Non-invasive assessment of single motor unit activity in relation to motor neuron level and lesion location in stroke and spinal muscular atrophy

Neuromuscular disorders e.g. spinal muscular atrophy (SMA) and stroke have a negative impact on functional movement capability. These disorders affect lower and upper motor neurons respectively. In this study high spatial resolution EMG (HSR-EMG) was used to record single motor unit (MU) activity in 3 groups: healthy subjects, an SMA group and a stroke group. 7 clinically sensitive parameters were used to analyse the activation patterns of single MUs. Results showed that for SMA there was no effect on single MU activation but on their number. Stroke was characterised by fewer active MUs, a significantly reduced firing rate with low variability. This suggests that information from the brain is modified thereby resulting in MUs firing at their natural frequency. Thus, HSR-EMG and the chosen parameters facilitate non-invasive, objective differentiation and analysis of muscle activation patterns in neuromuscular disorders.