sEMG Signal Acquisition Strategy towards Hand FES Control

Abstract

Due to damage of the nervous system, patients experience impediments in their daily life: severe fatigue, tremor or impaired hand dexterity, hemiparesis, or hemiplegia. Surface electromyography (sEMG) signal analysis is used to identify motion; however, standardization of electrode placement and classification of sEMG patterns are major challenges. This paper describes a technique used to acquire sEMG signals for five hand motion patterns from six able-bodied subjects using an array of recording and stimulation electrodes placed on the forearm and its effects over functional electrical stimulation (FES) and volitional sEMG combinations, in order to eventually control a sEMG-driven FES neuroprosthesis for upper limb rehabilitation. A two-part protocol was performed. First, personalized templates to place eight sEMG bipolar channels were designed; with these data, a universal template, called forearm electrode set (FELT), was built. Second, volitional and evoked movements were recorded during FES application. 95% classification accuracy was achieved using two sessions per movement. With the FELT, it was possible to perform FES and sEMG recordings simultaneously. Also, it was possible to extract the volitional and evoked sEMG from the raw signal, which is highly important for closed-loop FES control.

Figure 1

Electrode placement using a personalized template to find sEMG signal for acquisition task and stimulation location. After cleaning the skin and placing the electrodes, the isometric contraction (hand open, power grasp, fine pinch, pronation, and supination) was performed by the subject during 10 seconds, with 10 seconds for rest. The task was repeated 10 times. A session included a task for each movement.

Figure 2

OpenViBE flow diagram used to acquire raw sEMG signal (a); image cue synchronization control (b). This algorithm completes a movement task.

Figure 3

sEMG signal processing algorithm. The signal was filtered for 60 Hz, baseline was subtracted through DWT, and the envelope signal that selected the active pattern was obtained.

Figure 4

sEMG signal acquisition for tasks (hand open, power grasp, fine pinch, pronation, and supination) with FES stimulation. An isometric contraction was performed by the subject for each part of the trial. The motion was repeated 5 times per part. A session included 5 repetitions of volitional contraction, followed by 5 repetitions of sEMG evoked by FES, and finally, 5 repetitions of volitional contraction plus the evoked sEMG by the FES stimulation.

Figure 5

Subject 1, open hand/rest. Comparison of sEMG signal before and after processing using DWT. (a) Raw sEMG signal containing baseline drift and 60 Hz noise. (b) Processed sEMG signal drift-free and visible active and rest patterns.

Figure 6

For subject 1, (a) open hand and (b) power grasp, sEMG processed and envelope signal obtained for active pattern selection. Example for channels 1 and 2 of 8.

Figure 7

Analysis of window length for (a) 20 ms and (b) 3 s for all features (MAV, WL, SD, IAV, and V) and 8 channels, using data from the 6 subjects.

Figure 8

Subject 1 using FELT: (a) channels 1 and 2 for open hand and (b) channels 1 and 2 for power grasp.

Figure 9

(a) Power grasp sEMG signal recorded from trial (algorithm Figure 4), channel 1. Baseline has been eliminated using algorithm of Figure 2. (b) Spectogram of sEMG signal, where activity in the 30 Hz band for the 2nd and 3rd sets of motions and their harmonics can be observed.

Figure 10

Power grasp, subject 1, channel 1, sEMG signals of the 3 parts of the trial. (a) Set of 5 isometric contractions of the selected movement, each lasting 1 second with 3 seconds rest. (b) 5 FES stimulations of the selected movement, each lasting 1 second with 3 seconds rest. (c) 5 isometric contractions during FES stimulations of the selected movement, each lasting 1 second with 3 seconds rest.

Figure 11

Comparison of sEMG signals between 2 parts of the trial involving FES application. (a) Raw signal including FES (top) and sEMG signal evoked by FES free of the stimulus (bottom). (b) Raw signal including volitional sEMG and FES (top) and volitional sEMG signal and evoked by FES free of the stimulus (bottom).