methods and provides a promising technique for remote monitoring of hip joint angles.

Keywords: 3D joint kinematics; IMU; functional rotation axes; inertial measurement units; inertial sensors; joint angles; motion tracking; wearable sensors.

Figure 1

Marker set used, including calibration only markers, and close up views of the sensor clip used to create marker clusters (schematic—top right, example of clip attached to a leg—bottom right). Dotted lines indicate the sensor or marker is located on the posterior side.

Figure 2

Root mean square error (RMSE) values for all angles and trials from the proposed and functional calibration—strap down integration (FC-STI) methods. Red is for the proposed method results, blue for the FC-STI method results. FE: flexion/extension, AA: ad/abduction, IER: internal-external rotation.

Figure 3

Sample gait cycles for the right hip from example subjects who exhibited (a) good FE and IER angle tracking, though with larger offsets for the FC-STI method and (b) good angle tracking for all three angles. Data were sampled between 20 th and 26 th seconds of the 60s trial. For both subfigures: (top) FE (flexion/extension) angle traces. (middle) AA (ad/abduction) angle traces. (bottom) IER (internal-external rotation) angle traces. Red is the OMC trace, blue is the proposed method trace, purple is the FC-STI trace.

Figure 4

Sample regression plots for the treadmill fast walking trial for the right hip, comparing the proposed and FC-STI to OMC methods. The same trial and subject as shown in Figure 3a is shown. (top) Regressions for the proposed method, with FE, AA, and IER plotted from left to right. (bottom) Regressions for the FC-STI method, with FE, A, and IER plotted from left to right.