Area under the EMG envelope

Integrated EMG is defined as the area under the curve of the rectified EMG signal, which is one of the standard amplitude parameter to represent EMG signal characteristics [24]. It is a parameter that frequently used to compare EMG activation and is considered as a measure of voluntary muscle drive [25]. An increase in the integrated EMG signal period, amplitude, and power may represent an increase in firing frequency and a higher muscle fiber recruitment [26].

The selected raw EMG signals from each participant were high-pass filtered at 40 Hz with a zero-lag fourth-order Butterworth filter, demeaned, rectified, and low-pass filtered with a zero-lag fourth-order Butterworth filter at 10 Hz, resulting in the EMG envelope [24]. For each muscle, the filtered signal was normalized to its peak value from across all gait cycles then the area under the EMG envelope curve was calculated and used to represent the neuromuscular indicator as Ai where A can be EHL, TA, SO, GA, VL, RF, SM, or BF, and i can be the unaffected (US) or affected (AS) side. An example area under the EMG envelopes of all 16 muscles used in the study from a participant (P1) is depicted in Fig. 3.

Example area under the EMG envelope curves of all 16 muscles of bilateral legs during a single gait cycle from participant P1 (grey lines are rectified EMG data; A.U., arbitrary unit; RF, rectus femoris; VL, vastus lateralis; SM, semitendinosus; BF, biceps femoris; TA, tibialis anterior; EHL, extensor halluces longus; SO, soleus; GA, gastrocnemius)