Turning up the heat: can post-exercise hot water immersion be used to manipulate acute physiological responses & chronic adaptation following resistance training?

Resistance training is a modality of exercise that is a staple part of strength and conditioning programmes as it offers benefits to competitive performance. Despite the positive adaptations which occur through performing regular training sessions over time, a single bout of resistance exercise results in a series of acute physiological responses. These may negatively impact performers in the hours and days post-exercise, although several questions exist with regards to appropriately characterising the magnitude and time course of this response which relate to the sensitivity of the measures which are used to do so. The context-dependent dichotomy between recovery and adaptation has fuelled much discussion in the scientific literature and has recently been articulated within the concept of hormesis, with post-exercise strategies aiming to optimise the exercise stimulus. The complex interplay between acute physiological responses and recovery/adaptation requires further investigation as recovery remains one of the least understood aspects of the exercise-adaptation cycle. Hot water immersion (HWI) is a form of heat therapy which is anecdotally reported to be used by athletes, whilst the modern advent of Jacuzzis and immersion pools in an increasing number of leisure facilities make it an easily accessible strategy. HWI may influence acute physiological responses within the recovery/adaptation paradigm but has received limited attention, while no research has investigated the chronic use of HWI alongside a resistance training programme. Therefore, the aim of this course of investigations was to elucidate the effects of HWI on acute physiological responses as well as recovery/adaptation to resistance exercise in a trained cohort. This research initially critically evaluated the literature investigating the use of HWI to identify several gaps worthy of further investigation. Subsequently, three experimental chapters were designed and conducted to assess the impact of HWI to manipulate acute physiological responses following resistance exercise and the influence on recovery/adaptation.
Study 1: The aim of this investigation was to assess the usefulness of a variety of measures that are used to detail acute physiological responses following resistance exercise. The study utilised a crossover design, assessed measures through a relevant timescale (i.e. 2 h – 96 h post-exercise), recruited trained participants and employed a real-world exercise modality to enhance the ecological validity of the findings. The results suggest that several measures were able to demonstrate clear effects following resistance exercise. Additionally, the results provided a profile relating to the magnitude of change and time course for these measures with optimal sampling points identified which informed the acute physiological response measures used in subsequent chapters.
Study 2: The aim of this study was to investigate the effect of HWI on acute physiological responses and recovery following resistance exercise. The main findings demonstrated that HWI is a viable means of heat therapy that can support a greater intramuscular temperature following resistance exercise. The elevated intramuscular temperature may have manipulated inflammatory processes. Although changes in other acute physiological response markers were independent of changes in intramuscular temperature associated with HWI. These results represent the first investigation into the acute physiological responses of a ‘real-world’ HWI protocol following resistance exercise, alongside the use of a trained cohort, applied exercise session and utilising good nutritional practice.
Study 3: This chapter aimed to investigate the effect of HWI on acute physiological responses and training adaptation following a 10-week resistance training programme. The main findings demonstrated that HWI (i) augmented long-term gains in strength, (ii) had no effect on the post-training increase in lower body lean mass (iii) elicited an accelerated recovery of muscle function and soreness in the acute post-exercise period following training, and (iv) attenuated the increase of markers of inflammation and muscle cell disruption following training compared to passive recovery (PAS). Collectively, these findings suggest that at the end of a 10-week training programme, HWI manipulates acute physiological responses to hasten post-exercise recovery. This may have positively impacted an individual’s ability to train in subsequent sessions, leading to an accumulated training stimulus that induced small but worthwhile improvements in strength.
This course of investigation has provided novel information as to how HWI manipulates acute physiological responses and the subsequent impact on recovery/adaptation following resistance exercise. In addition to identifying sensitive measures and recommended sampling points for acute physiological responses, this research provides the first evidence which suggests (i) a ‘real world’ HWI protocol can maintain an elevated intramuscular temperature and blood flow following resistance exercise, (ii) acute physiological responses can be manipulated by HWI to enhance recovery during a training programme, and (iii) the HWI-associated benefits to training enabled small but worthwhile enhancements in strength adaptations following a resistance training programme. This series of studies utilised a ‘real-world’ HWI protocol, alongside the use of a trained cohort, applied exercise session and good nutritional practice, enhancing the ecological validity of this thesis. Further work is warranted to optimise the HWI protocol and widen the scope of application to other cohorts and with different exercise modalities, as well as deepen mechanistic knowledge. However, the positive findings from this thesis provide physiologists with rationale for utilisation of HWI alongside resistance training in their applied practice.