Firing rate trajectories of human motor units during activity-dependent muscle potentiation.

During activity-dependent potentiation (ADP), motor unit firing rates (MUFRs) are lower; however, the mechanism for this response is not known. During increasing torque isometric contractions at low contraction intensities, MUFR trajectories initially accelerate and saturate demonstrating a nonlinear response due to the activation of persistent inward currents (PICs) at the motoneuron. The purpose was to assess whether PICs are a factor in the reduction of MUFRs during ADP. To assess this, MUFR trajectories were fit with competing functions of linear regression and a rising exponential (i.e., acceleration and saturation). With fine-wire electrodes, discrete MU potential trains were recorded in the tibialis anterior during slowly increasing dorsiflexion contractions to 10% of maximal voluntary contraction following both voluntary [postactivation potentiation (PAP)] and evoked [posttetanic potentiation (PTP)] contractions. In eight participants, 25 MUs were recorded across both ADP conditions and compared with the control with no ADP effect. During PAP and PTP, the average MUFRs were 16.4% and 9.2% lower (both P ≤ 0.001), respectively. More MUFR trajectories were better fit to the rising exponential during control (16/25) compared with PAP (4/25, P < 0.001) and PTP (8/25, P = 0.03). The MU samples that had a rising exponential MUFR trajectory during PAP and PTP displayed an ∼11% lower initial acceleration compared with control ( P < 0.05). Thus, presumed synaptic amplification and MUFR saturation due to PIC properties are attenuated during ADP regardless of the type of conditioning contraction. This response may contribute to lower MUFRs and likely occurred because synaptic input is reduced when contractile function is enhanced. NEW & NOTEWORTHY During activity-dependent muscle potentiation (ADP), initial motor unit firing rate (MUFR) acceleration and the occurrence of MUFR trajectory saturation as a function of increasing contraction intensity were assessed. With no ADP (control), trajectories were more likely to accelerate and saturate (16/25 units) compared with voluntary- and stimulated-induced ADP conditions (4/25 and 8/25 units, respectively) that were fit better linearly. Therefore, during ADP, an attenuated intrinsic response to voluntary synaptic inputs occurs.