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Postural Muscle Unit Plasticity in Stroke Survivors: Altered Distribution of Gastrocnemius' Action Potentials

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Postural Muscle Unit Plasticity in Stroke Survivors: Altered Distribution of Gastrocnemius' Action Potentials

Taian M Vieira et al. Front Neurol.

Abstract

Neuromuscular adaptations are well-reported in stroke survivors. The death of motor neurons and the reinnervation of residual muscle fibers by surviving motor neurons, for example, seem to explain the increased density of muscle units after stroke. It is, however, unknown whether reinnervation takes place locally or extensively within the muscle. Here we combine intramuscular and surface electromyograms (EMGs) to address this issue for medial gastrocnemius (MG); a key postural muscle. While seven stroke survivors stood upright, two intramuscular and 15 surface EMGs were recorded from the paretic and non-paretic gastrocnemius. Surface EMGs were triggered with the firing instants of motor units identified through the decomposition of both intramuscular and surface EMGs. The standard deviation of Gaussian curves fitting the root mean square amplitude distribution of surface potentials was considered to assess differences in the spatial distribution of motor unit action potentials and, thus, in the distribution of muscle units between limbs. The median number of motor units identified per subject in the paretic and non-paretic sides was, respectively, 2 (range: 1-3) and 3 (1-4). Action potentials in the paretic gastrocnemius were represented at a 33% wider skin region when compared to the non-paretic muscle (Mann-Whitney; P = 0.014). Side differences in the representation of motor unit were not associated with differences in subcutaneous thickness (skipped-Spearman r = -0.53; confidence interval for r: -1.00 to 0.63). Current results suggest stroke may lead to the enlargement of the gastrocnemius muscle units recruited during standing. The enlargement of muscle units, as assessed from the skin surface, may constitute a new marker of neuromuscular plasticity following stroke.

Keywords: electromyogram; gastrocnemius; motor unit; standing; stroke.

Figures

Figure 1
Figure 1
Standing posture and electrode positioning. While individuals stood at a comfortable stance (A) the feet position was marked on the force plate. These drawings were considered to quantify the approximate center of pressure (CoP) position corresponding to a symmetric weight distribution between limbs (see Methods, section Experimental Protocol). (B) shows the relative position between medial gastrocnemius (MG) and surface and intramuscular electrodes. Wire electrodes were inserted at the MG region corresponding to the central location between the most proximal, surface electrode, and the distal extremity of the superficial aponeurosis (see dashed line); for the example illustrated in panel (B), this region roughly corresponds to the position of the sixth electrode from top to bottom. The length of MG superficial aponeurosis was estimated as the distance between its distal extremity and the popliteal crease.
Figure 2
Figure 2
Muscle activity and standing posture. (A) illustrates the CoP position along the lateral and anterior-posterior axes (top panel) and the surface (black traces) and intramuscular (gray traces) electromyograms (EMGs) detected during 10 s for a representative subject. EMGs are shown exclusively for the paretic (right) MG. The dashed line in the top panel denotes the CoP location where body weight distributes roughly symmetrically between limbs (Figure 1A). Note that action potentials are not present both in the surface and intramuscular recordings. When this subject shifted his CoP toward the paretic limb (B), action potentials could be clearly appreciated. An expanded view of EMGs (light gray rectangle) is shown in (D). Note there is a correspondence in the instants when action potentials were detected by the intramuscular electrodes and by the central though not by the distal nor proximal electrodes in the surface array. (C) shows the mean CoP position, calculated over the entire recording (60 s), while the subject stood at ease (black circle) and on his right leg (gray circle). Horizontal and vertical traces correspond to the standard deviation along the lateral and sagittal directions, respectively.
Figure 3
Figure 3
Motor unit action potentials in surface and intramuscular recordings. Short epochs (20 ms) of EMGs detected by wire and surface electrodes are shown. These epochs (thin traces) were obtained by triggering EMGs with the firing pattern of individual motor units, separately for each surface and intramuscular recordings. Thick traces were obtained by averaging the triggered EMGs. For this participant, three motor units were identified through decomposition of surface EMGs (cf. the first three columns from left to right). Note the similarity between waveforms obtained for the motor unit (MU 2) identified from decomposition of surface recordings and those obtained for the motor unit (MU 4) identified from decomposition of intramuscular EMGs.
Figure 4
Figure 4
Surface representation of motor unit action potentials. (A) schematically illustrates the relative position between surface electrodes and MG fascicles for a representative subject. (B) shows the spike triggered average potentials of motor units identified for EMGs detected from the paretic and non-paretic MG muscle, both from the skin surface and intramuscularly. The absence of an action potential in the bottom trace of the central panel indicates this motor unit was identified through decomposition of surface EMGs. Circles denote the root mean square (RMS) amplitude of each surface EMG. Note the greatest RMS values appear in correspondence of EMGs conveying the biggest potentials. Thick gray traces correspond to Gaussian curves fitted to the distribution of RMS values (11). These curves were estimated by considering the RMS values of surface EMGs detected by electrodes over the superficial aponeurosis (shaded area).
Figure 5
Figure 5
The amplitude distribution of surface, action potentials. (A) shows the standard deviation (sigma) of the Gaussian curves fitted to the RMS distribution of motor unit action potentials identified from the non-paretic (white boxes) and from the paretic (gray boxes) MG muscles. Sigma values normalized with respect to the length of the MG superficial aponeurosis (see Figure 1B) are shown in (B). Thick horizontal lines denote the median values. Boxes and whiskers correspond, respectively, to the interquartile interval and the range values. Ratios between sigma values obtained from the paretic and non-paretic MG (ordinate) and ratios between subcutaneous thickness values computed for the paretic and non-paretic limb (abscissa) are shown in (C). The skipped-Spearman correlation coefficient and its confidence interval (23) are shown within (C).
Figure 6
Figure 6
Differences in subcutaneous thickness between limbs. Ultrasound images recorded from the calf of two participants are shown in (A). Images in the right and left were, respectively, taken from the paretic and non-paretic limb. The white lines superimposed on each image indicate the thickness of fat and MG tissues. (B) shows the distribution of subcutaneous (top) and gastrocnemius (bottom) thicknesses estimated for the seven participants. Thick horizontal lines denote the median values. Boxes and whiskers correspond, respectively, to the interquartile interval and the range values.

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