Very low-amplitude muscle activity increases probability of motor evoked potentials in healthy individuals and in amyotrophic lateral sclerosis.

Muscle contraction increases motor evoked potential (MEP) amplitude, decreasing motor threshold (MT). Correspondingly, trials where baseline EMG amplitude exceeds a specified threshold are often rejected. We aimed to investigate the influence of motor activity below such a threshold of MEP amplitude. We retrospectively analysed TMS-EMG data collected during resting MT (RMT) measurement in 45 healthy control subjects (1794 data points) and 35 people with amyotrophic lateral sclerosis (ALS; 1229 data points). Trials with de-meaned root mean squared (RMS) EMG amplitude of >10 µV throughout the 200 ms prior to stimulation were rejected. Generalised linear mixed-effects models assessed effects of muscle activity below this rejection threshold on the probability of evoking an MEP with peak-to-peak amplitude of ≥50 µV. Greater sub-rejection-threshold activity significantly increases MEP probability in control subjects and people with ALS. Models predicted a 38%-43% increase in MEP probability when baseline RMS-EMG amplitude increased from 1 to 9 µV. Sub-rejection-threshold baseline activity was significantly greater in ALS than control subjects. Below a typical rejection threshold, greater baseline RMS-EMG amplitudes markedly increase the probability of evoking MEPs with peak-to-peak amplitude of ≥50 µV. Effects of sub-rejection-threshold muscle activity should be accounted for when comparing RMT measures, particularly between cohorts where such activity differs, such as ALS and control subjects.