Altered Motor Unit Discharge Coherence in Paretic Muscles of Stroke Survivors

Abstract

After a cerebral stroke, a series of changes at the supraspinal and spinal nervous system can alter the control of muscle activation, leading to persistent motor impairment. However, the relative contribution of these different levels of the nervous system to impaired muscle activation is not well understood. The coherence of motor unit (MU) spike trains is considered to partly reflect activities of higher level control, with different frequency band representing different levels of control. Accordingly, the objective of this study was to quantify the different sources of contribution to altered muscle activation. We examined the coherence of MU spike trains decomposed from surface electromyogram (sEMG) of the first dorsal interosseous muscle on both paretic and contralateral sides of 14 hemispheric stroke survivors. sEMG was obtained over a range of force contraction levels at 40, 50, and 60% of maximum voluntary contraction. Our results showed that MU coherence increased significantly in delta (1-4 Hz), alpha (8-12 Hz), and beta (15-30 Hz) bands on the affected side compared with the contralateral side, but was maintained at the same level in the gamma (30-60 Hz) band. In addition, no significant alteration was observed across medium-high force levels (40-60%). These results indicated that the common synaptic input to motor neurons increased on the paretic side, and the increased common input can originate from changes at multiple levels, including spinal and supraspinal levels following a stroke. All these changes can contribute to impaired activation of affected muscles in stroke survivors. Our findings also provide evidence regarding the different origins of impaired muscle activation poststroke.

Keywords: coherence; motor unit; stroke; surface electromyogram; synchronization.

Figure 1

Experimental apparatus, electromyogram recordings and motor unit decomposition. (A) Experimental setup. (B) The five-probe sensor array. (C) The four-channel surface electromyogram (sEMG) signals during a trapezoidal force production. In addition, motor unit spike trains and corresponding templates after sEMG decomposition are also displayed.

Figure 2

Block diagram for the entire data analysis. EMG, electromyography; STA, spike trigger average algorithm; CST, composite spike train; MU, motor unit.

Figure 3

Top: non-standardized coherence; bottom: z-coherence. An exemplar frequency-coherence plots from a representative stroke participant at 40% maximum voluntary contraction. The solid thick red line represents the coherence of the affected side, and the blue line represents the contralateral side. Thin lines present SE.

Figure 4

Comparison of the z-coherence of different frequency bands between affected side and contralateral sides. Each bar presents its corresponding average magnitude of mean band z-coherence (z-MBC), and the error bar presents its SE across 14 participants. Asterisks indicate the significant difference.

Figure 5

Top: paretic side; bottom: contralateral side. Post hoc pairwise comparison across four bands. Each red circle presents the mean value of one frequency band, and the red horizontal line is its corresponding 95% confidence interval. The vertical dashed lines show the overlap of two bands, which means no significant difference.