The preparation and initiation of movement has previously been described using a neural accumulation model; this model involves an increase of neural activation in the motor cortex (M1) from baseline to a subthreshold level following a warning signal, which is maintained until presentation of an imperative stimulus (IS). Activity then increases until reaching movement initiation threshold. This model predicts that variability in activation during preparation may influence reaction time (RT) and its variability. The purpose of this thesis project was to determine whether differences in RT/variability of RT during the completion of tasks with varying levels of complexity may be attributable to differences in neural excitability in M1. To test this prediction, transcranial magnetic stimulation (TMS) delivered concurrently with an IS was used to determine neural excitability for movements with different accuracy demands. It was hypothesized that higher accuracy demands would result in lowered amplitude and/or greater variability of neural activation, and consequently slower/more variable RT. Fifteen healthy participants completed a simple RT task involving a targeted wrist extension movement under three different accuracy conditions (easy, moderate, difficult). TMS was delivered concurrently with the IS on 50% of trials during each condition. While pilot testing showed RT differences between accuracy conditions (Appendix A), the data presented here failed to detect significant differences in RT latency (F(2, 28) = .074, p = .929) or variability (F(1.432, 20.053) = .633, p = .538) between conditions . Similarly, no difference in MEP amplitude was observed between difficulty conditions (F(2, 28) = 2.439, p = .106). However, a subset of participants (n = 7) did show significant RT increases between easy and hard conditions (t(6) = 2.531, p = .045), but this subset still failed to show differences in MEP amplitude (t(6) = 1.157, p = .291) or variability (t(6) = 1.545, p = .173), suggesting that preparatory levels at the IS may be similar for movements involving both high and low accuracy demands.

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The primary aim of the present study was to evaluate whether intra-individual variability in inhibition-related neural activation in response to sleep restriction is associated with eating behavior in adolescents aged 12-18 years. In addition, the potential moderating effects of sex and body mass index on the association between sleep and variability in neural activation were examined. This study employed a within-subjects crossover design that randomized subjects to both a 5 hours per night (sleep restricted) and 9 hours per night (well-rested) sleep condition for 5 nights, with experimental conditions separated by four weeks. On the 6th day of each study phase participants completed a 24-hour diet recall and a food-related inhibitory go/no-go task while undergoing functional magnetic resonance imaging. Repeated measures multilevel models examined individual differences attributable to sleep duration and a series of separate multivariate analysis of variance models examined the effect that vulnerability to sleep restriction has on eating behavior as well as the moderating impact of sex and weight status. Findings suggest that adolescents who exhibited greater efficiency in inhibitory and reward-related neural activation when sleep restricted demonstrated less pronounced decrements in neural activation when sleep restricted relative to when they were well-rested. These findings suggest that the effect of sleep restriction on inhibitory control may differ between individuals such that there are individuals who appear able to sustain inhibitory control comparable to when they are well-rested while other individuals show marked declines in executive functioning-related neural activation when sleep restricted. Results from separate exploratory models including regions of interest associated with reward valuation and across the whole brain were consistent with these findings. We also found that the effect of vulnerability to sleep restriction on inhibitory efficiency in the right inferior parietal lobule (R - IPL) and right middle frontal gyrus (R - MFG) differed by sex and was predictive of differences in overall eating behavior and sugar intake, respectively, when sleep restricted compared to well-rested. In addition, vulnerability in the inhibitory network was predictive of differences in individual eating behavior (i.e., total calories, added sugar, sugar, and total fat) for males and females across conditions. This finding demonstrates there is significant variability in the impact that sleep restriction has on inhibitory efficiency in adolescence relative to when they are well-rested, and vulnerability to inhibitory efficiency appears to effect male and female adolescent's dietary behaviors differently when they obtain insufficient sleep. Vulnerability to inhibitory efficiency when sleep restricted compared to well-rested may cause males and females to consume more energy dense foods when they obtain insufficient sleep and also differs for males and females irrespective of their sleep duration. Given the pervasiveness of chronic sleep restriction in adolescence, males who are unable to counter the effect that insufficient sleep has on palatable foods may be at greatest risk of obesity.

Neural activation is thought to be essential for the expression of maximal muscle performance, but the exact contribution of neural mechanisms such as the level of agonist, antagonist and stabiliser muscle activation to muscle strength is not fully understood. Explosive neuromuscular performance, including the ability to initiate (the electromechanical delay, EMD) and develop force rapidly (termed, rate of force development, RFD) are considered essential for the performance of explosive sporting tasks and joint stabilisation and thus injury avoidance. The thesis aimed to improve our understanding of the contribution of neural factors to muscle performance, with a specific focus on explosive neuromuscular performance. The work in this thesis utilised a range of approaches to achieve this aim. Initially, the association between muscle activation and rate of force development and EMD was established. Comparison of unilateral and bilateral actions was then undertaken. Finally interventions with the aim to both negatively affect and improve muscle strength, which included fatigue and resistance training (RT), respectively was undertaken and the neural contributions to changes in performance established. Agonist activation during the early phase of voluntary force production was shown to be an important determinant of voluntary EMD, explaining 41% of its inter-individual variability. Agonist activation was an important determinant of early, but not late phase RFD. Use of bilateral actions resulted in a reduction in explosive strength, which was thought to be due to differences in postural stability between unilateral and bilateral strength tasks. The level of stabiliser activation was strongly related to the level of agonist activation during the early phase of explosive force development and had a high association with explosive force production. Task-specific adaptations following isoinertial RT, specifically, the greater increase in isoinertial lifting strength than maximal isometric strength were due to training-specific changes in the level of agonist activation. High-intensity fatigue achieved a more substantial decline in explosive than maximal isometric strength, and this was postulated to be due to neural mechanisms, specifically decreased agonist activation. This work provides an in depth analysis of the neural contributions to maximal muscle performance.

Neuromuscular explosive strength (defined as rate of force development; RFD) is considered important during explosive functional human movements; however this association has been poorly documented. It is also unclear how different variants of strength training may influence RFD and its neuromuscular determinants. Furthermore, RFD has typically been measured in isometric situations, but how it is influenced by the types of contraction (isometric, concentric, eccentric) is unknown. This thesis compared neuromuscular function in explosive power athletes (athletes) and untrained controls, and assessed the relationship between RFD in isometric squats with sprint and jump performance. The athletes achieved a greater RFD normalised to maximum strength (+74%) during the initial phase of explosive contractions, due to greater agonist activation (+71%) in this time. Furthermore, there were strong correlations (r2 = 0.39) between normalised RFD in the initial phase of explosive squats and sprint performance, and between later phase absolute explosive force and jump height (r2 = 0.37), confirming an association between explosive athletic performance and RFD. This thesis also assessed the differential effects of short-term (4 weeks) training for maximum vs. explosive strength, and whilst the former increased maximum strength (+20%) it had no effect on RFD. In contrast explosive strength training improved explosive force production over short (first 50 ms; +70%) and long (>50 ms; +15%) time periods, due to improved agonist activation (+65%) and maximum strength (+11%), respectively. Explosive strength training therefore appears to have greater functional benefits than maximum strength training. Finally, the influence of contraction type on RFD was assessed, and the results provided unique evidence that explosive concentric contractions are 60% more effective at utilising the available force capacity of the muscle, that was explained by superior agonist activation. This work provides a comprehensive analysis of the association between athletic performance and RFD, the differential effects of maximum vs. explosive strength training, and the influence of contraction type on the capacity for RFD.

The main aim of this dissertation was to investigate the difference in neural language patternsrelated to language ability in healthy adults. The focus lies on unraveling the contributions of theright‐hemispheric homologues to Broca’s area in the inferior frontal gyrus (IFG) and Wernicke’s areain the posterior temporal and inferior parietal lobes. The functions of these regions are far from fullyunderstood at present. Two study populations consisting of healthy adults and a small group ofpeople with generalized epilepsy were investigated. Individual performance scores in tests oflanguage ability were correlated with brain activation obtained with functional magnetic resonanceimaging during semantic and word fluency tasks. Performance‐dependent differences were expectedin the left‐hemispheric Broca’s and Wernicke’s area and in their right‐hemispheric counterparts. PAPER I revealed a shift in laterality towards right‐hemispheric IFG and posterior temporal lobeactivation, related to high semantic performance. The whole‐brain analysis results of PAPER IIrevealed numerous candidate regions for language ability modulation. PAPER II also confirmed thefinding of PAPER I, by showing several performance‐dependent regions in the right‐hemispheric IFGand the posterior temporal lobe. In PAPER III, a new study population of healthy adults was tested.Again, the right posterior temporal lobe was related to high semantic performance. A decrease in lefthemisphericIFG activation could be linked to high word fluency ability. In addition, task difficultywas modulated. Increased task complexity showed to correlate positively with bilateral IFGactivation. Lastly, PAPER IV investigated anti‐correlated regions. These regions are commonly knownas the default mode network (DMN) and are normally suppressed during cognitive tasks. It wasfound that people with generalized epilepsy had an inadequate suppression of regions in the DMN,and showed poorer performance in a complex language test. The results point to neural adaptabilityin the IFG and temporal lobe. Decreased left‐lateralization of the IFG and increased rightlateralizationof the posterior temporal lobe are proposed as characteristics of individuals with highlanguage ability.