ECSS Paris 2023: OP-BM06
INTRODUCTION: Explosive strength is a key neuromuscular capability, measured as the rate of torque development (RTD) from a muscle group (1). In ballistic contractions, faster motor units recruitment and higher discharge rates are associated with higher RTD (2). The presence of a pre-contraction silent period, defined as the absence of muscle activity and motor units firing prior to contraction onset, leads to higher discharge rates and increased RTD (3). However, the role of the synchronisation of descending drive after the silent period in improving RTD remains debated, with lacking experimental evidence. In this study, we artificially silenced descending neural drive to motor units and thereafter synchronised it by using transcranial magnetic stimulation (TMS) before the onset of ballistic contractions. We hypothesized that explosive efforts preceded by TMS would exhibit higher RTD, muscle activity, and motor units discharge rate, compared to control contractions. METHODS: Nine participants performed 6 sets of 2 isometric ballistic ankle dorsiflexions (2 min rest between sets), separated by 20 s. Muscle activity of the tibialis anterior was recorded with high-density electromyography (EMG). Participants were instructed to contract as fast as possible when hearing the TMS discharge after a countdown. In the first contraction (w/SP), TMS was used (at rest) to induce a corticospinal silent period overlapping with the reaction time of the ballistic effort. For this reason, the TMS output was set up to induce a silent period recorded from tibialis anterior of ~180 ms during isometric contractions at 20% maximal voluntary contraction (MVC) torque. In the second condition (w/oSP), the TMS discharge sound was triggered, but no silent period was elicited. The order of w/SP and w/oSP was alternated between sets. RESULTS: Reaction time (TMS trigger to EMG onset) was longer w/SP (172 ± 31 ms) than w/oSP (112 ± 29 ms; P < 0.01). Root-mean-square of the EMG 0-50, 0-100, and 0-150 ms from onset was higher w/SP (129 ± 74, 94 ± 43, and 63 ± 33 %, respectively; P < 0.01). Peak RTD (in a 30 ms-window, divided by MVC torque) was 22 ± 9 % higher w/SP than w/oSP (8.5 ± 2.0 vs. 7.0 ± 1.9 MVC torque/s; P < 0.01). The discharge rate of motor units (average from the first four discharges) was higher w/SP than w/oSP (62 ± 15 vs. 56 ± 11 Hz; P < 0.01). CONCLUSION: By silencing corticospinal excitatory neurons in the primary motor cortex before ballistic efforts, and by delaying and synchronising descending neural drive, higher discharge rate of motor units and RTD was achieved. These findings highlight the simultaneous role of the silent period and following synchronised neural drive to motoneurons for muscle rapid force production.
Read CV Luca RuggieroECSS Paris 2023: OP-BM06
INTRODUCTION: Maximal rate of force development (RFD) is determined by neural (motor unit [MU] recruitment speed and discharge rate) and contractile properties (1,2). Chronic resistance training may confer neural adaptations including greater spinal cord output that facilitates faster RFD, whereas endurance trained (ET) individuals may have slower muscle contractile properties, resulting in limited increases in spinal cord output (3). Here, we assessed MU activity and intrinsic muscle contractile properties within RT, ET and UT individuals during maximal rapid contractions. METHODS: Sixty-six RT, ET and UT (22 per group, 6 females) individuals produced maximal and rapid voluntary isometric dorsiflexion force to determine maximal voluntary force (MVF) and RFD, respectively. Supramaximal percutaneous nerve stimulation (300 Hz) was administered to the common peroneal nerve at rest to record maximal evoked force (MEF) and RFD. A 64-channel grid electrode was placed on the tibialis anterior muscle of the dominant leg to assess myoelectrical activity (EMG). The EMG signals were decomposed into individual MU spike trains using Convolution Kernel Compensation algorithm to calculate MU recruitment speed, and discharge rate (DR) in the initial and plateau period of rapid contractions. RESULTS: MVF, MEF and maximal evoked RFD were significantly larger in RT (408.2 [358.0, 458.4] N, 151.3 [137.3, 165.3] N, 1998 [1776, 2220] N/s) compared to ET (346.6 [312.1, 381.0] N, 93.0 [78.2, 107.8] N, 1317 [1084, 1551] N/s, p<0.04) and to UT (301.4 [267.8, 335.0] N, 110.4 [96.7, 124.0] N, 1492 [1276, 1708] N/s, p<0.005), respectively. When maximal evoked RFD was normalised to MVF, both RT (482 [434, 529]%MVF/s) and UT (494 [448, 540]%MVF/s) were greater than ET (383 [333, 433]%MVF/s, p<0.0157). Greater absolute voluntary RFD was seen in RT compared to UT (1754 [1542, 1996] N/s vs. 1219 [1007, 1431] N/s, p=0.0031) but not compared to ET (1469 [1258, 1681] N/s, p=0.1486). However, when normalised to MVF, there were no differences between RT, ET or UT (437 [410, 464], 419 [392, 446], 403 [376, 430] %MVF/s, p=0.1984). Initial DR was greatest in RT (71.7 [67.3, 76.2] pps) compared to UT (63.9 [59.4, 68.5] pps, p = 0.0454), but not compared to ET (64.8 [60.4, 69.2] pps, p = 0.0773). DR during the plateau phase was the highest in RT (33.1 [30.5, 35.6] pps) vs. ET (27.8 [25.3, 30.4] pps, p=0.0136), but not compared to UT (31.0 [28.4, 33.6] pps, p=0.4949). No differences were noted in recruitment speed between RT, ET or UT (290 [224, 355], 316 [250, 381], 259 [184, 333] MUs/ms, p=0.767), respectively. CONCLUSION: RT individuals exhibited greater maximal strength and absolute RFD. However, despite greater initial MU discharge rate, relative RFD was similar between groups, likely due to between-group similarity in MU recruitment speed, a key determinant of RFD. 1) Del Vecchio et al 2019, J Physiol 2) Folland et al 2014, Scand J Med Sci Sports 3) Vila-Cha et al 2010, J Appl Physiol
Read CV Haydn ThomasonECSS Paris 2023: OP-BM06
INTRODUCTION: The ability to produce maximal force in minimal time is a key function of the neuromuscular system. While training can enhance rapid force generation, factors like ageing and muscle unloading can cause significant declines. Despite rapid force generation being more important for human function than maximal strength, there is limited research on its decline and potential countermeasures. The speed of motor neuron recruitment and maximal motor unit discharge rate are major contributors to rapid force generation. A startling cue, which activates the pontomedullary reticular formation neurons, can increase the maximal motor unit discharge rate. However, its unclear whether age-related physical decline is linked to reduced excitability of descending motor pathways, such as the reticulospinal tract, which plays a role in gross motor output. Transcranial Direct Current Stimulation (tDCS), a non-invasive brain stimulation technique, may be able to facilitate subcortical structures. If effective, tDCS could potentially mitigate the loss of rapid force production due to aging or bed rest, allowing for faster recovery and resumption of daily activities in older adults, for example. METHODS: Forty-four adults participated in a single-blind, two-arm crossover study. They were asked to generate isometric knee extensor forces as rapidly and forcefully as possible from rest to 75% of maximal voluntary force. This was done in response to visual (VC; LED), visual-auditory (VAC; 80 dB), or visual-startling cue (VSC; 110 dB), both before and after receiving either anodal or sham tDCS. Rate of force development and reaction time were measured using a rigid custom-built isometric leg extension dynamometer. Motoneuron activity was estimated by analyzing high-density surface electromyogram recordings over the vastus lateralis muscle. reticulospinal tract gain was calculated by comparing reaction times to each of the three cues. RESULTS: Twelve young males (means ± SD; age: 23 ± 4 yr, stature: 1.77 ± 0.07 m, body mass: 77.0 ± 10.7 kg), twelve young females (age: 21 ± 2 yr, stature: 1.67 ± 0.08 m, body mass: 59.6 ± 8.6 kg), ten old males (age: 71 ± 3 yr, stature: 1.78 ± 0.10 m, body mass: 83.2 ± 13.9 kg), and ten old females (age: 70 ± 4 yr, stature: 1.62 ± 0.04 m, body mass: 61.3 ± 7.5 kg) completed three visits to the School of Sport, Exercise and Rehabilitation Sciences, University of Birmingham. There were two to six days (2 ± 1 days) between experimental visits which were preceded by an initial familiarization visit and two to ten days (4 ± 1 days) before the first experimental visit. Further analysis will investigate key measures such as reaction time, rate of force development and motor unit decomposition. CONCLUSION: This study will provide valuable insights into the potential of non-invasive brain stimulation as a method to counteract age-related weakness. Additionally, it will contribute to the understanding of reticulospinal tract excitability in healthy aging research.
Read CV Eddie WigginsECSS Paris 2023: OP-BM06