Abstract
The objective of this study was to apply a novel multidimensional motor unit number index (MD-MUNIX) technique to examine hand muscles in patients with incomplete cervical spinal cord injury (SCI). The MD-MUNIX was estimated from the compound muscle action potential (CMAP) and different levels of surface interference pattern electromyogram (EMG) at multiple directions of voluntary isometric muscle contraction. The MD-MUNIX was applied in the first dorsal interosseous (FDI), thenar and hypothenar muscles of SCI (n = 12) and healthy control (n = 12) subjects. The results showed that the SCI subjects had significantly smaller CMAP and MD-MUNIX in all the three examined muscles, compared to those derived from the healthy control subjects. The multidimensional motor unit size index (MD-MUSIX) demonstrated significantly larger values for the FDI and hypothenar muscles in SCI subjects than those from healthy control subjects, whereas the MD-MUSIX enlargement was marginally significant for the thenar muscles. The findings from the MD-MUNIX analyses provide an evidence of motor unit loss in hand muscles of cervical SCI patients, contributing to hand function deterioration.
Highlights
Spinal cord injury (SCI) leads to various deficits in muscular function and affects the quality of daily life (Welch et al, 1986; Curt and Dietz, 1999)
It was observed that the motor unit number index (MUNIX) derived from the flexion mode was significantly larger than that from the abduction mode for the first dorsal interosseous (FDI) and thenar muscles of the healthy control subjects
There was no significant difference in MUNIX or motor unit size index (MUSIX) values between the two directions for the hypothenar muscles of the healthy control subjects and for all the three examined muscles of the spinal cord injury (SCI) subjects
Summary
Spinal cord injury (SCI) leads to various deficits in muscular function and affects the quality of daily life (Welch et al, 1986; Curt and Dietz, 1999). Lower muscle contractile speed, fatigability of force gradation and loss of precision are common in hand muscles after cervical SCI and collectively deteriorate hand function (Farmer et al, 1993; Thomas, 1997; Thomas et al, 1997; Zijdewind and Thomas, 2012). These symptoms could be the results of different mechanisms, including weaker synchrony of motor unit discharge, reduced maximum motor unit firing rate, trans-synaptic motor neuron degeneration, loss of central motor neuron trophic influences, etc. Different motor unit number estimation (MUNE) methods have been used to investigate changes in spinal motor neuron or motor unit population after SCI (Yang et al, 1990; Xiong et al, 2010)
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