Acute and Chronic Responses of Skeletal Muscle to Endurance and Sprint Exercise

Abstract

Skeletal muscle adapts to the stress of endurance and sprint exercise and training. There are 2 main types of skeletal muscle fibre--slow twitch (ST) and fast twitch (FTa, FTb, FTc). Exercise may produce transitions between FT and ST fibres. Sprint training has decreased the proportion of ST fibres and significantly increased the proportion of FTa fibres, while endurance training may convert FTb to FTa fibres, and increase the proportion of ST fibres (i.e. FTb----FTa----FTc----ST). However, the high proportion of ST fibres documented for elite endurance athletes may be simply the result of natural selection. ST fibres function predominantly during submaximal exercise, whereas FT fibres are recruited as exercise intensity approaches VO2max and/or glycogen stores are depleted. Long distance runners have greater ST and FT fibre areas than untrained controls. However, doubt remains as to whether the ST or FT fibre area is greatest in endurance athletes. Increases in FT fibre area seem to occur during the first 2 months of training whereas ST fibre areas appear to increase after 2 to 6 months of training. Sprint training leads to the preferential use of FT fibres and male, but not female sprinters have larger FT fibres than untrained controls. Mitochondrial proteins and oxidative enzymes, as opposed to VO2max, are important determinants of the duration of endurance exercise. Endurance training increases intramuscular glycogen stores in both FT and ST fibres and produces a 'glycogen-sparing' effect which is characterised by an increased free fatty acid (FFA) metabolism. The activity of glycogen synthase is also increased by endurance training. Sprint training increases glycogen concentrations similarly in all fibre types, reduces the rate of glycogen utilisation at submaximal workloads and allows supramaximal workloads to be maintained for longer periods of time. During endurance exercise the pattern of glycogen depletion varies between muscle fibre types and between muscle groups. Glycogen stores in ST fibres are utilised initially, followed by stores in FTa then FTb fibres. Sprint activities are associated with a much greater rate of glycogen depletion. However, it is unlikely that glycogen depletion causes fatigue during sprinting. Sprint work is associated with a preferential depletion of glycogen from FTb then FTa and ST fibres. Endurance training appears to increase triglyceride stores adjacent to mitochondria and ST fibres have greater triglyceride stores than FT fibres. Endurance exercise is associated with a preferential use of triglycerides from ST fibres and endogenous triglycerides may account for over 50% of the total lipid oxidised during exercise.(ABSTRACT TRUNCATED AT 400 WORDS)

Endurance Training Activates Pancreatic Islets Amp-Activated Kinase-Uncoupling Protein 2 Pathway and Reduces Insulin Secretion

“…Exercise training-induced increase in skeletal muscle and liver insulin signaling pathway results in higher blood glucose uptake (Zierath 2002, Hawley & Lessard 2008, Hoene et al 2009). In addition, endurance exercise also improves skeletal muscle and liver glycogen synthesis capacity by increased glycogen synthase activity (Abernethy et al 1990, Garrido et al 1996, enhancing blood glucose disposal, allowing faster glycemia reduction. Thus, faster training-induced blood glucose clearance by insulin-sensitive tissues, together with the low GLUT2 and glucokinase affinity, due to their elevated Km (Iynedjian 1993, Schuit 1997, might account for impaired GIIS signaling in pancreatic b-cells, reducing blood insulin content observed during ipGTT.…”

Ciclo de Krebs como fator limitante na utilização de ácidos graxos durante o exercício aeróbico

“…Postula-se que a reserva mais importante de AGL não plasmáticos, disponível para a oxidação durante exercício moderado e prolongado, é o TGIM (20). Esses são encontrados em concentrações diferentes conforme o tipo de fibra muscular, sendo armazenados em maior quantidade nas fibras musculares de contração lenta do que em fibras musculares de contração rápida (20).…”

“…Esses são encontrados em concentrações diferentes conforme o tipo de fibra muscular, sendo armazenados em maior quantidade nas fibras musculares de contração lenta do que em fibras musculares de contração rápida (20). Além disso, o próprio treinamento faz com que a deposição de TG seja diferente entre os tipos de fibra muscular (20).…”

Strategies to Enhance Fat Utilisation During Exercise

“…[17,18,37] However, with increasing exercise intensities, particularly above 70 to 80% maximal oxygen uptake (V . O 2max ), there is a progressive shift from fat to CHO [17,[38][39][40] indicating limitations in the rate of FA oxidation. Several explanations for this shift from fat to CHO have been proposed, including an increase in circulating catecholamines which stimulates both glycogen breakdown in the liver [41] and lipolysis.…”