Muscle adaptations to post-exercise cooling

Endurance training results in profound skeletal muscle adaptations that improve fatigue resistance and enhance exercise capacity. To maximise these adaptations, athletes often engage in extensive training regimes, involving 10 to 16 training sessions∙wk-1. The use of cold water immersion (CWI) as a recovery intervention has emerged as a strategy to maintain training performances between sessions. However, its concurrent influence on muscle aerobic adaptations to training is unclear. Thus, the overall purpose of the four research studies contained within this thesis was to determine the influence of post-exercise CWI on muscle metabolic activity, acute and long-term adaptations to exercise and exercise training, respectively. The first two studies of this thesis were designed to determine the reliability and between limb differences of the near infrared spectroscopy (NIRS)-derived indices describing muscle oxygenation and metabolic activity. Such information was necessary to substantiate the use of NIRS to monitor muscle aerobic adaptations to training/cooling as well as the onelegged cooling model utilised throughout this thesis. In study 1, it was found that there were considerable differences in reliability levels with regards to the analytical technique chosen. However the variables demonstrated CVs ranging from 3 to 35%, which is lower than currently reported changes in training-induced adaptations and/or group differences between athletes and sedentary controls (23% - 450%). In study 2, it was shown that there were no between limb differences in NIRS-derived variables. As such, studies 1 and 2 indicate that changes in NIRS-derived variables are suitable indices to monitor the influence of cooling on training-induced adaptations in the muscle, as well as substantiate that the exercise/training protocols induced similar physiological stimulus in both the intervention and control limbs. In study 2, it was also shown that cooling one leg (15 min at 10°C) from the gluteal fold downwards resulted in significant decreases in post-exercise vastus lateralis skin temperature (35.1 ± 0.6 vs. 16.9 ± 1.7°C, p \u3c 0.001), microvascular perfusion (20 ± 4%, p \u3c 0.01) and muscle metabolic activity (p \u3c 0.05) while not resulting in shivering thermogenesis. While these responses may improve local muscle recovery, its simultaneous effect is on muscle aerobic adaptations are unclear. Indeed, reduced muscle metabolism might attenuate mitochondrial biogenesis via inhibiting AMPK activation or via a decrease in the Q10 effect. Conversely, cooling in cell and rodent models has been shown to up-regulate the expression of the transcriptional coactivator PGC-1α, which is implicated in the regulation of non=shivering thermogenesis. As such, studies 3 and 4 investigated the acute and chronic influence of post-exercise cooling on muscle aerobic adaptations to exercise and exercise training, respectively. In study 3, it was shown that cooling resulted in significantly lowered intramuscular temperatures (28.9 ± 2.3°C vs. 37.0 ± 0.8°C, p \u3c 0.001). This change was associated with a significant increase in the mRNA content of PGC-1α in the cooled limb compared with control. However, associated PGC-1α targets related to vascular and metabolic adaptation, namely VEGF and nNOS, only demonstrated significant changes from baseline (i.e. time effects) with no significant differences between conditions evident. These data indicate that an acute post-exercise cooling intervention enhances the gene expression of PGC-1α and therefore may provide a valuable strategy to enhance exercise-induced mitochondrial biogenesis. However its influence on VEGF and nNOS expression and associated functional adaptations warrants further research. In study 4, we investigated the effect of regular post-exercise CWI on training induced AMPK activity and mitochondrial biogenesis. Ten males performed 3 sessions∙wk-1of endurance training for 4 wks, where following each session subjects immersed one leg in a cold water bath (10°C) to the level of their gluteal fold for 15 min, while the contra-lateral leg served as control. Subjects’ maximal oxygen consumption and maximal aerobic running speed were improved by 5.4% and 6.4% following training (p \u3c 0.05). Additionally, regular post-exercise cooling enhanced exercise-induced increases in basal AMPK activity. Despite an increase in AMPK activity, a concomitant increase in downstream targets PGC-1α and most mitochondrial electron chain subunits was not observed. However, a significant increase in COX3 protein content was evident and hence indicates that mitochondrial biogenesis may be enhanced. Regardless, we advocate caution with regards to regular use of this intervention as cold-induced mitochondrial biogenesis may concomitantly decrease mitochondrial efficiency. Further research should focus on muscle aerobic function following regular CWI also verify if increases in AMPK activity observed in this study translates to improved glucose disposal and fatty acid oxidation