The effect of tapering following a period of high-volume sprint interval training (SIT) and a basic volume of aerobic training on performance and muscle adaptations in moderately trained runners was examined. Eleven (8 males, 3 females) runners (maximum oxygen uptake (VO2-max): 56.8±2.9 mL·min(-1)·kg(-1); mean±SD) conducted high-volume SIT (HV; 20 SIT sessions; 8-12 x 30-s all-out) for 40 days followed by 18 days of tapering (TAP; 4 SIT sessions; 4 x 30-s all-out). Before and after HV as well as midway through and at the end of TAP, the subjects completed a 10-km running test and a repeated running test at 90% of vVO2-max to exhaustion (RRT). In addition, a biopsy from m. vastus lateralis was obtained at rest. Performance during RRT was better (P<0.01) at the end of TAP than before HV (6.8±0.5 vs 5.6±0.5 min; means±SE), and 10-km performance was 2.7% better (P<0.05) midway through (40.7±0.7 min) and at the end of TAP (40.7±0.6 min) than after HV (41.8±0.9 min). The expression of muscle Na(+),K(+)-ATPase (NKA)α1, NKAβ1, phospholemman (FXYD1) and sarcoplasmic reticulum calcium transport ATPase (SERCA1) increased (P<0.05) during HV and remained higher during TAP. In addition, oxygen uptake at 60% of vVO2-max was lower (P<0.05) at the end of TAP than before and after HV. Thus, tapering after a period of high-volume SIT improved 10-km performance and running economy, and short-duration exercise capacity was better than before the high-volume SIT period together with higher expression of muscle proteins related to Na(+)/K(+) transport and Ca(2+) re-uptake.