PMC7174775_01
The figure shows the process of the electrode placement. (A) The flexible custom-made sleeve that was used for marking the skin of the participant. The sleeve is flexible only around the circumferal direction and stiff along the longitudinal direction of the arm. (B) The marked skin of the participant. The longitudinal inter-electrode distance (LID) is fixed at 2 cm (L), while the circumferal inter-electrode distance (CID) depends on the forearm width of each participant. (C) The participant with all the 64 electrodes placed. The imaginary line (red line) that connects the lateral epicondyle and the styloid process of the ulna was used as the border between the dorsal and ventral side of the forearm. The placement of the electrodes starts right above this line, with electrode number one placed proximally (at 20% of forearm length from the elbow) and eight distally. The rest of the electrode rows are placed counterclockwise as someone is looking at his right arm. (D) This way, electrodes 1-32 were placed over the dorsal side (see sketch) and 33-64 over the ventral side of the forearm. The center of gravity (COG) is also shown for this gesture.
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F1
The experimental setup (Figure 1) included several components, and it was designed to record HD-sEMG signals from the forearm in a repeatable and systematic way. Then, we measured the forearm length from the lateral epicondyle until the styloid process of the ulna and the forearm circumference at 20% of the forearm length from the elbow (Figure 1). The participant had to wear a perforated sleeve (Figure 1) with equally placed holes and elastic only along the circumferal direction to ensure that the electrode placement was standardized for all participants. We used a non-permanent marker to mark the skin of the participant (Figure 1) and then visually inspect the markings before applying the electrodes. The first row of electrodes was placed above the imaginary line between the lateral epicondyle and the styloid process of the ulna and the last row below in such a way that the line lay in the middle between the two rows of electrodes (Figure 1).
The 10 repetitions of third Duchenne muscular dystrophy (DMD) participant (DP3) for wrist extension that were used to acquire the average normalized map. The lower part shows an example of the protocol followed to record the data. In this example, the participant was instructed to extend his wrist for 3 s and then rest for 3 s. This was repeated 10 times. The same procedure was followed for all the seven gestures.
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F2
Participants performed seven different gestures involving hand and wrist motions (Figure 2). For every gesture, 10 repetitions of 3 s contractions were performed, together with 10 repetitions of 3-s resting periods between the contractions (Figure 2). For every electrode, the average of this 1-s contraction was calculated and used to construct 10 heatmaps per gesture (Figure 2).
The heatmaps of two representative gestures for the three participants with Duchenne muscular dystrophy (DMD) and one healthy participant. (A-D) show wrist extension heatmaps for DP1 (A), DP2 (B), DP3 (C), and HP6 (D). (E-G) show wrist flexion heatmaps for DP1 (E), DP2 (F), DP3 (G), and HP6 (H). Regarding wrist extension, all participants exhibit similar activation patterns. However, for wrist flexion, there is higher variability in the activation patterns within participants. X marks show the center of gravity (COG) for each heatmap. Only the activations that are higher than 80% are used to calculate the COG.
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F3
For the visual inspection of the forearm activity per gesture, we constructed activity heatmaps by averaging the 10 repetition heatmaps (Figure 3). The COG was calculated over electrodes presenting activations equal or larger than 80% of the maximal value of the heatmap (Figure 3). The 10 heatmaps, one per gesture repetition, were used to construct one single average heatmap per gesture per participant (Figure 3) that was used for the motor control analysis.
The histogram of squared Pearson correlation between the 10 repetitions for all gestures and for all participants. High correlation shows similarity between the repetitions and thus high repeatability. Both healthy and Duchenne muscular dystrophy (DMD) participants achieved similarly high repeatability on the tasks. The full vertical lines represent the mean and the dashed the standard deviation. The number of unique comparisons between 10 repetitions is 45 multiplied by the seven gestures makes 315 unique comparisons per participant. That explains the total of 2,520 events in the healthy histogram compared to the 945 in the DMD.
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F4
For every gesture, this resulted in 45 unique comparisons between the 10 repetitions and thus 45 coefficients per gesture (Figure 4).
(A) The maximum normalized (left) and absolute (right) activation for each of the 10 repetitions of each gesture for all participants. Healthy participants generally performed the tasks with low levels of maximum normalized activation, while participants with Duchenne muscular dystrophy (DMD) showed higher levels of maximum normalized activation during the tasks. However, the maximum absolute activations were higher for the healthy participants. The full vertical lines represent the mean and the dashed the standard deviation. (B) The average center of gravity (COG) for the seven gestures for the healthy participants (black) and the participants with DMD (red). Healthy participants (gray shaded area) show on average a broader spatial distribution of the seven gestures than the participants with DMD (red shaded area). The red line represents the imaginary line that connects the lateral epicondyle and the styloid process of the ulna and was used as the border between the dorsal and ventral side of the forearm (see also Figure 1). The COG coordinates are normalized over the forearm circumference (COGx) and length (COGy).
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F5
The temporal distribution of activations between healthy and DMD was calculated via normalized and absolute activations per repetition of each gesture (Figure 5A). Figure 5B shows the average spatial distribution of the healthy and DMD participants.
(A) The percentage of variance explained as a function of the number of cumulative principal components (PCs). More than 90% of the variance (blue dashed line) of the data of the participants with Duchenne muscular dystrophy (DMD) is explained by three PCs, while for the healthy by five. The full lines represent the mean and the dashed the standard deviation. For clarity, we include only up to 10 of the 63 components, as those explain more than 99% of the variance explained. (B) The averaged squared Pearson correlation between the seven gestures of both groups of participants in the form of a similarity matrix. High correlation shows similarity between the gestures. Both healthy and DMD participants show correlated gestures; however, this phenomenon is more prominent in the DMD participants. A high value shows high correlation where one is the maximum (diagonal). The number of unique comparisons between the seven gestures is 21 per participant.
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F6
This number was used to explore the repertoire of orthogonal and uncorrelated sEMG patterns produced by the two groups of participants (Figure 6A). We averaged the correlation values of the healthy participants and the participants with DMD separately to identify which gestures are mostly correlated per population, and we presented this in the form of a similarity matrix (Figure 6B).