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Improved running economy following intensified training correlates with reduced ventilatory demands. Med. Sci. Sports Exerc., Vol. 30, No. 8, pp. 1250-1256, 1998.To compare the effects of three types of intensive run training on running economy (RE) during exhaustive running and to establish possible relationships with changes in ventilatory function and/or muscle fiber type distribution.Thirty-six male recreational runners were divided into three groups and assigned to either exhaustive distance training (DT), long-interval training (LIT), or short-interval training(SIT) three times 20-30 min·wk−1 for 6 wk. V˙O2max and RE were measured during treadmill running before and after training. Muscle fiber type distribution of the vastus lateralis muscle was established from biopsy material.V˙O2max(L·min−1) increased by 5.9% (P < 0.0001), 6.0%(P < 0.0001), and 3.6% (P < 0.01) in DT, LIT, and SIT, respectively, and running speed at V˙O2max by 9% (P < 0.0001), 10% (P < 0.0001), and 4% (P < 0.05), respectively. Time-to-exhaustion at 87% of pretraining V˙O2max (mean 3.83 m·s−1) increased by 94% in DT (P < 0.0001), 67% in LIT (P < 0.0001), and 65% in SIT (P < 0.001). Running economy improved by 3.1% in DT (P < 0.05), 3.0% in LIT (P < 0.01), and 0.9% in SIT (NS); pulmonary ventilation (V˙E) was on average 11 L·min−1 lower following training (P < 0.0001). The individual decrements in V˙E correlated with improvements in RE (r = 0.77; P < 0.0001) and may account for 25-70% of the decrease in aerobic demand. Muscle fiber composition, and respiratory exchange ratio, stride length, and stride frequency during running were unaltered with training.Recreational runners can improve RE and aerobic run performance by exchanging parts of their conventional aerobic distance training with intensive distance or long-interval running, whereas short-interval running is less efficient. The improvement in RE may relate to reduced ventilatory demands. Muscle fiber type distribution was unaltered with training and showed no association with RE.