## 3 Results
### Muscle Activities  
As expected, the mean muscle activities of the hip, knee, and ankle extensors generally increased with the increase of the load mass and grade (Figures 2-5 and Table S1). Both the grade and load mass had a significant effect on all muscles (\(p<0.05\)), and none of the muscles showed significant grade-load-mass interactions (\(p>0.05\)). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Normalized Step Length**} & \multicolumn{4}{c}{**Normalized Step Width**} \\ \cline{2-10}  & **0\({}^{\circ}\)** & **3\({}^{\circ}\)** & **5\({}^{\circ}\)** & **10\({}^{\circ}\)** & **0\({}^{\circ}\)** & **3\({}^{\circ}\)** & **5\({}^{\circ}\)** & **10\({}^{\circ}\)** \\ \hline \multirow{2}{*}{0 kg} & 1.00 & 1.07 & 1.03 & 1.08 & 1.00 & 1.03 & 1.04 & 1.02 \\  & (0.06) & (0.06) & (0.07) & 1.00 & (0.05) & (0.07) & (0.05) \\ \multirow{2}{*}{10 kg} & 1.06 & 1.10 & 1.06 & 1.05 & 0.98 & 0.98 & 1.04 & 1.02 \\  & (0.06) & (0.09) & (0.10) & (0.09) & (0.08) & (0.06) & (0.06) & (0.06) \\ \multirow{2}{*}{20 kg} & 1.09 & 1.13 & 1.10 & 1.10 & 1.04 & 0.98 & 0.95 & 1.07 \\  & (0.05) & (0.09) & (0.08) & (0.08) & (0.06) & (0.09) & (0.06) & (0.01) \\ \multirow{2}{*}{30 kg} & 1.10 & 1.08 & 1.06 & 1.02 & 1.09 & 1.00 & 0.97 & 1.03 \\  & (0.08) & (0.06) & (0.08) & (0.06) & (0.09) & (0.08) & (0.07) & (0.05) \\ \hline \hline \end{tabular} All the step lengths and step widths were divided by the values during level walking without backpack loads. Therefore, the normalized step lengths and step widths were “1.00”, and all the normalized step lengths and step widths were dimensionless in Table 1.  
\end{table} 
Table 1: Normalized step lengths and step widths during different grades of walking with different load masses. Figure 2: The mean EMG signals for muscles during different slope walking across all load masses in one gait from right heel strike to the next right heel strike, normalized to the mean activity during level walking without backpack loads. * Representing the mean EMG was significantly different from level walking across all the backpack loads, according to post hoc comparisons with a Bonferroni adjusted level of significance (\(p<0.0072\)). The red line represented the normalized mean EMG of each muscle at different grades without backpack loads. The yellow one represented the normalized mean EMG of each muscle at different grades with 10-kg backpack loads. The green one represented the normalized mean EMG of each muscle at different grades with 20-kg backpack loads. The blue one represented the normalized mean EMG of each muscle at different grades with 30-kg backpack loads. Figure 3: The muscle activity of leg muscles in one gait during different slope walking across all backpack loads, normalized to the mean activity during level walking without backpack loads: (**a**) The muscle activity of the hip extensor muscles. (**b**) The muscle activity of the knee extensor muscles. (**c**) The muscle activity of the ankle muscles. The different colors of the curves had the same representation as those colors in Figure 2. The gray area represented the higher muscle activation duration in one gait. RMS: root mean square. Figure 4: Mean EMG signals for muscles while walking across all grades with different backpack loads during one gait from right heel strike to the next right heel strike, normalized to the mean activity during level walking without backpack loads. * Representing the mean EMG was significantly different from walking without backpack loads across all the grades, according to post hoc comparisons with a Bonferroni adjusted level of significance (\(p<0.0072\)). The line in dark gray represented the normalized mean EMG of each muscle with different load masses during level walking. The line in orange represented the normalized mean EMG of each muscle with different load masses during slope walking at grade 3\({}^{\circ}\). The line in purple represented the normalized mean EMG of each muscle with different load masses during slope walking at grade 5\({}^{\circ}\).