Characterization of motor unit behavior in patients with amyotrophic lateral sclerosis

In this study, we investigated the behavior of active motor units identified via analysis of electromyographic (EMG) signals recorded from the first dorsal interosseous (FDI) muscle using a quadrifilar needle electrode. Data was collected from control subjects and patients with both lower (LMN) and upper (UMN) motor neuron dominant forms of amyotrophic lateral sclerosis (ALS). EMG recordings were gathered during isometric contractions reaching 20 or 50 % of the force output produced during a maximum voluntary contraction (MVC). Recordings were analyzed using available EMG decomposition software (EMGLAB). Results showed differences in mean motor unit firing rates between patients with ALS and control subjects. Differences were also observed between patients with LMN- and UMN-dominant forms of ALS. Motor unit substitution was observed in patients despite the contractions lasting just a few seconds. Finally, we observed that motor unit action potential (MUAP) waveforms recorded from patients were more complex than those recorded from control subjects as often observed in motor neuron diseases.