# Effects of Arm Swing in Lower Limb Muscle Activation and Coordination During Treadmill Walking
## 4 Discussion
These patterns include: the eccentric action of TA to control the descent of the foot to the ground; the eccentric contraction of RF, which moderates knee flexion and facilitates the absorption of energy from the trunk; and pelvic obliquity controlled by the hip adductor muscles, such as GLM, to prevent the elevation of the center of gravity induced by the body mass over the hip [58]. Therefore, this phase, corresponding to Synergy 1, may exhibit individual variations due to its complexity. As to the spatial time-independent coefficients W of Synergy 2, 3, and 4, they were composed of GaM and SOL; BF and ST; and TA, respectively, which is consistent with their functional contributions. In Synergy 2, GaM and SOL are activated concentrically during midstance and late stance, supporting body weight and aiding propulsion. During WNA, their increased activation throughout stance reflects compensatory demands for stability due to the absence of arm swing. In addition, in Synergy 3, BF and ST, key for knee flexion during swing, also decelerate the leg before heel strike and stabilize the ankle during early stance. During WNA, their earlier and lower activation at -75% of the gait cycle suggests an anticipatory adjustment to shorten swing and maintain balance. Finally, in Synergy 4, TA activates progressively in the second half of the gait cycle to ensure dorsiflexion during swing. During WWA, an earlier peak at -75% of the cycle likely compensates for reduced stability in treadmill walking with limited arm swing. Time-dependent coefficients H were also comparable to literature [3,24,38,63], as the peaks of the four synergies occurred at the beginning of the GC for Synergy 1, at around 40% of GC for Synergy 2, and at the end of the GC for Synergy 3. Differences were observed in Synergy 4, consistent with the differences observed in the activity of TA. [PERSON] et al. [24] demonstrated that decreased cadence delays the peak of the synergy related to TA (Synergy 4, in our case). This is consistent with the results observed in the literature: [PERSON] et al. found that the peak occurred at around 70% of GC when participants walked at 0.5 m/s [38]; we observed, as did [PERSON] et al. and [PERSON] et al., that the peak occurred at 90% of GC when walking at 1.0 m/s [63] or with slow cadence (42 strides/min) [24], whereas when walking at 1.25 m/s [3] or with faster cadences [24], the peak occurred at the beginning of the GC. These results suggest that the timing of TA activation in Synergy 4 is highly dependent on gait speed and cadence. Specifically, at slower speeds or cadences, the prolonged contact time between the foot and the ground may necessitate greater and later TA activation to prevent abrupt foot-ground collisions and maintain ankle stability during the initial stance phase. We also calculated the FWHM, which indicates the percentage of GC at which a synergy can be considered active. Our results are consistent with the observations of [PERSON] et al. [37] for the WWA task, as all synergies were active around 15% of GC, with a longer duration for the synergy related to GaM and SOL (Synergy 2, in our case). Though the data cannot be directly compared to the literature, [PERSON] et al. also did not find meaningful changes between WWA and Nordic walking [37]. Moreover, we observed a high correlation between synergies during WWA and WNA, both in the spatial time-independent (\(dot^{W}>0.97\)) and time-dependent coefficients (\(crosscorr^{H}>0.99\), \(RMSE_{FWHM}^{H}<0.12\)). Therefore, considering all performance indicators, our results demonstrate that WWA and WNA present very similar muscle coordination, even closer than WWA and Nordic walking [37] or cycling [24]. In the current work, the sample size was relatively small, consisting of ten young, healthy subjects.