| Abstract | Caffeine (CAF) is the most widely consumed ergogenic substance in sport and has been reported to improve various attributes associated with successful soccer performance including, endurance capacity, gross motor skill performance and cognitions. These benefits are typically ascribed to pharmacological mechanisms (i.e. central nervous and peripheral tissue stimulation). However, the psychological and perceptual permutations that are associated with CAF expectancies are largely unaddressed in most experimental designs but could be as important as CAF pharmacology in understanding if/how CAF elicits an ergogenic response on sport performance. As the consumption of pharmacologically active CAF may not be necessary in observing a CAF associated ergogenic response, this body of work may prove beneficial to individuals suffering from pre-existing health concerns (e.g. hypertension, genetic polymorphisms, depression, insomnia etc.), CAF habituation, and those participating in late evening sports competitions whereby CAF consumption may impair sleep quality/duration.
The main aim of this thesis is to evaluate and explore the psychobiological effects of expectancies associated with oral CAF consumption on various facets of simulated soccer performance and perceptual states. This was achieved via completion of the following objectives: (1) conducting a systematic review and meta-analyses pertaining to literature exploring the potential psychobiological effects of CAF on sport and exercise (A) and cognitive performance (B) (2) exploration of the potential psychobiological effects of CAF on high-intensity intermittent endurance capacity, reaction time and soccer skill proficiency (3) exploration of the potential psychobiological effects of CAF on perceptual fatigue and mood states (4) exploration of the potential psychobiological effects of CAF on subjective perceptions using the double-dissociation design but in particular the mixed methods approach (Brooks et al., 2015). There remains a severe under representation of the mixed methods design in the literature pertaining to the phenomenon of CAF expectancies on sport, exercise, and cognitive performance. The mixed methods design and associated triangulation of quantitative and qualitative data is fundamental to validly compare CAF’s psychological versus pharmacological impetus. Prior to this thesis, only two other studies (Beedie et al., 2006; Duncan et al., 2009) implemented the mixed methods design, with neither providing a rigorous account of methodological decisions, researcher reflexivity, and/or evidence of applying an epistemological framework. These factors were incorporated within the current thesis and improved the quality of data collection, analysis, and reflection.
The results of our, novel, meta-analyses demonstrated that all studies exploring the psychobiological effects of CAF on sport and exercise performance displayed a beneficial effect (p=0.01) with an overall moderate effect size (Cohen’s d (ES): 0.40) observed. In contrast, no significant effect was observed for studies exploring the psychobiological effects of CAF on cognitive performance (p=0.142) with a small effect size (ES=0.1) observed. Though, due to significant methodological heterogeneity associated with studies exploring the psychobiological effects of CAF on cognitive performance, any associated implications here should be taken with caution.
Experimental study 1 explored the influence of CAF expectancies on facets of simulated soccer performance (e.g. exercise capacity, reaction time and passing ability (LSPT)) and perceptual states via utilisation of a mixed-methods approach and double-dissociation design. Exercise capacity was greater (p<0.05) for CAF psychology (given placebo (PLA)/told CAF) (623 ± 117 s) versus pharmacology (given CAF/told PLA) (578 ± 99 s) with all participants running longer during psychology. This benefit appeared to be driven by CAF expectancies and reductions in perceptual effort. Interestingly, positive perceptions for told CAF conditions appeared to impair BATAK performance via potential CAF over reliance. In contrast, negative perceptions possibly facilitated BATAK performance via augmented conscious effort. A similar trend to BATAK was observed for LSPT performance.
Following the completion of experimental study 1 it became apparent that the techniques used to modulate expectancies across experimental conditions (i.e. told PLA/CAF groups) here or any other study with a primary aim of exploring the influence of CAF expectancies on sport, exercise and/or cognitive performance, require validation. This was the premise of experimental study 2. No meaningful findings were observed from baseline to post-intervention across any outcome measure during experimental study 2. This lack of effect may be related to environmental factors, whereby individuals completed trials in classrooms and/or a home cinema, prior to lectures/seminars and/or social interactions, respectively. In contrast, participants in experimental study 1 were administered the appropriate expectancy modulating techniques after they had perceived to consume PLA or CAF within an environment that necessitated an immediate importance for CAF (e.g. prior to exercise performance).
In summary, the novelty and original contribution of the current body of research entails: completion of a systematic review and meta-analyses pertaining to the influence of CAF expectancies on sport, exercise and cognitive performance; assessment of the influence CAF expectancies may have on simulated soccer performance; the implementation of a mixed methods approach and double dissociation design; an in depth rationale, description and set of instructions for the utilisation of the mixed methods approach in any future research, including the use of an epistemological framework; a summary of ecological factors that are fundamental in understanding the phenomenon of CAF expectancies across sport and exercise performance. With respect of the main findings from the experimental data contained in this thesis: the benefits associated with CAF expectancies may better suit tasks that entail lesser cognitive/skill specific attributes but greater gross motor function (e.g. cycling, weightlifting, running etc.) and this is likely due to reduced perceptual effort. Moreover, future studies aiming to validate expectancy modulating techniques or generally assessing expectancies should provide a greater immediate importance for CAF and this may be achieved by replicating environmental and/or psychosocial conditions associated with sport performance (e.g. the utilisation of a performance measure) and the perception for CAF consumption. |
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