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

Abstract

Enhanced untrained muscle strength and force steadiness following unilateral resistance training (i.e., cross-education) is typically attributed to neural responses. However, the mechanisms of these adaptations for spinal motoneurons remain unexplored. 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 9 controls. High-density surface electromyography 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 ({Delta}F) and motor unit recruitment thresholds (MURT) were assessed during the ramp forces (35%MVF). MVF increased in both the trained (+14%, p0.70, p0.65, p0.70, p View larger version (33K): org.highwire.dtl.DTLVardef@1a9e7feorg.highwire.dtl.DTLVardef@1bdd9a6org.highwire.dtl.DTLVardef@11c5ba0org.highwire.dtl.DTLVardef@c39a42_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOC_FLOATNO unilateral training intervention reduced the variance in common synaptic input (CSI-V), which was associated with decreased variability in force steadiness (CovF) in both the trained and contralateral untrained limbs. On the exercised side, the increase in maximal voluntary force (MVF) was accompanied by a higher proportion of common synaptic input (CSI), a lower motor unit recruitment threshold (RT), enhanced persistent inward current (PIC) amplitude, and increased neural drive. In contrast, the contralateral untrained limb exhibited higher shared synaptic input and a lower RT but with unaltered PIC amplitude and neural drive ({leftrightharpoons}). Overall, these adaptations resulted in a 14% increase in MVF in the exercised limb and a 6% increase in the untrained limb. C_FIG