IMU and Smartphone Camera Fusion for Knee Adduction and Knee Flexion Moment Estimation During Walking
By Tian Tan and Dianxin Wang
This repository includes the code and models for an IEEE Transactions on Industrial Informatics publication. See the paper When implementing our models, please place the cameras and IMUs according to Hardware and store the data according to Data Format. An example implementation is provided.
Python 3.8; Pytorch 1.7.0; Cuda 11.0; Cudnn 8.0.4; matplotlib 3.3.2; numpy 1.19.4; h5py 3.0.0; Scikit-learn 0.23.2
Versions different from ours may still work.
iPhone 11 (Apple Inc., Cupertino, CA, USA) were placed vertically on the right side (90 degree from walking direction) and back of the subject (180 degree from walking direction) (see the figure below). The distance from treadmill center to each camera was 3m and the height from treadmill plane to each camera was 1.2m.
Eight SageMotion IMUs (SageMotion, Kalispell, MT, USA) were placed on each subject. For each IMU, its z-axis is aligned with the body segment surface normal, y-axis points upwards, and x-axis being perpendicular to the y and z axes following the right-hand rule.
IMU anatomical locations:
| Segement name | Anatomical definition |
|---|---|
| trunk | mid-point between sternum jugular notch and sternum xiphisternal joint |
| pelvis | mid-point between left and right anterior superior iliac spine |
| both thighs | mid-point between anterior superior iliac spine and femur medial epicondyle |
| both shanks | mid-point between femur medial epicondyle and tibia apex of medial malleolus |
| both feet | second metatarsal |
Ground reaction force (GRF) and center of pressure (CoP) were collected with an instrumented treadmill with two split belts (Bertec Corp., Worthington, OH, USA). CoP are calibrated with marker trajectories that the left-back corner was the origin of both force plates and optical motion capture. The z-axis is aligned with the vertical direction pointing upwards, y-axis is aligned with the walking direction (in general aligned with anterior-posterior direction) pointing forwards, and x-axis being perpendicular to the y and z axes following the right-hand rule (in general aligned with medio-lateral direction).
32 reflective markers were placed on each subject and collected by an optical motion capture system (Vicon, Oxford Metrics Group, Oxford, UK). Markers' corresponding body landmarks are visualized in the following figures.
Each subject's data should be stored as a 3-dimensional matrix. The first dimension is walking steps. The second dimension is time step from heel-strike - 20 samples to toe-off + 20 samples. Both heel-strike and toe-off are detected using right foot IMU data. Its length is 152, which is the lenghth of the longest step. Zeros were appended in the end of shorter steps. The third dimension contains 256 data fields, which are described in the following section. All 17 subjects' data are available on SimTK.org https://simtk.org/projects/imukinetics. Also, marker and force data have been processed by AddBiomechanics and is available in https://addbiomechanics.org/download_data.html
example_data.h5 is a correctly formatted example data file containing 10 walking step of 2 subjects. The dimension of each subject's data is: 10 steps * 152 samples * 256 data fields.
| Field name | Explanation |
|---|---|
| force_phase | ground-truth stance phase |
| body weight | weight of the subject, being constant for the whole step |
| body height | height of the subject, being constant for the whole step |
The external moments of the right knee were computed using cross product function [1]
| Field name | Explanation |
|---|---|
| EXT_KM_X | external knee moment in sagittal plane, also known as knee flexion moment |
| EXT_KM_Y | external knee moment in frontal plane, also known as knee adduction moment |
| EXT_KM_Z | external knee moment in transverse plane |
IMU fields are named in the form of "Measurement(Axis)_Segment".
| Measurement name | Explanation |
|---|---|
| Accel | acceleration |
| Gyro | gyroscope |
| Mag | magnetometer |
| Quat | quaternion |
| Segments name | Explanation |
|---|---|
| L_FOOT | left foot |
| R_FOOT | right foot |
| R_SHANK | right shank |
| R_THIGH | right thigh |
| WAIST | pelvis |
| CHEST | upper trunk |
| L_SHANK | left shank |
| L_THIGH | left thigh |
Videos recorded by two smartphone cameras were processed by OpenPose [2], a body keypoint detection library, a body keypoint detection library. 2D positions of left/right shoulders, left/right hip, mid-hip, left/right knees, left/right ankles, and left/right heels detected by OpenPose's Body_25 model.
2D joint position fields are named in the form of "Joint_Axis_Camera". For example, LShoulder_x_90 means pixel position (1080 pixel in total) of left shoulder joint in horizontal direction of right camera view. RShoulder_y_180 means pixel position (1920 pixel in total) of right shoulder joint in vertical direction of back camera view.
GRFs' and CoPs' field names are in the form of "plate_2_force/cop_Axis". Plate 2 corresponds to right foot.
Markers' field names are in the form of "MarkerName_Axis".
Five types of models are provided:
(1) a fusion model based on eight IMUs and cameras
(2) a fusion model based on three IMUs (pelvis and feet) and cameras
An implementation (a_load_model_and_predict.py) is provided as an example of using the trained model.
[1] D. J. Rutherford and M. Baker, “Knee moment outcomes using inversedynamics and the cross product function in moderate knee osteoarthritisgait: A comparison study,”Journal of Biomechanics, vol. 78, pp. 150–154, 2018.
[2] Z. Cao, G. Hidalgo Martinez, T. Simon, S. Wei, and Y. A. Sheikh,“Openpose: Realtime multi-person 2d pose estimation using part affinityfields,”IEEE Transactions on Pattern Analysis and Machine Intelligence,vol. 43, no. 1, pp. 172–186, 2019
