## 4 Discussion  
Ankle (TA-SOL and TA-GM muscle pairs) and thigh (RF-BF muscle pairs) antagonist coactivation in forward translation and antigravity sub-phases in the SitTS task in post-stroke subjects exhibit a specific trend toward higher antagonist coactivation. This finding agrees with previous studies involving walking, specifically in the double-support phase of walking [31]. On both, in SitTS and in the double-support phase of walking, antagonist coactivation observed in the two lower limbs seems to reflect a bilateral postural control dysfunction in these subjects. The influence of supra-spinal structures on antagonist coactivation [32] reinforces the value of this synergy in joint stability [33] through precise control of joint position [15]. Previous studies have demonstrated that ankle muscles play a determinant role in influencing proximal muscles activity for stability in both healthy [34,35] and post-stroke subjects [36,37]. Like in walking, SitTS involves sub-phases with specific coactivation patterns across joints and the role of ankle muscles in stability has also been highlighted in elders [38]. The results obtained in the present study showed that it is possible to explore post-stroke subjects' incapacity in performing the SitTS task [39] based on the impairment in the antagonist muscle coactivation modulation against the environment. The results also revealed that during the antigravity sub-phase, IPSI presented the most relevant alteration of antagonist coactivation. These findings are in accordance with other studies in SitTS muscle synergies [37], and also during double support in walking [31], and may be related to the ipsilateral postural control dysfunction often evidenced in lesions affecting the middle cerebral artery territory [26]. In this sub-phase, the plantar flexors act as agonists for stability, and, particularly, the motoneurons of the SOL are more dependent on output from the ventromedial systems than from the reticulospinal system [26,40,41]. Given these assumptions, it is important to think about the mechanisms underlying IPSI vs. CONTRA weakness, while suggesting that IPSI presents the most relevant alteration in antagonist coactivation. Future studies could improve the clarity regarding this area. Globally, the more variable pattern of ankle muscles antagonist coactivation compared to the thigh in both IPSI and CONTRA legs in post-stroke subjects seems to suggest that the distal ones may present with more accuracy in the postural control impairment in both sides of these subjects when performing functional tasks with higher postural control demand. However, as no significant differences were observed in the antigravity sub-phase, in the antagonist coactivation of the thigh, it may be thought that during double support, similarly to studies carried out in gait, the role of ankle coactivation for stabilization during movement may have a major influence on the proximal adjustment [34,36]. This might explain the differences obtained where a higher performing postural control demand was expressed. The non-existence of proximal statistically significative differences may demonstrate the lower requirement of postural performance during the task when compared to  
Figure 3: Antagonist coactivation (%) in healthy and post-stroke subjects (CONTRA and IPSI) in both forward (**A**) and antigravity (**B**) sub-phases of the SitTS task in ankle (TA-GM and TA-SOL) and thigh (RF-BF). distal, which was observed in the results. It would be interesting to complement the linear analysis performed with the combination of a non-linear treatment of the variables under study to explore their behaviour. This might show the trend of existing behaviours in a more detailed spatiotemporal way, describing the variability inherent in human movement in order to better characterize it [11]. The variations expressed during coactivations may also suggest improvement in knowledge regarding the biomechanical characteristics of postural tone and its evaluation [7].