Neural determinants of the increase in muscle strength and force steadiness of the untrained limb following a 4 week unilateral training.

Enhanced untrained muscle strength and force steadiness following unilateral resistance training (i.e. cross-education) are attributed to neural responses. However, the mechanisms of these adaptations for spinal motoneurons remain underexplored. Therefore, we examined maximal voluntary force (MVF), steady force variability (CovF) and longitudinally tracked motor unit adaptations in 10 individuals completing a 4 week unilateral strength intervention compared to nine controls. High-density surface EMG was recorded from the biceps brachii during steady (10% MVF) and trapezoidal (35% MVF) contractions. The relative proportion of common synaptic input (CSI) to motoneurons and its variability (CSI-V) were estimated using coherence and spectral analysis. Indirect estimates of persistent inward currents using firing rate hysteresis (∆F) and motor unit recruitment thresholds (MURTs) were assessed during ramps (35% MVF). MVF increased in both the trained (+14%, P < 0.001) and untrained limbs (+6%, P = 0.004), and CovF decreased in both limbs (P < 0.001). Greater CSI was observed on both sides (P < 0.01), concomitant with reduced CSI-V (P < 0.01). ∆F increased exclusively in trained limbs [+1.61 ± 0.71 pulse per second (pps); P < 0.001], and both sides exhibited lower MURTs (P < 0.001). In trained limbs, MVF gains were strongly associated with changes in CSI, MURT and ∆F (R² > 0.70, P < 0.01), while the contralateral muscle MVF increase was associated exclusively with CSI and MURT (R² > 0.65, P < 0.01). In both limbs, lower CovF was strongly associated with reduced CSI-V (R² > 0.70, P < 0.01). Our findings suggest that enhanced untrained muscle force and steadiness are mediated by increased relative strength of shared synaptic input with respect to independent noise and decreased variability of this shared input, with trained muscle MVF gains being associated with ∆F. KEY POINTS: Unilateral resistance training improves strength and force steadiness in the contralateral untrained limb, suggesting neural adaptations without directly overloading the muscle. Despite established force-related modifications, specific untrained limb responses in the relative shared synaptic input distribution and intrinsic motoneuron properties remain largely unknown. A 4 week unilateral training intervention enhanced muscle strength and force steadiness in the untrained limbs of 10 individuals, alongside a greater proportion of shared synaptic input, reduced variance in common input and lower motor unit recruitment thresholds. We demonstrated that the neural mechanisms underlying improved strength and force steadiness in muscles without mechanical overloading are associated with a higher relative shared input to motoneurons and reduced variance in these common input components.