Jacobs, Robert Acton3; Flueck, Daniela3; Bonne, Thomas Christian7; Buergi, Simon5; Christensen, Peter Moller8; Toigo, Marco3; Lundby, Carsten3
1 PhD, Department of Nutrition, Exercise and Sports, Faculty of Science, Københavns Universitet2 Department of Nutrition, Exercise and Sports, Faculty of Science, Københavns Universitet3 Zurich Center for Integrative Human Physiology (ZIPH), University of Zurich, Zurich4 Department of Exercise and Sport Sciences, Faculty of Science, Københavns Universitet5 unknown6 Eyepath Lab, Department of Neuroscience and Pharmacology, Faculty of Health and Medical Sciences, Københavns Universitet7 Department of Exercise and Sport Sciences, Faculty of Science, Københavns Universitet8 Eyepath Lab, Department of Neuroscience and Pharmacology, Faculty of Health and Medical Sciences, Københavns Universitet
Six sessions of high-intensity interval training (HIT) are sufficient to improve exercise capacity. The mechanisms explaining such improvements are unclear. Accordingly, the aim of this study was to perform a comprehensive evaluation of physiologically relevant adaptations occurring after six sessions of HIT to determine the mechanisms explaining improvements in exercise performance. Sixteen untrained (43 +/- 6 ml.kg(-1).min(-1)) subjects completed six sessions of repeated (8-12) 60 s intervals of high-intensity cycling (100% peak power output elicited during incremental maximal exercise test) intermixed with 75 s of recovery cycling at a low intensity (30 W) over a 2-wk period. Potential training-induced alterations in skeletal muscle respiratory capacity, mitochondrial content, skeletal muscle oxygenation, cardiac capacity, blood volumes, and peripheral fatigue resistance were all assessed prior to and again following training. Maximal measures of oxygen uptake ((V)O-2peak; similar to 8 P = 0.026) and cycling time to complete a set amount of work (similar to 5 P = 0.008) improved. Skeletal muscle respiratory capacities increased, most likely as a result of an expansion of skeletal muscle mitochondria (similar to 20 P = 0.026), as assessed by cytochrome c oxidase activity. Skeletal muscle deoxygenation also increased while maximal cardiac output, total hemoglobin, plasma volume, total blood volume, and relative measures of peripheral fatigue resistance were all unaltered with training. These results suggest that increases in mitochondrial content following six HIT sessions may facilitate improvements in respiratory capacity and oxygen extraction, and ultimately are responsible for the improvements in maximal whole body exercise capacity and endurance performance in previously untrained individuals.
Journal of Applied Physiology, 2013, Vol 115, Issue 6, p. 785-793