ECSS Paris 2023: OP-BM04
INTRODUCTION: Maximal rate of force development (RFD) is determined by neural (motor unit [MU] recruitment speed and discharge rate) and muscular properties (Del Vecchio, 2019; Folland et al. 2013) . Though resistance trained (RT) individuals typically exhibit greater absolute RFD, the findings are equivocal regarding relative RFD (i.e., normalised to maximal voluntary force [MVF]), with studies reporting greater, similar, or even lower relative RFD in RT individuals (Balshaw et al. 2022). Nevertheless, chronic resistance training may confer neural adaptations including greater spinal cord output in RT compared to untrained individuals (UT), that facilitates faster RFD. Here, we assessed MU discharge characteristics and intrinsic muscle contractile properties within RT and UT individuals during maximal rapid contractions. METHODS: Twenty-two RT and 22 UT (6 females per group) individuals produced maximal and rapid voluntary isometric dorsiflexion force (up to ~80% of MVF), to determine MVF and RFD (maximal slope of the force-time curve from force onset), respectively. Percutaneous nerve stimulation (25 pulses at 100 Hz and 8 pulses at 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. Discharge rate in the initial (first five spikes) and plateau period (20 spikes) of rapid contractions, and recruitment speed of the identified MUs were calculated. RESULTS: MVF and MEF were significantly larger in RT (393 [337, 449] N and 245 [212, 278] N) compared to UT (296 [260, 331] N, p=0.014, and 174 [151, 198] N; p<0.001), respectively. Greater absolute RFD was observed in RT compared to UT during voluntary (1792 [1550, 2035] vs 1307 [1064, 1550] N/s; p=0.007) and evoked (2001 [1751, 2252] vs 1492 [1247, 1736] N/s; p=0.005) contractions. However, when normalised to MVF, there were no differences between groups for either voluntary (454 [429, 480] vs 420 [394, 445]%MVF/s, p=0.056) or evoked RFD (513 [447, 579] vs 493 [429, 557]%MVF/s, p=0.665). Compared to UT, RT exhibited greater discharge rate in the initial period of rapid contractions (73 [68, 79] vs 65 [59, 71] pps; p=0.022), but not during the contraction plateau (31 [28, 34] vs 28 [26, 31] pps; p=0.176). Both groups displayed similar MU recruitment speed (302 [203, 401] vs 281 [170, 392] MU/ms; p=0.767). CONCLUSION: RT individuals exhibited greater maximal strength and absolute RFD. However, despite greater initial MU discharge rate, relative RFD was similar between groups. The lack of differences in relative RFD likely stems from between-group similarity in intrinsic contractile properties and MU recruitment speed, two key determinants of maximal RFD.
Read CV Haydn ThomasonECSS Paris 2023: OP-BM04
INTRODUCTION: The influence of the mind on physical performance has been extensively demonstrated in the last decades (1). In line with these findings, we highlighted in a previous study how unconsciously manipulating time perception significantly slowed down the accumulation of neuromuscular fatigue (2). Here, we move forward by addressing two important yet unresolved questions: 1) What are the neural correlates of this effect, and 2) Is this effect persistent in the absence of motivational confounds (i.e., performance goals)? METHODS: 24 subjects participated in four separate sessions in which they performed 100 isometric knee extensions against a fixed resistance (20% of their maximal torque). While the rest time between contractions was identical (5s) for each session, each contractions real (R) and perceived (P) time were independently manipulated. In each session, the contractions’ time was either short (10s) or long (12s), while the digital clock displayed in front of the participants was either Normal (N) or Biased (B). This led to 4 counterbalanced conditions: N10 (10s P, 10s R), N12 (12s P, 12s R), B10 (10s P, 12s R), B12 (12s P, 10s R). Using electroencephalographic recordings, we measured power changes in motor (beta [13-31Hz]) and frontal (theta [4-8Hz], alpha [8-12Hz] and beta [13-31Hz]) areas over the 100 contractions. Simultaneously, the root mean square of the quadriceps electromyographic activity (EMG) was computed. For each measurement, we extracted the integral over all contractions. Finally, pre to post-exercise changes in the quadriceps Maximal Voluntary Torque (MVT) were assessed. Repeated measures ANOVAs and paired t-tests were performed using frequentist and Bayesian frameworks to evaluate both the difference and absence of difference between sessions. RESULTS: Pre to post-MVT decrease was larger in N12 (-24.2±2.3%) compared to N10 (-20.3±1.7%), B10 (-21.5±1.8%), and B12 (-20.1±1.5%) (p<.05). Similarly, EMG increase over the 100 contractions was greater in N12 (187.8±13.1a.u.) compared to N10 (180.1±9.8a.u.), B10 (181.0±9.7a.u.), and B12 (182.3±12.4a.u.) (p<.05). Importantly, there was no difference between N10 and B10 for both MVT and EMG (p>.05, Bayes Factor (BF)<.3). Regarding the underlying neural correlates, no difference was observed between the sessions in beta power of the motor area (p>.05, BF<.3). At the same time, frontal power in theta and beta bands exhibits a significant difference between N10 and N12 (p<.05), but no difference (p>.05, BF<.3) between B10 and N10, and N12 and B12, respectively. CONCLUSION: Our study shows a subjective time-dependent accumulation of physical fatigue irrespective of motivational factors. Interestingly, this effect is observed only when the clock is slowed down, suggesting that fatigue accumulation can be reduced but not accelerated. This effect is mediated by an oscillatory dynamic that follows subjective time in frontal but not motor areas. 1. Noakes, Appl Physiol Nutr Metab, 2011 2. Matta et al., Psychophysiology, 2023
Read CV Pierre-Marie MattaECSS Paris 2023: OP-BM04
INTRODUCTION: During high-intensity exercise, pain arises from the accumulation of metabolites (e.g., H+, K+) and increased intramuscular pressure (1). While the contribution of pain on impaired exercise performance is poorly understood, studies using experimental pain models have suggested that the increased inhibitory afferent feedback and augmented feedforward central drive can accelerate the achievement of the sensory tolerance limit to compromise performance (2). While prior studies have investigated the effect of experimental pain on isometric exercise tasks, little is known about corticomotor modulations to painful stimuli during ecological dynamic tasks such as cycling. Accordingly, this study aimed to examine the effect of experimental pain on corticospinal excitability, intracortical inhibition, and neuromuscular function during cycling. METHODS: Nine healthy, adult participants (2 females) completed a counterweighted single-leg cycling ramp incremental test to obtain peak power output (PPO) followed by two experimental sessions wherein single-leg cycling at 60% of PPO was performed to task failure. Sessions were performed either without (CTRL) or with (PAIN) an experimental pain intervention consisted of a complete vascular occlusion of the contralateral resting leg combined with electrical stimulation of thigh muscles. To measure corticospinal excitability and intracortical inhibition, 2 femoral nerve stimuli, 5 unconditioned single-pulse transcranial magnetic stimuli (TMS), and 5 short- and 5 long-interval paired TMS pulses were elicited every five minutes during cycling to evoke Mmax, MEP, SICI, and LICI, respectively. Following each stimulation epoch, the bike pedals were locked and participants performed a brief maximal voluntary contraction (MVC) combined with femoral nerve stimuli to characterize neuromuscular function. Subjective ratings of leg pain, fatigue, and effort were also measured every 5 min. RESULTS: Despite a shortened time to task failure in the PAIN condition (37±12min vs. 61±21min; P=0.015), no differences in neuromuscular function (i.e., MVC force, voluntary activation, and quadriceps twitch force) were present between conditions. Additionally, there were no between-condition differences in Mmax, MEP, SICI, or LICI during cycling (all P>0.05). Contralateral leg pain ratings were higher in PAIN than CTRL (P<0.001), but exercising leg pain was higher in CTRL than PAIN (P=0.015). No differences between fatigue or effort appeared between conditions. CONCLUSION: Pain impaired exercise performance, but this impedance occurred independently from the intracortical, corticospinal, and neuromuscular pathways governing voluntary movement. Instead, regardless of the amount of inhibitory afferent feedback, exercise terminated once individuals achieved a maximal tolerable limit which was accelerated and primarily determined by perceived pain. 1. Cook et al., 1997, MSSE 2. Azevedo et al., 2022, APNM
Read CV Jenny ZhangECSS Paris 2023: OP-BM04