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BALDWIN, J., R. J. SNOW, and M. A. FEBBRAIO. Effect of training status and relative exercise intensity on physiological responses in men. Med. Sci. Sports Exerc., Vol. 32, No. 9, pp. 1648–1654, 2000.This study examined the effect of training status and relative exercise intensity on physiological responses to endurance exercise in humans.Seven endurance trained (TR: peak oxygen uptake [V̇O2peak] = 65.8 ± 2.4 mL·kg−1·min−1) and six untrained (UT: V̇O2peak = 46.2 ± 1.9 mL·kg−1·min−1) men cycled for 60 min, either at a work rate corresponding to ∼ 70% V̇O2peak or ∼ 95% lactate threshold (LT).The work rate and relative exercise intensity (i.e., % V̇O2peak) for UT 95% LT were lower (P < 0.01) than for all of the other trials. Although the work rate for UT 70% V̇O2peak was lower (P < 0.001) than for TR 70% V̇O2peak and TR 95% LT, average heart rate (HR) for the trial was higher (P < 0.01) throughout exercise in UT 70% V̇O2peak compared with all of the other trials. Plasma lactate and ammonia concentrations were greater (P < 0.01) during exercise in UT 70% V̇O2peak compared with all of the other trials. There was a tendency (P = 0.077) for plasma hypoxanthine to be greater at 60 min in UT 70% V̇O2peak compared with the other trials. At no time were any of the plasma metabolite concentrations different between the UT 95% LT, TR 95% LT and TR 70% V̇O2peak trials.These data demonstrate that HR and plasma markers of metabolic stress were greater in UT compared with TR when exercise was performed at 70% V̇O2peak but were similar during exercise at 95% LT.