*3.3. Signal Pre-Processing*

Knee angles in the sagittal plane measured by electrogoniometers were low pass filtered with cut-o ff frequency of 15 Hz. Each signal was min-max normalized within each subject, thus mapping the values in the [0–1] interval. An example of normalized flexion-extension knee angle in a representative subject is reported in Figure 2. Footswitch signals were processed for identifying the di fferent gait cycles and phases (stance and swing), according to the approach discussed in [26]. sEMG signals were amplified, high-pass filtered (linear-phase FIR filter, cut-o ff frequency: 20 Hz) and low-pass filtered (linear-phase FIR filter, cut-o ff frequency: 450 Hz) for removing motion artefacts and high frequency noise, respectively. After a full-wave rectification, a second-order Butterworth low-pass filter (cut-o ff frequency: 5 Hz) was applied to extract the envelope of the signal, following the classic indication provided by acknowledged studies by Hermens et al. and Winter et al. [31,32]. Winter proposed a cut-o ff frequency of 3 Hz, while Hermens suggested a cut-o ff frequency of 10 Hz. The cut-o ff frequency of 5 Hz adopted in the present paper seems to be a good compromise between the two approaches. Finally, each sEMG signal was min-max normalized within each subject and for each muscle.
