A systematic review of the literature was conducted to determine if physical activity in outdoor natural environments ("green exercise") has greater physical and mental health benefits than physical activity indoors. For inclusion in this review, studies had to compare the effects of outdoor exercise initiatives with those conducted indoors and report on at least one physical or mental well-being outcome in adults or children. A total of eleven papers met this criteria and were included in the review. All the studies involved adults, totaling 833 in all. None of the studies involved children, as no such studies were identified. A wide range of data sources (including electronic databases, relevant Web sites, and hand searching of appropriate journals) was used to identify related research. The identified studies were published in a variety of publications.
All studies reported on walking or running indoors with the same activity conducted outdoors on a separate occasion. Thirteen different outcome measures were used to evaluate the effects of exercise on mental well-being, and four to assess attitude to exercise. The measures of mental well-being were administered soon after participating in the activity.
Compared with exercising indoors, exercising in natural environments was associated with greater feelings of revitalization and positive engagement, decreases in tension, confusion, anger, and depression, and increased energy. Additionally, participants reported greater enjoyment and satisfaction with outdoor activity and declared a greater intent to repeat the activity at a later date. While there were trends toward reduced feelings of calmness and tranquility associated with the outdoor walk compared with the indoor walk, this was only shown to be statistically significant in the male participants of one study.
The authors noted a paucity of high quality studies on which to base recommendations and called for further, more rigorous research in this area. Specifically, larger, better controlled, studies in which the effects of long-term physical activity conducted indoors and in nature on mental and physical wellbeing, and which compare different groups of people, are necessary.