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Interval training at V˙O2max: effects on aerobic performance and overtraining markers. Med. Sci. Sports Exerc., Vol. 31, No. 1, pp. 156-163, 1999.Between inefficient training and overtraining, an appropriate training stimulus (in terms of intensity and duration) has to be determined in accordance with individual capacities. Interval training at the minimal velocity associated with V˙O2max (vV˙O2max) allows an athlete to run for as long as possible at V˙O2max. Nevertheless, we don't know the influence of a defined increase in training volume at vV˙O2max on aerobic performance, noradrenaline, and heart rate.Eight subjects performed 4 wk of normal training (NT) with one session per week at vV˙O2max, i.e., five repetitions run at 50% of the time limit at vV˙O2max, with recovery of the same duration at 60% vV˙O2max. They then performed 4 wk of overload training (OT) with three interval training sessions at vV˙O2max.Normal training significantly improved their velocity associated with V˙O2max (20.5 ± 0.7 vs 21.1 ± 0.8 km·h−1, P = 0.02). As a result of improved running economy (50.6 ± 3.5 vs 47.5 ± 2.4 mL·min−1·kg−1, P = 0.02), V˙O2max was not significantly different (71.6 ± 4.8 vs 72.7 ± 4.8 mL·min−1·kg−1). Time to exhaustion at vV˙O2max was not significantly different (301 ± 56 vs 283 ± 41 s) as was performance (i.e., distance limit run at vV˙O2max: 2052.2 ± 331 vs 1986.2 ± 252.9 m). Heart rate at 14 km·h−1 decreased significantly after NT (162 ± 16 vs 155 ± 18 bpm, P < 0.01). Lactate threshold remained the same after normal training (84.1 ± 4.8% vV˙O2max). Overload training changed neither the performance nor the factors concerning performance. However, the submaximal heart rate measured at 14 km·h−1 decreased after overload training (155 ± 18 vs 150 ± 15 bpm). The maximal heart rate was not significantly different after NT and OT (199 ± 9.5, 198 ± 11, 194 ± 10.4, P = 0.1). Resting plasma norepinephrine (veinous blood sample measured by high pressure liquid chromatography), was unchanged (2.6 vs 2.4 nm·L−1, P = 0.8). However, plasma norepinephrine measured at the end of the vV˙O2max test increased significantly (11.1 vs 26.0 nm·L−1, P = 0.002).Performance and aerobic factors associated with the performance were not altered by the 4 wk of intensive training at vV˙O2max despite the increase of plasma noradrenaline.