**5. Conclusions**

The work contributes to classifying the characteristic joint movement patterns of the hip and the knee joints during the walking. The study conducted the following three steps to recognize the phase difference angle changes: i) calculation of 3-D joint angles of the hip and knee from the two-IMU sensor network system; ii) calculation of the normalized gait phase diagrams; iii) estimation of the gait phase difference angles for classifying abnormality in gait patterns. The work contributes to IMU signal analysis by introducing the following techniques: instantaneous signal normalization to reduce noise from the acquisition and the gait phase difference angle as a technique for the decomposition of signals. The methods proposed in the paper are suitable for establishing the patterns of different gait movements. The proposed techniques contribute in many ways to state-of-the-art identification, modeling, decomposition, and prediction of IMU signal analysis for various body motion applications.

**Author Contributions:** Conceptualization, K.S.M.; methodology, K.S.M. and S.Q.L.; software, K.S.M. and Y.O.; validation, A.G., and J.A.C.; investigation, K.S.M. and S.Q.L.; resources, K.S.M. and S.Q.L.; data curation, K.S.M. writing—original draft preparation, K.S.M.; writing—review and editing, Y.O. and S.Q.L.; supervision, K.S.M.; project administration, K.S.M.; funding acquisition, K.S.M. and S.Q.L.

**Funding:** This work was partially supported by R&D program of Ministry of Trade, Industry & Energy (N015800182), and NIH/NIMHD (U54 MD012397 -01A1) grant.

**Acknowledgments:** The authors would like to thank Woosub Youm and Kim Phan for their kind assistance.

**Conflicts of Interest:** The authors declare no conflicts of interest.

## **References**

1. Taj-Eldin, M.; Ryan, C.; O'Flynn, B.; Galvin, P. A Review of Wearable Solutions for Physiological and Emotional Monitoring for Use by People with Autism Spectrum Disorder and Their Caregivers. *Sensors (Basel)* **2018**, *18*, 4271. [CrossRef] [PubMed]


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