The present study examined muscle adaptations and alterations in performance of highly trained soccer players with intensified training or training cessation. Eighteen elite soccer players were for a 2-wk period assigned to either a group which performed high intensity training with a reduction in the amount of training (HI, n=7) or an inactivity group without training (IN, n=11). HI improved (P<0.05) performance of the fourth, sixth and tenth sprint in a repeated 20-m sprint test (RST) and IN reduced (P<0.05) performance in the fifth to the tenth sprint after the 2-wk intervention period. In addition, the Yo-Yo IR2 test performance of IN was lowered from 845+/-48 to 654+/-30 m. In HI the protein expression of the Na(+)/K(+) pump alpha2 isoform was 15% higher (P<0.05) after the intervention period, whereas no changes were observed in alpha1 and beta1 isoform expression. In IN, Na(+)/K(+) pump expression was not changed. In HI, the FXYD1ser68/FXYD1 ratio was 27% higher (P<0.01) after the intervention period, and in IN the AB_FXYD1ser68 signal was 18% lower (P<0.05) after inactivity. The change in FXYD1ser68/FXYD1 ratio was correlated (r(2)=0.35; P<0.05) with change in performance in RST. The present data suggest that short-term intensified training even for trained soccer players can increase muscle Na(+)/K(+) pump alpha2 isoform expression and that cessation of training for two weeks does not affect the expression of Na(+)/K(+) pump isoforms. Resting phosphorylation status of the Na(+)/K(+) pump is changed by training and inactivity and may play a role for performance during repeated intense exercise. Key words: Fatigue, Training cessation, Performance, MCT1.