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Title: The influence of intermittent exercise and heat exposure on neuromuscular and cognitive function
Author: Malcolm, Rachel
ISNI:       0000 0004 7959 9393
Awarding Body: Nottingham Trent University
Current Institution: Nottingham Trent University
Date of Award: 2018
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Performance in team sport exercise requires athletes to maintain cognitive and neuromuscular function whilst completing intermittent bouts of exercise, at varying intensities. This ability enables athletes to perform skills effectively throughout a match, whilst achieving a high physical output. However, the influence of this type of exercise, particularly in the heat, on neuromuscular and cognitive function is not well known. Therefore, the aim of this thesis was to establish the neuromuscular and cognitive responses to intermittent exercise in the heat, providing potential mechanisms involved for any changes. Chapter 4 highlights the detrimental cognitive response to passive heat exposure, mediated largely by negative perceptual feelings. Due to the short duration (60 min) of heat exposure (40º C & 50 % Rh), core temperature did not show a physiologically significant increase during passive heat exposure (baseline: 37.1 ± 0.3º C vs end: 37.6 ± 0.4º C). This allowed the effect of skin temperature and perceptual feeling upon cognition to be established. Response times were slower in the hot trial on the simple (main effect of trial, P < 0.001) and complex (main effect of trial, P < 0.01) levels of the Stroop test (Hot: 872 ± 198 ms; Moderate: 834 ± 177 ms) and the simple level of the visual search test (Hot: 354 ± 54 ms; Moderate: 331 ± 47 ms) (main effect of trial, P < 0.01). Participants demonstrated superior accuracy on the simple level of the Visual Search test in the hot trial (Hot: 98.5 ± 3.1%; Moderate: 97.4 ± 3.6%) (main effect of trial, P=0.04). Participants also demonstrated an improvement in accuracy on the complex level of the visual search test following 1 h passive heat exposure (Pre: 96.8 ± 5.9 %; Post: 98.1 ± 3.1 %), whilst a decrement was seen across the trial in the moderate condition (Pre: 97.7 ± 3.5; Post: 97.0 ± 5.1 %) (trial*time interaction, P=0.029). The findings of chapter 4 suggest that response times for perception and executive function tasks are the most likely to be altered when exposed to heat stress. Chapter 5 found that a competitive hockey match facilitated response times for simple perception tasks at full-time (trial*time interaction, P < 0.01). Response times on the complex executive function task also improved from baseline to half-time (Pre: 827 ± 168; HT: 787 ± 163ms) (trial*time interaction, P < 0.01). However, working memory declined at full-time on the match trial (Pre: 6.3 ± 1.0; FT: 5.9 ± 1.1 %) (trial*time interaction, P < 0.01). The beneficial effects were likely a result of higher serum BDNF (Control: 23787 ± 899; Match: 28113 ± 2115 pg/ml) (main effect of trial, P=0.03) on the match trial, as well as increased arousal, demonstrated by the increases in noradrenaline shown during the match (Pre: 329 ± 82; Post: 451 ± 156 pg/ml) (trial*time interaction, P < 0.01). The study was the first to highlight the beneficial effect a competitive team sports match can have on response times in the domains of perception and executive function, whilst the detriment in working memory may influence tactical recall. Chapter 6 assessed the reliability of a number of the neuromuscular function measures to be used in chapter 7. Maximal cortical voluntary activation was not different between-day (94.2±4.1% versus 93.4±4.6%, P=0.06) or within-day (93.1±5% versus 92.9±4.5%, P=0.45). Systematic error (95% limits of agreement) for maximal cortical voluntary activation was −0.78% (−4.92%, 3.36%) for between-day and -0.28% (−4.12%, 3.57%) for within-day. ICC and CV values demonstrated high reliability between-day (ICC=0.927, CV=2.32%) and within-day (ICC=0.953, CV=2.19%). These results indicate that TMS can reliably estimate the output of the motor cortex to the knee extensors, both between-day and within-day. Chapter 7 found that both cortical (Hot: 89.0 ± 5.8 %; Moderate 91.7 ± 4.1 %) (main effect of trial, P = 0.04) and peripheral (Hot:84.2 ± 6.7 %; Moderate: 87.7 ± 4.3 %) (main effect of trial, P < 0.01) voluntary activation were worse in the heat, likely as a result of the increase in core temperature (trial*time interaction, P < 0.01). A positive effect of intermittent exercise in the moderate condition was seen for the simple perception task, however this effect was reversed in the heat where response times slowed (Pre: 303 ± 28; Full-time (FT): 333 ± 56 ms) (trial*time interaction, P = 0.01), and accuracy was worse throughout the hot trial (Main effect of trial, P < 0.01). Response times improved across the moderate trial for the complex executive function task (Pre: 865 ± 162; FT: 813 ± 174 ms), however no positive effect was seen in the hot (Pre: 902 ± 171; FT: 906 ± 222 ms) (trial*time interaction, P = 0.02). The findings of this thesis suggest perception and executive function are the most vulnerable cognitive domains in response to heat and exercise, where heat has a negative effect and exercise has a positive one. However, the final study suggests that when combined intermittent exercise and heat stress detrimentally influence performance in these domains, reversing the beneficial effect seen with exercise. Similarly, both cortical and peripheral voluntary activation are worse in the heat, which will limit muscular function of athletes competing in hot conditions. Therefore, protective strategies must be investigated, focusing specifically on the protection of cognition and neuromuscular function during intermittent exercise in the heat.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available