## 2 Materials and Methods
### Procedures  
The skin of both IPSI and CONTRA limbs was prepared through standard procedures (shaving, removing dead skin cells and non-conductor elements with alcohol and with an abrasive pad) to reduce the electrical resistance to a level equal to or less than 5 K\(\Omega\), monitored with the electrode impedance checker, before electrode placement in the muscle's mid-belly according to anatomical references [19,20].  
All individuals maintained a sitting position with 2/3 of the femur supported on the seat, whose height was adjusted to 100% of the lower leg length. Then, they were asked to stand up at a self-selected speed without using upper limbs, maintaining them comfortably along the body, or moving the feet [21]. At the end of the task, they remained quietly standing. A one-minute rest period was provided between each trial, and sufficient repetitions were performed to obtain three valid trials. In the post-stroke subjects' group, both lower limbs were analysed, while in the control group, just one lower limb was randomly selected.  
The raw EMG signal was filtered with a band-pass filter (20 Hz-500 Hz), and the root mean square was calculated with a sliding window of 100 ms. The EMG signals have been normalized to the maximum voluntary contraction of the subjects' different muscles. The signal from the force plate was also filtered using a low pass filter of 10 Hz, and the values were normalized to the weight of each subject. The onset of the task, defined as time zero (T0), and the definition of forward translation and antigravity sub-phases were performed through GRF signals. These events were identified through the antero-posterior component of the GRF (FAP) and the vertical component of the GRF (FV) as represented by the schematic strategy illustrated in Figure 1[3,22,23,24].  
Figure 1: Illustrative identification of forward translation (A) and antigravity sub-phases (B) of the SitTS task through FAP and FV signals.  
T0 was defined as the instant when the FAP value was greater or less than the mean of its baseline value plus 2 standard deviations (SD) for at least 50 milliseconds (ms). The forward translation sub-phase was identified in the interval between the end of the previous phase and the maximum value of the FV. As an example, Figure 2 demonstrates a force plate and EMG signals in a 2 s representative window of the SitTS task from two (healthy and stroke) analysed participants.  
In each sub-phase, the mean of the EMG signal was used to assess antagonist coactivation through the following Formula (1) [25]:  
\[\text{C (\%)}=\text{ antagonist activity/agonist}+\text{ antagonist activity}\times 100 \tag{1}\]  
Muscles were classified according to their role during the task. Specifically, during the forward translation sub-phase, the TA and BF muscles were considered the agonists, while in the antigravity sub-phase, this role was attributed to the SOL, GM and RF muscles. The TA muscle is likely to be the most representative for postural adjustments considering that it is activated early on to ensure foot stability and it assists in centre-of-mass forward displacement [4; 26; 27]. When acting in a synergistic pattern with coactivation, the more proximal muscles, despite their secondary role in the task [28], play an important role in thigh stability during trunk-forward translation and forward rotation of the tibia over the foot [29]. In the antigravity sub-phase, the SOL and GM muscles play an agonist role in body support and stability against gravity [30], while the proximal muscles behave as prime movers [27].