Author Identifiers

ORCID: 0000-0002-8543-9818

Date of Award

5-23-2019

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Medical and Health Sciences

First Advisor

Associate Professor Chris R. Abbiss

Second Advisor

Dr Dale W. Chapman

Field of Research Code

110602

Abstract

Acute dehydration is a common method of weight loss in many combat sports, including Mixed Martial Arts (MMA). Acute dehydration has been shown to negatively affect exercise performance but minimal research has examined the mechanisms influencing exercise performance and potential strategies to mitigate them. The present thesis aimed to: i) examine the prevalence of weight loss methods and magnitude of weight lost in MMA and other combat sports, ii) examine the physiological and psychobiological mechanisms (neuromuscular function, mood and cognitive performance) that may be responsible for acute dehydration-induced decreases in performance, and iii) determine if heat acclimation with and without fluid restriction can minimise the effects of acute dehydration. In Study One, 637 combat sports athletes completed a questionnaire to determine their weight loss practices used prior to competition. MMA athletes reported the highest usage of sauna (76%) and water loading (67%) whilst also reporting the second highest use of training in rubber/plastic suits (63%). In Study Two, fourteen MMA athletes completed a familiarisation session, followed by two experimental sessions during which physiological and psychobiological responses were assessed following acute dehydration or a control trial. During the dehydration session athletes lost 3% of their body mass while in a heated chamber (40˚C and 60% relative humidity (RH)) wearing a sweat suit for 3 h. Participants were then provided with a 3 h recovery period to consume food/fluids, after which neuromuscular fatigue, mood and cognitive performance were assessed. Strength-endurance was impaired following acute dehydration despite no influence on markers of central and peripheral fatigue. However, athlete fatigue perception increased which could indicate impaired performance resulted from mental fatigue. In Study Three, a total of 20 recreationally trained athletes were randomly assigned into two groups and completed 3 weeks (12 sessions) of heat acclimation (passive exposure at 40˚C and 60% RH), either with (HAW) or without (HANW) fluid replacement. On two occasions prior to (i.e. double- baseline) and one following heat acclimation, participants also performed experimental trials. These trials involved 3 h of passive heating (45°C, 38% relative humidity) to induce dehydration followed by 3 h of ad libitum food and fluid intake after which participants performed a repeat sled-push test to assess physical performance. No meaningful differences in performance, physiology or psychobiology were observed between the HAW and HANW groups at any time point. Pooled data revealed that mean sprint speed was faster following heat acclimation compared with baseline data. Heat acclimation appeared to improve mood following dehydration, which could have improved repeat-effort performance via altered fatigue perception. This research provides a better understanding of the prevalence of acute weight loss in combat sports and the mechanisms by which such weight loss impairs exercise performance. The results support heat acclimation as a possible strategy to reduce the negative performance effects of rapid weight loss achieved through acute dehydration.

Available for download on Sunday, May 26, 2024

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