*3.4. Variability Index*

The CV index is used to measure the variability of muscles rhythmic activation during walking. According to the definition provided in Table 1, the value of this index within a cycle is computed as the ratio of the sEMG signal standard deviation (<sup>σ</sup>*i*) to the mean value (*Xi*) in a single *i-th* interval [15,16]:

$$CV\_{i} = \frac{\sigma\_{i}}{\overline{X}\_{i}}, \ i = 1 \ldots k \tag{2}$$

As anticipated in Table 1, definition of interval depends on the specific study target (e.g., gait analysis, swimming, walking). The interval considered in the present study is the gait cycle, assessed from the basographic signal. After the evaluation of *CVi* index in each single stride, the average over all the *k* strides of a single walking task gives the global CV. High CV values indicate a large range of variability for a muscle, characterized by periods of contraction and periods of relaxation; lower values identify a more uniform and constant muscle activity.
