Author(s): LECCE, E., AMORUSO, P.1, DEL VECCHIO, A.2, CASOLO, A.3, FELICI, F.1, BAZZUCCHI, I.1, Institution: UNIVERSITY OF ROME 'FORO ITALICO': UNIVERSITA DEGLI STUDI DI ROMA FORO ITALICO, Country: ITALY, Abstract-ID: 1458

INTRODUCTION:
Neural determinants of cross-education during submaximal contractions have been associated with increased common input to spinal motoneurons and enhanced motor unit (MU) recruitment, without changes in spinal motoneuron drive [1]. However, the characterisation of MU behaviour during maximal voluntary contractions (MVCs) is generally unfeasible in surface EMG studies, due to limitations in decomposing the full population of active MUs [2]. Therefore, we adopted a novel approach to decompose MU activity across multiple MVCs (up to four), each including a 5-s hold phase. This approach increases the number of independent sources available to the decomposition algorithm, enabling more accurate MU identification [3] and obtaining a relatively large sample of MUs (> 6 per participant).
METHODS:
High-density surface EMG was recorded from the biceps brachii in the trained (TL) and contralateral untrained limb (UL) during MVCs to assess maximal voluntary force (MVF) and MU properties. Ten participants (BMI: 21.8 ± 2.5 kg∙m-2) completed a 4-week unilateral strength training program (4x6 repetitions at 80% MVF; 3 sessions/week) and were compared with a control group (n=10, BMI: 23.1 ± 4.8 kg∙m-2). Analyses were performed on longitudinally tracked MUs over the intervention. The strength of the neural drive was estimated as the mean discharge rate (DR) of the MU pool. Cumulative MU recruitment and spiking activity were also assessed with EMG amplitude and muscle fibre conduction velocity (MFCV) [4].
RESULTS:
Training produced significant MVF increases in the TL (+14 %, p<0.001) and UL (+6 %, p=0.004). These gains were accompanied by increased MU-DR (TL, +4.89 pps, p<0.001; UL,+2.19, p=0.001) and concurrent increases in EMG amplitude (TL, +281 μV; UL,+195 μV) and MFCV (TL, +0.43 m∙s-1; UL,+0.35 m∙s-1), suggesting greater cumulative neural activity and activation of larger MUs. Higher MFCV does not necessarily indicate increased muscle cross-sectional area, but rather a higher proportion of larger muscle fibres activated by the central nervous system. No changes were observed in the control group in any parameter (p>0.05).
CONCLUSION:
This study provides the first in vivo evidence that unilateral strength training increases spinal motoneuron output to untrained muscles during MVCs. These findings extend previous observations from submaximal contractions by demonstrating that cross-transfer of maximal force is mediated by a higher MU-DR rather than by MU recruitment alone [1]. These results also indicate that cross-education relies on distinct neural mechanisms at submaximal vs maximal force output, plausibly reflecting differences in supraspinal drive during maximal-effort contractions [5].

References
1. https://doi.org/10.1113/jp288954
2. https://doi.org/10.1016/j.jelekin.2020.102426
3. https://doi.org/10.1088/1741-2560/11/1/016008
4. https://doi.org/10.1152/japplphysiol.00810.2024
5. https://doi.org/10.1523/JNEUROSCI.0627-21.2022