The present study examined muscle adaptations and alterations in work capacity in endurance-trained runners after a change from endurance to sprint training. Fifteen runners were assigned to either a sprint training (ST, n=8) or a control (CON, n=7) group. ST replaced their normal training by 30-s sprint runs 3-4 times a week, whereas CON continued the endurance training (about 45 km/week). After the 4-wk sprint period the expression of the muscle Na⁺,K⁺ pump 1 subunit and Na⁺/H⁺-exchanger isoform 1 was 29 and 30% higher (P<0.05), respectively. Furthermore, plasma [K⁺] was reduced (P<0.05) during repeated intense running. In ST, performance in a 30-s sprint test (S), Yo-Yo intermittent recovery (Yo-Yo IR) test and two supra-maximal exhaustive runs was improved (P<0.05) by 7%, 19%, 27% and 19%, respectively, after the sprint training period, whereas VO₂-max and 10-K time were unchanged. No changes in CON were observed. The present data suggest a role of the Na⁺,K⁺ pump in the control of K⁺ homeostasis and in the development of fatigue during repeated high-intensity exercise. Furthermore, performance during intense exercise can be improved and endurance performance maintained even with a reduction in training volume if the intensity of training is very high.