Purpose: The present study examined the effect of additional speed endurance training (SET) during the season on muscle adaptations and performance of trained soccer players. Methods: Eighteen subelite soccer players performed one session with six to nine 30-s intervals at an intensity of 90%-95% of maximal intensity (SET) a week for 5 wk (SET intervention). Before and after the SET intervention, the players carried out the Yo-Yo intermittent recovery level 2 (Yo-Yo IR2) test, a sprint test (10 and 30 m), and an agility test. In addition, seven of the players had a resting muscle biopsy specimen taken and they carried out a running protocol on a motorized treadmill before and after the SET intervention. Results: After the SET intervention, the Yo-Yo IR2 test (n = 13) performance was 11% better (P < 0.05), whereas sprint (n = 15) and agility (n = 13) performances were unchanged. The expression of the monocarboxylate transporter 1 (n = 6) was 9% higher (P < 0.05). and the expression of the Na+/K+ pump subunit ß1 (n = 6) was 13% lower (P < 0.05) after the SET intervention. The Na+/K+ pump subunits a1, a2, as well as the monocarboxylate transporter 4 and the Na+/H+ exchanger 1 (n = 6) were unchanged. After the SET intervention, the relative number of Type IIx fibers and oxygen consumption at 10 km·h-1 were lower (P < 0.05), whereas VO2max was unchanged. Conclusions: In conclusion, adding ~30 min of SET once a week during the season for trained soccer players did lead to an improved ability to perform repeated high-intensity exercise, with a concomitant increase in the expression of monocarboxylate transporter 1 and an improved running economy.
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