## 2 Methods
### Performance Indexes  
The proposed EMG-driven game concept enables the creation of a set of game-related performance indexes that can be used to assess the rehabilitation process. The following paragraphs show the available indexes both for the _state navigator_ and the _Myo-Pong_ tool. The _state navigator_ has a set of targets located by the user. Once the targets are set, the user chooses a target, navigates to it, and sets on it by keeping a steady effort over a given period of time. Afterwards, the user relaxes the muscle and goes back to the rest state. Then, the same sequence repeats for the rest of the targets. This tool enables the evaluation of these performance measurements:  
* _Time to reach the target_ (TRT), the time to achieve a defined effort from the rest state, i.e., the time to reach each target with the pointer from the top left corner of the tool;
* _Time to rest_ (TR), the time to reach the rest state, i.e., the top left corner;
* _Time on target_ (ToT), the time the user can maintain the pointer on a given target;
* _Fatigue time_ (FT), defined as the elapsed time before the user cannot continue to keep lock on the targets. This time is defined since the user starts navigating, although other definitions are possible depending on the task.  
The _Myo-Pong_ tool training configuration is set on the basis of two parameters: (i) The speed of the ball: Constant (slow/fast) or incremental, and (ii) the length of the player bar (paddle). The system allows us to evaluate the training efficiency by means of the following scores:  
* _Success rate_ (SR), the ratio of successful hits to unsuccessful hits, the latter being defined as the total amount of collisions against the player's wall (left or right side of the window). The complementary is the _Error Rate_ (ER), which is the failure ratio related to the total amount of collisions;
* _Admissible speed_ (AS), measures the maximum ball speed the user can track, without exceeding a given error rate. The AS can be related to the ability to perform rapid actions with a prosthetic limb;
* _Precision control_ (PC), measures the accuracy in the actions. The measurement is performed by changing the bar length, i.e., the shorter the bar, the more difficult the hits. The bar size setting is suitable for practicing accuracy in proportional control of prosthetic limbs;
* _Fatigue time_ (FT), measures the time interval that maintains a steady ER once this rate is stabilised. The measure is finished when the ER is increased due to muscle fatigue.  
From the aforementioned performance measurements and scores, we defined two performance indexes to evaluate quantitatively the evolution in the rehabilitation process. For the _state navigator_,  
Figure 1: Tools in the UVa-NTS. **Top left**: _signal viewer_ split in two graphs displaying the electromyography (EMG) signals from two muscles; **top right**: _state navigator_, where the rest point is the upper left corner and the pointer is centred due to the muscles effort; **bottom right**: _virtual arm_ that moves when the _state navigator_ pointer reaches a target; **bottom left**: _Myo-Pong_ where the two paddles, due to the muscles’ activity, are displaced from the rest sites located at the top of each side.  
the selected performance index was the _navigation time_ (NT), which is a combination of the TRT plus the TR plus a predefined ToT for each target. This index evaluates the capability to perform controlled effort changes using muscle contractions. As on the _Myo-Pong_, the selected performance index was the SR, which assesses the muscle control in a continuous effort range. These indexes assess the ability to command EMG-driven systems such as a prosthetic robotic arm, whether they work with proportional control or with discrete control, as each target represents a state within a set of discrete states.