Purpose: This study examined the effect of training status and relative exercise intensity on physiological responses to endurance exercise in humans. Methods: Seven endurance trained (TR: peak oxygen uptake [V(dot)O2peak] = 65.8 ± 2.4 mL·kg-1·min-1) and six untrained (UT: V(dot)O2peak = 46.2 ± 1.9 mL·kg-1·min-1) men cycled for 60 min, either at a work rate corresponding to ~ 70% V(dot)O2peak or ~ 95% lactate threshold (LT). Results: The work rate and relative exercise intensity (i.e., % V(dot)O2peak) for UT 95% LT were lower (P < 0.01) than for all of the other trials. Although the work rate for UT 70% V(dot)O2peak was lower (P < 0.001) than for TR 70% V(dot)O2peak and TR 95% LT, average heart rate (HR) for the trial was higher (P < 0.01) throughout exercise in UT 70% V(dot)O2peak compared with all of the other trials. Plasma lactate and ammonia concentrations were greater (P < 0.01) during exercise in UT 70% V(dot)O2peak 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% V(dot)O2peak 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% V(dot)O2peak trials. Conclusions: These data demonstrate that HR and plasma markers of metabolic stress were greater in UT compared with TR when exercise was performed at 70% V(dot)O2peak but were similar during exercise at 95% LT.
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