Chandrasiri, Shavin (2021) Effects of long term concurrent heat stress on performance and muscle molecular response to resistance exercise: is hot really hot or is it just smoke? PhD thesis, Victoria University.

Abstract

Natural performance enhancement, muscle hypertrophy and muscle rehabilitation on an expedited timeframe is highly coveted in sports and fitness industries. Heat stress (HS) can enhance muscle hypertrophy and strength when used in combination with resistance exercise (RE), however, the additive effects of HS on power, speed and agility have not been investigated. Furthermore, the efficacy of previously tested heating methods in achieving muscle heating is highly variable and inconsistent, which probably contributes to the inconsistent effects of heating reported in the literature. Therefore, primarily, this thesis aimed to investigate the effects of HS on performance adaptations to RE. Secondarily, how the cellular signal transduction pathways regulating skeletal muscle adaptations to RE are altered with long term concurrent HS were also investigated. Thirdly, we aimed to develop a reliable localised heating method. We hypothesised that full body HS applied concurrently to heavy progressive RE may improve upon the phenotypic and molecular muscle adaptations to RE. In the first study, we aimed to develop a localised heating method that is capable of raising core muscle temperature above 38-5-39°C, by testing three different models of localised heating. However, none of the models were able to raise core muscle temperature (CMT) to the desired levels. Therefore, in the second study, eighteen recreationally active males were assigned to two groups, HEAT (n=8, 40°C, 30% RH), and CON (n=10, 23°C, 20% RH). Each group undertook an identical, ten week, full body RE program three days a week. Peak core body temperature (HEAT 38.18 ± 0.27 °C; CON 37.97 ± 0.32 °C), as well as vastus lateralis muscle temperature 3.5 cm under the skin (HEAT 36.79 ± 1.55 °C; CON 35.94 ± 1.51 °C) were measured. Strength, peak force, speed, agility and body composition (DXA) were measured pre-, mid (week 5), and post-intervention. Muscle biopsies were obtained from the vastus lateralis pre-intervention at rest, one hour and 48 hours post the first resistance training session. An identical biopsy trial was performed ~72-96h after the last training session of the intervention. In study three, the muscle samples at each time point were analysed via western blots for key markers of muscle protein synthesis. Fibre cross sectional area (CSA), satellite cell (SC) and myonuclear density were quantified via immunofluorescence. In study four, muscle samples were analysed for the heat shock protein (HSP) response via western blots. Study five investigated the mitochondrial and angiogenic adaptations via western blots and capillarisation response via immunofluorescence. Ten weeks of RE improved lower body strength and relative upper body strength, however had no effect on power, agility or speed. Lean muscle mass, fibre CSA, SC content and myonuclear density improved in response to RE. Concurrent full body HS applied at 40°C failed to increase CMT in the vastus lateralis. Therefore, full body HS applied at 40°C did not improve upon performance, molecular or phenotypic adaptations to RE.

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