Study 1: This study investigated the effects of high intensity interval running (HIIR) and diet on the activation of signalling pathways involved in mitochondrial biogenesis. HIIR did not increase AMPK or p38MAPK phosphorylation. Therefore, it was unclear whether feeding carbohydrate (CHO) during recovery from this acute bout of high intensity exercise would blunt the activation of these pathways during a subsequent bout of HIIR. An increase in ACCβ phosphorylation was found after HIIR and was not blunted by CHO feeding during a subsequent bout of HIIR. Study 2: This study compared the changes in mitochondrial biogenesis and endurance performance in response to two weeks of HIIR or endurance running (ER) in Gaelic games players. Compared to 6 sessions of ER, HIIR induced a similar improvement in endurance performance and a more pronounced increase in mitochondrial biogenesis.