# Effects of Arm Swing in Lower Limb Muscle Activation and Coordination During Treadmill Walking
## 4 Discussion  
This study analyzed the influence of arm swing on lower limb (LL) muscle activity and coordination during comfortable treadmill walking. Humans exhibit interlimb coordination between upper limbs (ULs) and LLs during walking, with arm swing being an integral part of this coordination [1,6]. Previous research examined the effects of walking speed, walking surface, or body-weight support on muscle activity and coordination [39,40,55] but, to the best of our knowledge, this is the first time that muscle synergies have been compared between walking with arms (WWA) and walking with no arms (WNA). The ultimate objective of this research was to verify to what extent the absence of arm swing during robot-based treadmill gait therapy may affect muscle activation, which might have an impact on the design of novel devices that allow normal arm swing. First, we measured the EMG activity of 8 LL muscles from ten young, healthy subjects during treadmill WWA and WNA at comfortable speeds, following a similar protocol to those carried out in previous studies [18,24,38,52,56]. Both the EMG envelopes and the timing of peak occurrence (\(max_{x}^{EMG}\)) were similar to those presented in the literature: the activation peak occurred before 20% of the GC for GLM, RF, and VL; at around 90% of the GC for BF and ST; at around 40% of the GC for GaM and SOL; and at the beginning of the GC for TA (at around 0%). As most of the previous studies presented normalized EMG data, the EMG amplitudes obtained in this study cannot be directly compared. Finally, to the best of our knowledge, only a few studies have analyzed EMG activity during WWA and WNA [18,57]. [PERSON] et al. [18] showed that the largest statistical differences in EMGs were found in ST and SOL at the beginning of the GC (between 0% and 30%) and for ST at the end of the GC (between 80% and 100%). These results are consistent with our observations. However, we also observed significant differences in the GLM at 50% of GC and sporadic statistical differences for GaM and VL. In our study, we used SPM analysis to examine individual differences in muscle activation patterns [31] between WWA and WNA, but no significant differences were observed. It is important to emphasize that, while SPM compares the mean of the linear envelopes of the EMG signals for each subject and task, the analysis of muscle synergies takes into account the stride-by-stride variability within each subject and task. Moreover, although differences in GLM, VL, ST, GaM, and SOL, as well as in the H components, were identified using the Wilcoxon signed-rank test following the methodology of previous studies [38,51], no significant differences were found in the recruitment timing of each synergy throughout the gait cycle (Hs) when analyzed with SPM using a non-parametric paired t-test. These findings highlight the complementary nature of the two approaches and underscore the need for appropriate statistical methods to minimize false-positive rates. In terms of EMG amplitude, the increased variability and activation of RF and VL during WNA, particularly at the beginning of the stance phase, may reflect compensatory adaptations to the altered stability demands caused by the absence of arm movement [8]. These muscles play a key role in motor control of the load response phase and ensure balance during initial stance [58]. This effect varies between individuals, underscoring inter-subject variability and the limitations of our small sample size. This trend is also reflected in Synergy 1, where greater and earlier quadriceps muscles activation during WNA was observed at 0%, 5%, and 90% of the GC. Interestingly, these findings align with those of [PERSON] et al. [37], who reported a decrease in RF activity during Nordic walking, a task requiring increased arm movement and effort. This suggests that changes in arm activity, whether increased or absent, can significantly influence LL muscle activation and coordination.