# Lower Limb Muscle Activation in Young Adults Walking in Water and on Land
## 4 Discussion  
This study aimed to investigate lower-limb muscle activation during walking gaits performed by young adults in water versus on land. Our hypothesis that TA activation would be greater during the swing phase of aquatic treadmill walking compared to land treadmill walking at matched speeds was confirmed. Additionally, we hypothesized that the difference in TA activation between aquatic treadmill walking and land treadmill walking would be more pronounced at 3.5 mph compared to 2.5 mph, which was also confirmed. We observed a significant 15% increase in TA RMS sEMG during the swing phase while walking in water compared to walking on land at 2.5 mph ([PERSON]'s \(d\) = 0.42). The increase in TA RMS sEMG was considerably more pronounced for walking in water compared to on land at 3.5 mph (123%; [PERSON]'s \(d\) = 1.13). We also observed meaningful effects on swing phase TA activation between water walking conditions. For instance, TA RMS sEMG increased 94% ([PERSON]'s \(d\) = 1.77) during walking in the water at 3.5 mph compared to 2.5 mph. Additionally, TA RMS sEMG significantly increased by 52% ([PERSON]'s \(d\) = 1.11) for walking in water at 3.5 mph performed with 75% jet resistance compared to without jet resistance. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Measure** & **Water 3.5 mph** & **Water 3.5 mph + Jets** \\ \hline Stance time (s) & 0.63 (0.04) & 0.60 (0.05) a  \\ Swing time (s) & 0.57 (0.05) & 0.57 (0.06) \\ Stride length (m) & 1.87 (0.13) & 1.83 (0.14) a  \\ Stride rate (strides\(\times\)s\({}^{-1}\)) & 0.84 (0.06) & 0.86 (0.09) a  \\ TA RMS Stance (\(\mu\)V) & 101.2 (40.3) & 152.7 (61.9) a  \\ GM RMS Stance (\(\mu\)V) & 110.4 (41.5) & 206.2 (98.2) a  \\ Co-A Stance (\%) & 110.4 (56.8) & 92.4 (60.7) \\ TA RMS Swing (\(\mu\)V) & 217.4 (81.0) & 330.4 (121.7) a  \\ GM RMS Swing (\(\mu\)V) & 18.1 (7.8) & 27.7 (10.2) a  \\ Co-A Swing (\%) & 9.1 (5.3) & 9.1 (4.0) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Central tendency and dispersion results were compared between water walking conditions at 3.5 mph with and without jet resistance. Indeed, our findings regarding the heightened TA activation during the swing phase of water walking compared to land walking are compatible with the effects of fluid drag. Fluid drag is known to have a linear relationship with fluid density and a quadratic relationship with relative speed. In our study, the water temperature was carefully regulated and maintained at approximately 29.5 \({}^{\circ}\)C, resulting in a water density estimate of 995-996 kg/m\({}^{3}\)[36]. This density is approximately 800 times greater than that of ambient air [37]. In addition, by increasing the walking speed in water or by adding jet resistance, fluid drag (water's resistance to the motion of the foot) increases quadratically, further amplifying the muscle activation necessary to maintain the position of the foot [23]. Our findings indicate that TA activation during water walking is modulated by either increasing the walking speed or by introducing jet resistance opposing the direction of the walking gait. We observed an increase in stride rate both on land and in water when walking speed was raised to 3.5 mph compared to 2.5 mph (+15%, [PERSON]'s \(d\) = 1.40), which coincided with decreases in both stance and swing time, particularly in water. These findings align with the notion that increased lower limb velocity was accompanied by increased fluid drag, attributable to the quadratic relationship between the velocity of the foot relative to the surrounding water. Stride rate also increased for the 3.5 mph water walking condition when jet resistance was added; however, this increase appeared to be largely due to a reduction in stance time as there was no significant difference in swing time between the 3.5 mph water walking and 3.5 mph water walking with added jet resistance. This indicates that the application of jets led to greater motor drive as participants exerted more effort to maintain a stationary walking position on the aquatic treadmill.