Dependence of the paired motor unit analysis on motor unit discharge characteristics in the human tibialis anterior muscle

The paired motor unit analysis provides in vivo estimates of the magnitude of persistent inward currents (PIC) in human motoneurons by quantifying changes in the firing rate (ΔF) of an earlier recruited (reference) motor unit at the time of recruitment and derecruitment of a later recruited (test) motor unit. This study assessed the variability of ΔF estimates, and quantified the dependence of ΔF on the discharge characteristics of the motor units selected for analysis. ΔF was calculated for 158 pairs of motor units recorded from nine healthy individuals during repeated submaximal contractions of the tibialis anterior muscle. The mean (SD) ΔF was 3.7 (2.5) pps (range −4.2 to 8.9 pps). The median absolute difference in ΔF for the same motor unit pair across trials was 1.8 pps, and the minimal detectable change in ΔF required to exceed measurement error was 4.8 pps. ΔF was positively related to the amount of discharge rate modulation in the reference motor unit (r2 = 0.335; P < 0.001), and inversely related to the rate of increase in discharge rate (r2 = 0.125; P < 0.001). A quadratic function provided the best fit for relations between ΔF and the time between recruitment of the reference and test motor units (r2 = 0.229, P < 0.001), the duration of test motor unit activity (r2 = 0.110, P < 0.001), and the recruitment threshold of the test motor unit (r2 = 0.237, P < 0.001). Physiological and methodological contributions to the variability in ΔF estimates of PIC magnitude are discussed, and selection criteria to reduce these sources of variability are suggested for the paired motor unit analysis