INTRODUCTION: IntroductionPhysical exercise has been proven beneficial for health in all ages. In elderly, physical exercise, may contribute to prolonged life with maintained high quality and less costs associated with health care for society. In this study we explore if exercise out of doors may provide extra benefits as compared to exercise indoors and we measure power output, lactate levels and perceived effort in 49 senior citizens performing 20 minutes of moderate physical activity.METHODS:MethodThe HERO project is a randomized cross-over experiment with three different treatments <indoors, simulated="" outdoors="" and="">. The study sample consisted of 49 healthy senior citizens. They performed 20 minutes moderate intensity physical activity on an ergometer bike in all three conditions. Before immediately after and at minutes 10, 20, 30, 60 and 120 we sampled blood lactate. Power output was computed at 7 occasions during the 20 minutes of cycling, as was the rating of perceived exertion . Data was analyzed using mixed linear models. RESULTS:ResultsOur findings show that there were no differences between the two indoor treatments in any of the measured variables. In the outdoor treatment, however, there were significant effects on blood lactate levels and power output but not in perceived exertion, suggesting that exercise out of doors makes the test person more efficient although the experience of effort stay unaffected.CONCLUSION:Discussion/conclusionsOur results support the notion that the outdoors may provide extraordinary conditions for exercise, not only because it appear to results in more exercise/effort, the outdoors is also an “arena” with high availability and most often totally for free. Previous results from the HERO project also suggests that white blood cell counts are affected by the outdoor treatment, something that may be related to the observed increased power output herein