Resistance training tempo selectively modulates corticospinal and reticulospinal excitability in humans

The neural mechanisms underlying resistance training (RT) adaptations remain incompletely understood, particularly the contribution of cortico-reticular and reticulospinal tracts (RSTs), which have rarely been examined in humans. One factor that may critically shape these adaptations is training pace, an overlooked yet physiologically relevant variable, as externally paced and self-paced contractions impose distinct demands on cortical and subcortical motor circuits. This study examined (1) how RT affects RST excitability and (2) whether different types of RT produce distinct changes in descending motor pathway excitability. Thirty healthy participants were randomized to metronome-paced RT (MP-RT), self-paced RT (SP-RT) or control. Cortical and corticospinal excitability were assessed with transcranial magnetic stimulation, while cortico-reticulospinal and RST excitability were measured using the ipsilateral to contralateral motor-evoked potential amplitude ratio (ICAR) and the StartReact effect. Training consisted of dumbbell exercises performed three times weekly for three weeks, with repeated neurophysiological testing. Both RT protocols improved one-repetition maximum and maximal voluntary force (P < 0.001). SP-RT resulted in greater increases in the StartReact effect (P < 0.001), rate of force development (0.0453) and cortico-reticulospinal excitability (ICAR; P = 0.0351), whereas MP-RT elicited larger increases in corticospinal excitability (P = 0.00420) and greater reductions in short-interval intracortical inhibition (P < 0.001). This study provides the first evidence in humans that RT modifies cortico-reticular and RST excitability. Importantly, adaptations were pacing-dependent: SP-RT selectively targets cortico-reticular circuit and RST, whereas MP-RT engaged corticospinal and inhibitory intracortical circuits. This pathway-specific plasticity underscores training modality as critical determinant of neural adaptation, with implications for rehabilitation and performance. KEY POINTS: This study examined the effects of resistance training (RT) on descending motor pathways using transcranial magnetic stimulation and the StartReact protocol. Given the inaccessibility of the reticulospinal tract in humans, its excitability was inferred indirectly via the StartReact effect and ipsilateral to contralateral MEP amplitude ratio (ICAR), offering novel insight into cortico-reticulospinal modulation. Both metronome-paced and self-paced RT increased muscle strength but induced distinct neural adaptations. Self-paced RT engaged cortico-reticular and reticulospinal pathways, as indicated by elevated ICAR and StartReact effect values, while metronome-paced RT increased corticospinal excitability and reduced intracortical inhibition. These findings demonstrate, for the first time in humans, that RT pacing can differentially modulate corticospinal and reticulospinal excitability, two parallel descending motor systems with distinct roles. This selective pathway engagement suggests that training tempo is not merely behavioural but a physiologically meaningful factor for directing neuroplasticity.