**5. Conclusions**

In this study, we explored the effects of backpack loads on leg muscle activations during slope walking. It was concluded that the hip, knee, and ankle extensor muscle activations increased during slope walking, and the hip muscle increased the most among the hip, knee, and ankle muscles. Moreover, muscle activations increased pronouncedly with loads during slope walking, and the knee extensor muscle activations increased more than the hip and ankle muscles. The results in our study imply that the hip and knee muscles play an important role during slope walking with loads. Our results are important for the design of assistant devices, such as exoskeleton robots, to enhance people's walking ability, especially for hikers and soldiers. The hip and knee extension movements during slope walking should be considerably assisted to lower the muscle activations. Future studies could explore the effects of loads and grades on more muscles and involve more participants, including female subjects, to expand the insights into the muscle strategy for providing more suggestions for the design of assistant devices.

Full postal address: Room A1043, Lee Shao Kee S&T Building, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2076-3417/10/14/4890/s1: Table S1: Normalized mean (mean ± SD) EMG activities during inclined walking with a range of backpack loads, Table S2: The ratio of the EMG between inclined walking and level walking across all backpack loads, Table S3: The ratio of the EMG between walking with backpack loads and without backpack loads across all grades.

**Author Contributions:** Conceptualization, Y.L. and Q.S.; methodology, X.G.; software, Y.L. and M.Z.; validation, Y.L. and M.Z.; formal analysis, X.G.; investigation, Y.L. and L.Q.; resources, M.Z.; data curation, Y.L., L.Q., and M.Z.; writing—original draft preparation, Y.L.; writing—review and editing, Q.S. and X.G.; visualization, Y.L., L.Q., Q.S., and X.G.; supervision, X.G.; project administration, Y.L.; and funding acquisition, Q.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** The study was funded by grants from the National Natural Science Foundation of China (Grant No. 51905035 and 51905291), the China Postdoctoral Science Foundation-funded project (Grant No. 2019M660478), and the Ministry of Science and Technology national key R&D program (Grant Number: 2017YFB1300500).

**Acknowledgments:** We thank Yue Zhou and Peidong Ma from Beijing Sports University for providing the lab experiments and giving suggestions for the analysis. We thank all the participants in this study.

**Conflicts of Interest:** The authors have no conflict of interest concerning this manuscript.
