Fig. 3.

Firing frequency profiles of 2 low-threshold motor units (below threshold of test unit) that continued to fire after a voluntary contraction in a spastic-paretic muscle. Overlayed waveforms denote the accuracy in discrimination of the motor units. Note that the firing rates of the 2 low-threshold units were modulated in a parallel manner. Inset shows the firing rate of the 2 motor units plotted against one another in 500-ms bins during the duration that they were firing in parallel. Note the high r² value from the linear regression fit through the rate–rate plot, confirming that the 2 motor units were modulated in parallel (P < 0.001, r² = 0.64).