@article{LaineMartinezValdesFallaetal.2015, author = {Laine, Christopher M. and Martinez-Valdes, Eduardo Andr{\´e}s and Falla, Deborah and Mayer, Frank and Farina, Dario}, title = {Motor Neuron Pools of Synergistic Thigh Muscles Share Most of Their Synaptic Input}, series = {The journal of neuroscience}, volume = {35}, journal = {The journal of neuroscience}, number = {35}, publisher = {Society for Neuroscience}, address = {Washington}, issn = {0270-6474}, doi = {10.1523/JNEUROSCI.0240-15.2015}, pages = {12207 -- 12216}, year = {2015}, abstract = {Neural control of synergist muscles is not well understood. Presumably, each muscle in a synergistic group receives some unique neural drive and some drive that is also shared in common with other muscles in the group. In this investigation, we sought to characterize the strength, frequency spectrum, and force dependence of the neural drive to the human vastus lateralis and vastus medialis muscles during the production of isometric knee extension forces at 10 and 30\% of maximum voluntary effort. High-density surface electromyography recordings were decomposed into motor unit action potentials to examine the neural drive to each muscle. Motor unit coherence analysis was used to characterize the total neural drive to each muscle and the drive shared between muscles. Using a novel approach based on partial coherence analysis, we were also able to study specifically the neural drive unique to each muscle (not shared). The results showed that the majority of neural drive to the vasti muscles was a cross-muscle drive characterized by a force-dependent strength and bandwidth. Muscle-specific neural drive was at low frequencies (<5 Hz) and relatively weak. Frequencies of neural drive associated with afferent feedback (6 - 12 Hz) and with descending cortical input (similar to 20 Hz) were almost entirely shared by the two muscles, whereas low-frequency (<5 Hz) drive comprised shared (primary) and muscle-specific (secondary) components. This study is the first to directly investigate the extent of shared versus independent control of synergist muscles at the motor neuron level.}, language = {en} } @article{MartinezValdesFallaNegroetal.2017, author = {Martinez-Valdes, Eduardo Andr{\´e}s and Falla, Deborah and Negro, Francesco and Mayer, Frank and Farina, Dario}, title = {Differential Motor Unit Changes after Endurance or High-Intensity Interval Training}, series = {Medicine and science in sports and exercise : official journal of the American College of Sports Medicine}, volume = {49}, journal = {Medicine and science in sports and exercise : official journal of the American College of Sports Medicine}, publisher = {Lippincott Williams \& Wilkins}, address = {Philadelphia}, issn = {0195-9131}, doi = {10.1249/MSS.0000000000001209}, pages = {1126 -- 1136}, year = {2017}, abstract = {Purpose Using a novel technique of high-density surface EMG decomposition and motor unit (MU) tracking, we compared changes in the properties of vastus medialis and vastus lateralis MU after endurance (END) and high-intensity interval training (HIIT). Methods Sixteen men were assigned to the END or the HIIT group (n = 8 each) and performed six training sessions for 14 d. Each session consisted of 8-12 x 60-s intervals at 100\% peak power output separated by 75 s of recovery (HIIT) or 90-120 min continuous cycling at similar to 65\% VO2peak (END). Pre- and postintervention, participants performed 1) incremental cycling to determine VO2peak and peak power output and 2) maximal, submaximal (10\%, 30\%, 50\%, and 70\% maximum voluntary contraction [MVC]), and sustained (until task failure at 30\% MVC) isometric knee extensions while high-density surface EMG signals were recorded from the vastus medialis and vastus lateralis. EMG signals were decomposed (submaximal contractions) into individual MU by convolutive blind source separation. Finally, MU were tracked across sessions by semiblind source separation. Results After training, END and HIIT improved VO2peak similarly (by 5.0\% and 6.7\%, respectively). The HIIT group showed enhanced maximal knee extension torque by similar to 7\% (P = 0.02) and was accompanied by an increase in discharge rate for high-threshold MU (50\% knee extension MVC) (P < 0.05). By contrast, the END group increased their time to task failure by similar to 17\% but showed no change in MU discharge rates (P > 0.05). Conclusions HIIT and END induce different adjustments in MU discharge rate despite similar improvements in cardiopulmonary fitness. Moreover, the changes induced by HIIT are specific for high-threshold MU. For the first time, we show that HIIT and END induce specific neuromuscular adaptations, possibly related to differences in exercise load intensity and training volume.}, language = {en} } @article{MartinezValdesNegroFallaetal.2018, author = {Martinez-Valdes, Eduardo Andr{\´e}s and Negro, Francesco and Falla, Deborah and De Nunzio, Alessandro Marco and Farina, Dario}, title = {Surface electromyographic amplitude does not identify differences in neural drive to synergistic muscles}, series = {Journal of applied physiology}, volume = {124}, journal = {Journal of applied physiology}, number = {4}, publisher = {American Chemical Society}, address = {Bethesda}, issn = {8750-7587}, doi = {10.1152/japplphysiol.01115.2017}, pages = {1071 -- 1079}, year = {2018}, abstract = {Surface electromyographic (EMG) signal amplitude is typically used to compare the neural drive to muscles. We experimentally investigated this association by studying the motor unit (MU) behavior and action potentials in the vastus medialis (VM) and vastus lateralis (VL) muscles. Eighteen participants performed isometric knee extensions at four target torques [10. 30. 50, and 70\% of the maximum torque (MVC)] while high-density EMG signals were recorded from the VM and VL. The absolute EMG amplitude was greater for VM than VL (P < 0.001), whereas the EMG amplitude normalized with respect to MVC was greater for VL than VM (P < 0.04). Because differences in EMG amplitude can be due to both differences in the neural drive and in the size of the MU action potentials, we indirectly inferred the neural drives received by the two muscles by estimating the synaptic inputs received by the corresponding motor neuron pools. For this purpose. we analyzed the increase in discharge rate from recruitment to target torque for motor units matched by recruitment threshold in the two muscles. This analysis indicated that the two muscles received similar levels of neural drive. Nonetheless, the size of the MU action potentials was greater for VM than VL (P < 0.001), and this difference explained most of the differences in EMG amplitude between the two muscles (similar to 63\% of explained variance). These results indicate that EMG amplitude, even following normalization, does not reflect the neural drive to synergistic muscles. Moreover, absolute EMG amplitude is mainly explained by the size of MU action potentials. NEW \& NOTEWORTHY Electromyographic (EMG) amplitude is widely used to compare indirectly the strength of neural drive received by synergistic muscles. However, there are no studies validating this approach with motor unit data. Here, we compared between-muscles differences in surface EMG amplitude and motor unit behavior. The results clarify the limitations of surface EMG to interpret differences in neural drive between muscles.}, language = {en} } @article{MartinezValdesFarinaNegroetal.2018, author = {Martinez-Valdes, Eduardo Andr{\´e}s and Farina, Dario and Negro, Francesco and Del Vecchio, Alessandro and Falla, Deborah}, title = {Early motor unit conduction velocity changes to high-intensity interval training versus continuous training}, series = {Medicine and science in sports and exercise : official journal of the American College of Sports Medicine}, volume = {50}, journal = {Medicine and science in sports and exercise : official journal of the American College of Sports Medicine}, number = {11}, publisher = {Lippincott Williams \& Wilkins}, address = {Philadelphia}, issn = {0195-9131}, doi = {10.1249/MSS.0000000000001705}, pages = {2339 -- 2350}, year = {2018}, abstract = {Purpose Moderate-intensity continuous training (MICT) and high-intensity interval training (HIIT) are associated with different adjustments in motor output. Changes in motor unit (MU) peripheral properties may contribute to these adjustments, but this is yet to be elucidated. This study evaluated early changes in MU conduction velocity (MUCV) and MU action potential amplitude after 2 wk of either HIIT or MICT. Methods Sixteen men were assigned to either an MICT group or HIIT group (n = 8 each), and participated in six training sessions over 14 d. HIIT: 8 to 12 x 60-s intervals at 100\% peak power output. Moderate-intensity continuous training: 90 to 120 min continuous cycling at similar to 65\% VO2peak. Preintervention and postintervention, participants performed maximal voluntary contractions (MVC) and submaximal (10\%, 30\%, 50\%, and 70\% of MVC) isometric knee extensions while high-density EMG was recorded from the vastus medialis (VM) and vastus lateralis (VL) muscles. The high-density EMG was decomposed into individual MU by convolutive blind-source separation and tracked preintervention and postintervention. Results Both training interventions induced changes in MUCV, but these changes depended on the type of training (P < 0.001). The HIIT group showed higher values of MUCV after training at all torque levels (P < 0.05), MICT only displayed changes in MUCV at low torque levels (10\%-30\% MVC, P < 0.002). There were no changes in MU action potential amplitude for either group (P = 0.2). Conclusions Two weeks of HIIT or MICT elicit differential changes in MUCV, likely due to the contrasting load and volume used in such training regimes. This new knowledge on the neuromuscular adaptations to training has implications for exercise prescription.}, language = {en} }