Purpose: To determine the role of noradrenergic modulation in the control of motor output, we compared the acute effect of reboxetine (REB), a noradrenaline reuptake inhibitor, to a placebo (PLA) on knee extensors motor performance and cortical and spinal excitability.
Methods: Eleven young men took part in two randomized experiments during which they received either 8 mg of REB or a PLA. The torque produced during a maximal voluntary contraction (MVC) and its variability (i.e., coefficient of variation) during submaximal contractions ranging from 5% to 50% MVC were measured. Paired electrical (PES) and transcranial magnetic stimulation (TMS) were used to assess changes in voluntary activation during MVC, and corticospinal (motor-evoked potential (MEP)) and spinal excitability (Hoffmann (H) reflex) during contraction at 20% MVC.
Results: MVC torque and torque steadiness increased respectively by 9.5% and 24% on average in REB compared with PLA condition (P < 0.001). Voluntary activation tested by TMS and PES was greater (~3%; P < 0.05) in REB than PLA condition. The increase in voluntary activation in REB condition was significantly correlated with subjects' initial voluntary activation level when tested by TMS (r = -0.62; P = 0.048) and PES (r = -0.86; P < 0.01). The maximal amplitudes of H reflex and MEP and the slope of their recruitment curves were enhanced by REB (P < 0.05). The ratio between the TMS-induced EMG silent period and the corresponding MEP (silent period/MEP) was reduced in REB condition (P < 0.01).
Conclusions: The present findings indicate that voluntary activation and accuracy in force control can be increased by an enhanced level of noradrenaline concentration. This improvement in motor performance is accompanied by changes located at both cortical and spinal levels.