Submitted on May 21, 2008
Revised on August 17, 2008
Accepted on October 1, 2008
Four weeks of speed endurance training reduces energy expenditure during exercise and maintains muscle oxidative capacity despite a reduction in training volume
F. Marcello Iaia1, Ylva Hellsten1, Jens Jung Nielsen1, Maria Fernström2, Kent Sahlin3, and Jens Bangsbo1*
1 University of Copenhagen
2 Karolinska Institutet, and Stockholm University College of Physical Education and Sport
3 GIH, The Swedish School of Sport and Health Sciences
* To whom correspondence should be addressed. E-mail: jbangsbo{at}ifi.ku.dk.
We studied the effect of an alteration from regular endurance to speed endurance training on muscle oxidative capacity, capillarization as well as energy expenditure during submaximal exercise and its relationship to mitochondrial uncoupling protein (UCP3) in humans. Seventeen endurance-trained runners were assigned to either a speed endurance training (SET, n=9) or a control (CON, n=8) group. For a 4-wk intervention (IT) period, SET replaced the ordinary training (~45 km/wk) with frequent high-intensity sessions each consisting of 8-12 30-s sprint runs separated by 3 min of rest (5.7±0.1 km/wk) with additional 9.9±0.3 km/wk at low running speed, whereas CON continued the endurance training. After the IT period, VO2 was 6.6, 7.6, 5.7 and 6.4% lower (P<0.05) at running speeds of 11, 13, 14.5 and 16 km.h-1 respectively, in SET, while remained the same in CON. No changes in blood lactate during submaximal running were observed. After the IT period the protein expression of skeletal muscle UCP3 tended to be higher in SET (34±6 vs. 47±7, P=0.06). Activity of muscle citrate synthase and 3-hydroxyacyl-CoA dehydrogenase, as well as VO2-max and 10-km performance time remained unaltered in both groups. In SET, the capillary-to-fibre ratio was the same before and after the IT period. The present study showed that speed endurance training reduces energy expenditure during submaximal exercise, which is not mediated by lowered mitochondrial UCP3 expression. Furthermore, speed endurance training can maintain muscle oxidative capacity, capillarization and endurance performance in already trained individuals despite significant reduction in the amount of training.
