## 2 Methods
### Evaluation Protocol  
Two antagonistic muscles were selected for the experiment, the flexor and the extensor carpi radialis. Surface myoelectric signals were acquired by the UVa-NTS platform by means of silver/silver chloride recessed electrodes placed over the muscles and a reference electrode on the elbow, as shown in Figure 2. The evaluation protocol was carried out in two different sessions, one week apart each other: The first one aimed at familiarisation, the second one at consolidation.  
The system requires a _calibration routine_ aimed at normalising the signal. Thus, in order to check the quality of the EMG, the subject must contract the flexor and the extensor carpi radialis. The EMG strength is checked with the _signal viewer_ and the gain is adjusted if required. Afterwards, a three-step calibration is performed: (i) A baseline calibration with the muscles at rest, (ii) a desired maximum voluntary contraction (DMVC) calibration for the flexor carpi radialis, and (iii) a DMVC calibration for the extensor carpi radialis. Rather than working with a maximal voluntary contraction (MVC), the DMVC is chosen as an isometric sub-maximal MVC close to the MVC, yet comfortable. The MVC is not always a true maximum, it can be fatiguing and uncomfortable [13; 14].  
The _familiarisation session_ started with the _calibration routine_. Afterwards, the _state navigator_ was opened so the patient could locate five targets, as shown in Figure 3, by contracting his or her muscles isometrically: One at rest (located on the top left), one at DMVC for flexor (on the right), one at DMVC for extensor (at the bottom), and the remaining two targets were intended for the desired medium contraction (DMC) of the muscles. The patient was given the option to combine both muscles in order to fix the two DMC targets comfortably. Crosstalk was usually detected during the DMVC target location as a deviation from the axes. We define the crosstalk as the interference in the EMG from the antagonistic muscle through the limb tissues. Likewise, a combination of EMG activity from both muscles and crosstalk was also detected during simultaneous flexor and extensor contraction when locating the DMC targets.  
Figure 2: Patient working with the UVa-NTS platform. Surface EMG electrodes were located on the forearm: On the flexor carpi radialis, on the extensor carpi radialis, and the reference electrode was on the elbow.  
Once the targets were located, the patient went through the five targets. This routine started on the rest target. Then, the patient went to the first target where he or she had to achieve a stay of three seconds (ToT). Once the target was achieved, the patient relaxed to the rest target and stayed there again for ToT, i.e., three seconds. This sequence was repeated for the remaining three targets to complete the routine. The routine was performed four times (four rounds) and the whole duration, NT, was timed. After a five-minutes rest interval, the whole exercise was repeated and timed again. NT is defined in Equation (1), so \(n\) is the target number and \(m\) is the round number.  
\[NT=\sum_{m=1}^{4}\sum_{n=1}^{4}\left(TRT_{mn}+TR_{mn}+2\cdot ToT\right) \tag{1}\]  
The _familiarisation session_ ended with the _Mayo-Pong_ tool. The game was played for five minutes with a paddle size of 0.25 (1 is the size of the side wall) and a speed of 8.4 bounces per minute on each side. After these five minutes, the achieved success rate was displayed and annotated for each of the two muscles.  
The _consolidation session_ repeated the _familiarisation session_ activities one week later and ended with a new task. This task consisted in playing the _Mayo-Pong_ game with an increased level of difficulty for five minutes. Thus, the ball speed was 1.3 times faster and the paddle size was decreased 1.25 times, i.e., to a size of 0.20. After the elapsed time, the success rate achieved was also annotated for evaluation.