The present study examined muscle adaptations and alterations in work capacity in endurance-trained runners as a result of a reduced amount of training combined with speed endurance training. For a 6- to 9-wk period, 17 runners were assigned to either a speed endurance group with a 25% reduction in the amount of training but including speed endurance training consisting of six to twelve 30-s sprint runs 3–4 times/wk (SET group n = 12) or a control group (n = 5), which continued the endurance training (∼55 km/wk). For the SET group, the expression of the muscle Na+-K+ pump α2-subunit was 68% higher (P < 0.05) and the plasma K+ level was reduced (P < 0.05) during repeated intense running after 9 wk. Performance in a 30-s sprint test and the first of the supramaximal exhaustive runs was improved (P < 0.05) by 7% and 36%, respectively, after the speed endurance training period. In the SET group, maximal O2 uptake was unaltered, but the 3-km (3,000-m) time was reduced (P < 0.05) from 10.4 ± 0.1 to 10.1 ± 0.1 min and the 10-km (10,000-m) time was improved from 37.3 ± 0.4 to 36.3 ± 0.4 min (means ± SE). Muscle protein expression and performance remained unaltered in the control group. The present data suggest that both short- and long-term exercise performances can be improved with a reduction in training volume if speed endurance training is performed and that the Na+-K+ pump plays a role in the control of K+ homeostasis and in the development of fatigue during repeated high-intensity exercise.
- running economy
- Na+-K+-Cl− cotransporter isoform 1
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