# Effects of Arm Swing in Lower Limb Muscle Activation and Coordination During Treadmill Walking
## 4 Discussion
Previous studies also found that the activity of the GLM increased when removing arm swing [57], which was not observed either in our study or in another similar study [59]. In addition, the literature has shown that the activity of latissimus dorsi decreases when removing arm swing [57] and increases when addingweight to the arms [59]. Therefore, it might be interesting to also analyze the activity of low-back muscles in future studies comparing WWA and WNA. To compare WWA and WNA, we calculated the cross-correlation between EMG patterns and found a minimum between-subject cross-correlation of 0.99. Cross-correlation was also very high (0.96) for the LL muscles evaluated by [PERSON] et al., who compared WWA and Nordic walking [37], whereas higher variability was observed in the UL muscles. Nordic walking is a task that actively involves the ULs by integrating walking poles. In our study, we did not ask the subjects to specifically swing the arms in a controlled manner, nor did we control arm swing amplitude or ask them to add weights to the wrists, which are all conditions that have been shown to have an impact on LL muscle activity, intermuscular coherence, walking speed, and other spatiotemporal parameters [15; 45; 59; 60; 61]. However, considering only natural arm swing such as that that involved in this study, some studies have concluded that WNA only leads to shorter and more frequent steps [18; 47], as most did not find any statistical differences in spatiotemporal parameters and kinematics under both uncontrolled and controlled cadences [10; 61]. Though measuring additional parameters such as metabolic consumption, spatiotemporal parameters, or kinematics might be interesting in order to obtain a global picture of the effects of arm swing in walking efficiency, stability, and symmetry, we believe that the added complexity does not compensate for the expected results. Therefore, in future studies we plan to keep a comfortable walking speed with a controlled cadence, but we will increase the number of measured muscles, e.g., by including UL muscles, to quantify muscle effort during arm swing. Muscle synergies were extracted using NNMF, which is the most commonly used factorization method for analysis of muscle synergies [62] and, as has been recently demonstrated by [PERSON] et al. [48], highlights the potential of this method in accurately estimating LL joint movements. We considered four synergies, consistent with previous studies [3; 24; 38; 63; 64], and obtained VAFglobal and VAFmuscle values above 90% and 75%. These thresholds are used in the literature to consider EMG reconstruction acceptable [24]. Our analysis revealed that spatial time-independent coefficients W were comparable to those found in previous studies [3; 24; 38; 63]. Some differences were observed in the synergies in which TA is involved: we identified a single synergy that accounted for most of the variability of TA, which is similar to previous research [38; 63]. In contrast, [PERSON] et al. reported that hamstring muscles (BF and ST) work together with TA at slow cadences [24], while [PERSON] et al. found a synergistic behavior between RF and TA, mainly due to the higher activation of RF and TA during late stance (at around 70%) [3]. These differences might be due to the different walking speeds and cadences used in each protocol. As observed by [PERSON] et al. [24], the main changes in spatial time-independent W coefficients depending on speed are related to TA, which changes from working with the hamstring muscles in late swing to working with the GLM during the initial stance when increasing speed. During this phase, different motor patterns contribute to the controlled transfer of body weight to the support limb.