The effect of an alteration from regular endurance to interval (10-20-30) training on the health profile, muscular adaptations, maximum oxygen uptake (VO(2)-max) and performance of runners was examined. Eighteen moderately trained individuals (6 females and 12 males; VO(2)-max: 52.2±1.5 ml(.)kg(-1)(.)min(-1)) (means±SE) were divided into a high intensity training (10-20-30; 3 females and 7 males) and a control (CON; 3 females and 5 males) group. For a 7-week intervention period the 10-20-30 replaced all training sessions with 10-20-30 training consisting of low, moderate and high speed running [90% of maximal intensity] for 30, 20 and 10 s, respectively, in 3-4 5-min intervals interspersed by 2 min of recovery, reducing training volume by 54% (14±0.9 vs. 30.4±2.3 km(.)week(-1)) while CON continued the normal training. After the intervention period VO(2)-max in 10-20-30 was 4% higher, and performance in a 1500-m and a 5-K run improved (p