# Lower Limb Muscle Activation in Young Adults Walking in Water and on Land
## 4 Discussion
Anecdotally, many participants self-reported localized fatigue in the TA after completing the 3.5 mph water walking condition with jets, which supports the observed increase in TA activation. Our hypothesis regarding greater GM activation in water compared to land during the stance phase of walking gait was not supported by the findings of this study. GM RMS sEMG was lower during the stance phase of water walking compared to land walking, with a more substantial effect observed at the matched 2.5 mph speed condition (+52%; [PERSON]'s \(d\) = 1.82) relative to the 3.5 mph speed condition (+24%; [PERSON]'s \(d\) = 0.65). While we expected the GM to play a larger role in propelling the body forward in the presence of increased fluid drag, the findings suggest that the buoyancy effect of water immersion, which offloads body weight, reduces GM activation during stance. Interestingly, as fluid drag increases with walking speed, differences in GM activation between water and land walking decrease. We also hypothesized that there would be no significant difference in GM activation during the swing phase of walking in water compared to on land. This hypothesis was partially supported, with no significant difference observed between water and land walking at the matched 3.5 mph speed condition. Curiously, however, GM RMS sEMG was significantly reduced for the 2.5 mph water walking condition relative to all other conditions. This observation may reflect a differential balance of contributions from buoyancy and fluid drag expressed at different walking speeds in water. While our study did not specifically hypothesize about Co-A indices, we did observe that the Co-A of the GM during the swing phase was greater on land compared to in water, irrespective of the matched speed conditions at 2.5 mph (56%; [PERSON]'s \(d\) = 0.88) and 3.5 mph (37%; [PERSON]'s \(d\) = 0.70). Swing phase Co-A of the GM was influenced by a combination of increased TA RMS sEMG across both speed conditions and reduced GM RMS sEMG for the environment-matched 2.5 mph speed condition. This suggests the potential that net ankle torque is shifted toward dorsiflexion when walking is performed in water compared to on land at 2.5 mph. We also observed that the Co-A of the TA during the stance phase was greater in water compared to on land (44%; [PERSON]'s \(d\) = 1.05), a finding that was explained by reduced GM activation, given that there was no significant difference in TA activation between land and water conditions. Collectively, the medium-to-large effects observed in TA and GM activation during water walking compared to land support the notion of accentuated TA activation when walking is performed in water. However, it is important to contextualize these findingswithin the existing body of research. Prior studies have not observed significant differences in TA activation between land and water walking performed at self-selected speeds [24; 25]. This lack of disparity may be attributed to participants' tendency to opt for slower walking speeds in water [24]. Consequently, mismatched speeds resulting in a greater speed of walking on land, paired with a reduction in fluid drag in the water, likely contribute to the absence of a significant effect of the environment on TA activation [24; 25; 26; 27]. To target TA activation through aquatic treadmill walking exercise, it is important to optimize walking intensity through variables such as speed, water resistance application, and water depth. Moreover, further research is warranted to determine whether TA activation magnitude, duration, or a combination of both has a greater impact on longitudinal biomechanical and neural adaptations. While our study focused on RMS sEMG, future studies could explore muscle activation in terms of absolute duration and total activation [38]. Furthermore, it is essential to customize optimal walking conditions for specific populations, especially those with neurological or mobility limitations.