M/RMS: an EMG method for quantifying upper motoneuron and functional weakness

A new method for quantifying upper motoneuron (UMN) weakness using standard EMG equipment is presented and compared to the twitch occlusion method. Control subjects and patients with stroke, spinal cord injury (SCI), and peripheral nerve injury (PNI) were evaluated. Force, EMG, and twitch force from nerve electrical stimulation were recorded during isometric contractions of the biceps and triceps. Ratios of the elicited CMAP amplitude (M)/voluntary-root-mean-square EMG activity (RMS) and of the voluntary contraction twitch force (Ti)/the resting twitch force (Tmax) were the two measures of UMN weakness studied. Both ratios are linearly correlated with force for controls, log M/RMS (r = 0.96) and Ti/Tmax (r = 0.86). Log M/RMS ratio was abnormally high (> mean + 2 SD; i.e., > 1.09) for weak muscles affected by stroke and incomplete SCI, but was normal in muscles weakened by PNI. An elevated M/RMS ratio, may aid in quantification of deficient supraspinal activation from UMN or functional weakness.