Biology subjects

Felici, F.

Publications and source records attributed to Felici, F..

4 recordsLinked to original sources

bioRxiv2026

Unilateral resistance training induces greater rate coding adaptations in high-threshold motor units during maximal voluntary contractions

Resistance training lasting a few weeks increases maximal force mainly through neural adaptations that enhance the drive from the nervous system to muscle. While these adaptations have been well documented at the motor unit (MU) level during submaximal force contractions, the mechanisms underlying force increases during maximal voluntary contractions are poorly understood. This is due to a classic technical limitation in tracking MUs longitudinally during maximal force tasks. Here, we solved this technical challenge, enabling the investigation of MU adaptations during MVCs in both the trained and untrained limbs following unilateral resistance training. High-density surface electromyography was recorded from the biceps brachii of both limbs before and after a 4-week unilateral resistance-training intervention, and the same MUs were longitudinally tracked across sessions during MVCs by concatenation of three MVC trials of [~]5-s each. Unilateral training increased maximal force in the trained limb (+16%) and induced strength transfer to the untrained limb (+8%). In both limbs, maximal contractions after training were characterized by greater EMG amplitude, faster muscle-fiber conduction velocity, and higher MU discharge rates, indicating enhanced neural drive to the motoneuron pool. These adaptations were strongly associated with improvements in maximal force (R2 > 0.7 for all). Importantly, longitudinal MU tracking revealed a non-uniform adaptation across the MU pool: MUs with higher baseline conduction velocity, indicative of higher recruitment threshold, exhibited the largest pre-post increases in discharge rate, whereas lower-threshold units showed smaller changes. Collectively, these findings demonstrate that gains in maximal force and their transfer to the untrained limb are primarily mediated by enhanced rate coding of higher-threshold MUs during MVCs.

physiology↗

bioRxiv2025

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) 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%, 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). {Delta}F increased exclusively in the trained limbs (+1.61{+/-}0.71 pps; p<0.001), and both sides exhibited lower MURT (p<0.001). In trained limbs, MVF gains were strongly associated with changes in CSI, MURT, and {Delta}F (R{superscript 2}>0.70, p<0.01), while the contralateral muscle MVF increase was associated exclusively with CSI and MURT (R{superscript 2}>0.65, p<0.01). In both limbs, lower CovF was strongly associated with reduced CSI-V (R{superscript 2}>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 gains in trained muscle MVF being associated with {Delta}F. Key pointsO_LIUnilateral resistance training improves strength and force steadiness in the contralateral untrained limb, suggesting neural adaptations without directly overloading the muscle. C_LIO_LIDespite established force-related modifications, the specific neural mechanisms mediating these functional responses remain largely unknown. C_LIO_LI4-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. C_LIO_LIWe demonstrate that the neural mechanisms underlying improved strength and force control in muscles without mechanical overloading are associated with a higher relative shared input among motoneurons and reduced variance in these common input components. C_LI O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=148 SRC="FIGDIR/small/645159v1_ufig1.gif" ALT="Figure 1"> 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

physiology↗

bioRxiv2022

Non-invasive muscle biopsy: estimation of muscle fibre size from a neuromuscular interface

Because of the biophysical relation between muscle fibre diameter and the propagation velocity of action potentials along the muscle fibres, motor unit conduction velocity (MUCV) could be a non-invasive index of muscle fibre size in humans. However, the relation between MUCV and fibre size has been only assessed indirectly in animal models and in human patients with invasive intramuscular EMG recordings, or it has been mathematically derived from computer simulations. By combining advanced non-invasive techniques to record motor unit activity in vivo, i.e., high-density surface EMG, with the gold standard technique for muscle tissue sampling, i.e., muscle biopsy, here we investigated the relation between the conduction velocity of populations of motor units identified from the biceps brachii muscle, and muscle fibre diameter. Moreover, we demonstrate the possibility to predict muscle fibre diameter (R2 = 0.66) and cross-sectional area (R2 = 0.65) from conduction velocity estimates with low systematic bias (~2% and ~4% respectively) and a relatively low margin of individual error (~8% and ~16%, respectively). The proposed neuromuscular interface opens new perspectives in the use of high-density EMG as a non-invasive tool to estimate muscle fibre size without the need of surgical biopsy sampling. The non-invasive nature of high-density surface EMG for the assessment of muscle fibre size may be useful in studies monitoring child development, aging, space and exercise physiology. SIGNIFICANCE STATEMENTOur study explored the relation between the conduction velocity of populations of motor units and muscle fibre size in healthy humans. Our results provide in vivo evidence that a high-density surface EMG-derived physiological parameter, i.e. motor unit conduction velocity, can be adopted to estimate muscle fibre size, without the need of surgical biopsy sampling. Here we propose a neuromuscular interface that opens new perspectives not only in the study of neuromuscular disorders, but also in other fields where the non-invasive and painless determination of muscle fibre and motor unit size becomes a priority, such as in aging, space and exercise physiology.

physiology↗

bioRxiv2021

Why humans are stronger but not faster after isometric strength training: specific neural, not muscular, motor unit adaptations

While maximal force increases following short-term isometric strength training, the rate of force development (RFD) may remain relatively unaffected. The underlying neural and muscular mechanisms during rapid contractions after strength training are largely unknown. Since strength training increases the neural drive to muscles, it may be hypothesized that there are distinct neural or muscular adaptations determining the change in RFD independently of an increase in maximal force. Therefore, we examined motor unit population data during the rapid generation of force before and after four weeks of strength training. We observed that strength training did not change the RFD because it did not influence the number of motor units recruited per second or their initial discharge rate during rapid contractions. While strength training did not change motoneuron behaviour in the force increase phase of rapid contractions, it increased the discharge rate of motoneurons (by [~]4 spikes/s) when reaching the plateau phase ([~]150 ms) of the rapid contractions, determining an increase in maximal force production. Computer simulations with a motor unit model that included neural and muscular properties, closely matched the experimental observations and demonstrated that the lack of change in RFD following training is primarily mediated by an unchanged maximal recruitment speed of motoneurons. These results demonstrate that maximal force and contraction speed are determined by different adaptations in motoneuron behaviour following strength training and indicate that increases in the recruitment speed of motoneurons are required to evoke training-induced increases in RFD.

neuroscience↗