Skeletal muscles have a great ability to adapt to changes in physical activity. Recently, Burgomaster et al. (2005) have reported a marked increase in muscle oxidative potential and cycle endurance capacity of untrained males after only six session of sprint interval training. The aim of the present study was to examine the physiological response to exercise and changes in performance as a result of a change to high-intensity training in trained endurance runners. Methods Fifteen sub-elite endurance runners (mean±S.E.M; age: 34.4±1.5 yrs; body mass: 74.0±2.3 Kg; height: 180.4±2.0; experience: 6.3±1.2 yrs; VO2 peak: 54.2±1.4 ml/kg/min) were matched and randomly assigned to either an experimental (EG, N=8) or control group (CG, N=7). The CG continued the training as during the period prior to the study (mainly aerobic moderate intensity training, 3-4 times a week), while the EG for a 4-wk period replaced their normal training by performing 3-4 sessions a week of high-intensity intermittent exercise (HIT) each consisting of 8-12 repeated 30-s supra-maximal running efforts separated by 3 min of rest. Before (PRE) and after (POST) the 4-wk period the athletes completed: 1) four 6-min sub-maximal treadmill running stages followed by an incremental test to exhaustion to determine VO2 max (T1); 2) Two supra-maximal intense exercise bouts to exhaustion on a treadmill (T2), 3) a 10,000-m race on a track (10K); 4) the Yo-Yo intermittent recovery test - level 2 (Yo-Yo IR2); 5) a 30-s "all out" running effort. Heart rate and pulmonary oxygen uptake were measured, and blood samples were collected frequently during T1 and T2. Results In T1, EG improved (P<0.05; Students Paired t-test) running economy (ml/kg/km) at speeds of 13.0, 14.5 and 16.0 km/h (7.3%, 5.2% and 6.4% respectively), whereas no significant changes were observed in CG at either speeds. For EG, time to exhaustion in the 1st and 2nd bout of T2POST were 24.6% and 18.0% longer (P<0.05), respectively, than T2PRE. No changes were observed in CG. Furthermore, in EG the Yo-Yo IR2 performance was 18.2% better (P<0.05) after HIT, with no significant change for CG (+4.3%). In EG the distance covered during the 30-s all-out test was 6.8% longer (P<0.05) after the training period. VO2 peak and 10K performance remained unaltered in both groups. Conclusion The present study showed that the anaerobic training improved performance during supra-maximal and intermittent exercise, and maintained the aerobic fitness and endurance performance of trained runners. Furthermore, running economy at medium and high sub-maximal intensities was enhanced. These changes occurred despite a dramatic reduction in training volume.
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