Testing a Quaternion Conversion Method to Determine Human Three-Dimensional Tibiofemoral Angles During an In Vitro Simulated Jump Landing

Abstract

Lower limb joint kinematics have been measured in laboratory settings using fixed camera-based motion capture systems; however, recently inertial measurement units (IMUs) have been developed as an alternative. The purpose of this study was to test a quaternion conversion (QC) method for calculating the three orthogonal knee angles during the high velocities associated with a jump landing using commercially available IMUs. Nine cadaveric knee specimens were instrumented with APDM Opal IMUs to measure knee kinematics in one-legged 3-4x bodyweight simulated jump landings, four of which were used in establishing the parameters (training) for the new method and five for validation (testing). We compared the angles obtained from the QC method to those obtained from a commercially available sensor and algorithm (APDM Opal) with those calculated from an active marker motion capture system. Results showed a significant difference between both IMU methods and the motion capture data in the majority of orthogonal angles (p<0.01), though the differences between the QC method and Certus system in the testing set for flexion and rotation angles were smaller than the APDM Opal algorithm, indicating an improvement. Additionally, in all 3 directions both the limits of agreement and root mean square error between the QC method and the motion capture system were smaller than between the commercial algorithm and the motion capture.

A Comparison of Inertial Measurement Unit and Motion Capture Measurements of Tibiofemoral Kinematics during Simulated Pivot Landings

“…Such phase differences likely had a minor effect on our results because the angle and angular velocities of each system were calculated relative to the peak values for each trial. Finally, we employed the generic APDM software algorithm to process these data, but a custom-tuned algorithm can improve the results slightly, particularly by reducing the phase lag in the generic APDM results due to the rapid acceleration profiles [36] associated with HAS cycles. In summary, we do not believe that these limitations materially affected the overall results in comparing the two measurement systems.…”

Can Wearable Sensors Provide Accurate and Reliable 3D Tibiofemoral Angle Estimates during Dynamic Actions?

“…Briefly, informed consent was obtained before a participant was fitted with 46 retro-reflective spherical markers; the first 30 markers were placed according to the 30 Rizzoli lower body protocol, with the remaining markers placed in clusters of four on the lateral thigh and gastrocnemius ( Figure 1 ). Motions were collected using a 12-camera motion capture system (Prime 13 Optitrack, Corvallis, OR, USA) and two APDM wearable sensors (APDM Opal, APDM Wearable Technologies, Portland, OR, USA); the wearable sensors were placed similarly to previous works [ 37 ]. All data were collected at 200 Hz and a sync device was used to synchronize the recordings.…”

“…IMU data were filtered using a fourth-order, zero-lag, low-pass Butterworth filter. Optimum cutoff frequencies for the Butterworth digital filter were obtained by applying Winter’s method; this method has been used in previous works [ 37 , 38 ]. Only the training set was utilized in determining the optimal frequencies ( Table 3 ).…”

“…Following filtering, a nine-axis indirect Kalman filter using the quaternion conversion of the IMU accelerometer, gyroscope, and magnetometer data was used. This process is detailed at greater length in our previous work and in the paper by Stanley et al [ 37 , 39 ]. Briefly, this process began through the estimation of a sensor’s current orientations from the angular changes of the previous orientation with the initial alignment estimated to be north-east-down (NED).…”

“…Therefore, the data attributed to the tibial IMU sensor was subtracted from the femoral IMU sensor. The resulting values were then converted into Euler angles, similar to what was done before [ 37 ].…”