PMC6288233_01
The experimental apparatus includes the stimulation of the ulnar/median nerve bundles, and the recordings of high density EMG at the forearm flexor muscles and the finger flexion forces (A). Sample stimulation trains for the LF and HF stimulation mode (B). The stimulation train of a trial on the contralateral side in the HF mode (C). The EMG signal of a representative channel recorded on the contralateral side in the HF mode (D).
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F1
The hand was restrained, and the forearm was supported (Figure 1A). An 8 x 16 channel high density EMG electrode pad (OT Bioelettronica) was placed at the finger flexor muscles to obtain the EMG activities (Figure 1A). The sample stimulation trains for the LF and HF mode are illustrated in Figure 1B. Muscle fatigue was evoked by delivering 20 (for the affected side) or 30 (for the contralateral side) 3-s stimulation trains with 1-s intervals (Figure 1C). Stimulation artifacts were then identified with a threshold-crossing method based on the EMG, and the EMG segment between two consecutive stimulation artifacts were extracted to calculate the EMG area, estimated as the integral of the absolute EMG over time (Figure 1D).
The summated force across fingers for the affected (A) and the contralateral (B) side. The changes of the force level with the sequential stimulation trains for the affected (C) and the contralateral (D) side, and the corresponding fitting results with the exponential function. The changes of the normalized force fluctuation with the successive stimulation trains for the affected (E) and the contralateral (F) side. Comparison of the normalized force between the affected and contralateral sides under the HF stimulaion (G) and the LF stimulaion (H).
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F2
Figures 2A,B illustrate the summated force of all four fingers between the two stimulation modes for the affected and contralateral sides. Figures 2C,D for the affected and contralateral sides, respectively. For all conditions, the force decreased exponentially with the stimulation train. Figures 2G,H illustrate the relative force decay between the affected and contralateral sides under different stimulation modes. The normalized force fluctuation increased with successive stimulation trains (Figures 2E,F), which was consistent with our previous study.
The initial (first stimulation train) strength distribution of the EMG activity under the HF (A) and the LF (B) stimulation, respectively, on the affected side. The initial strength distribution of the EMG activity under the HF (F) and the LF (G) stimulation, respectively, on the contralateral side. The white area in (F) and (G) was eliminated from analysis because of motion artifact contamination. The EMG activity of the channel with the maximum EMG area on the affected (C) and the contralateral sides (H), respectively. Only the EMG activity of the first and the last stimulation train were illustrated. The changes of the EMG area with the successive stimulation trains for the affected (D) and contralateral sides (I), respectively. The changes of the F-EMG ratio with the successive stimulation trains for the affected (E) and the contralateral sides (J), respectively.
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F3
The initial EMG activity distribution and the average EMG of the channel with the highest intensity from the first and the last 3-s stimulation train are shown in Figure 3. The EMG activity intensity decreased substantially over time under the LF stimulation (Figures 3D,I) while the EMG activity under the HF stimulation did not show an obvious declining trend.