The effects of outdoor versus indoor exercise on psychological health, physical health, and exercise behaviour: a systematic review of longitudinal trials

Background A growing body of research is exploring the potential added health benefits of exercise when it is performed outdoors in nature, as compared with indoors. We systematically reviewed longitudinal trials comparing the effects of exercise in outdoor versus indoor environments on psychological and physical health, and exercise behaviour. Methods We searched the following nine databases from inception to March 2021 for English‐language, peer‐reviewed articles: MEDLINE, Embase, PubMed, Scopus, Web of Science, CINAHL, SportsDiscus, GreenFile, CENTRAL. We included randomized and non‐randomized trials that compared multiple bouts of exercise in outdoor versus indoor environments, and that assessed at least one of the following main outcomes of interest: physical health, psychological health, or exercise behaviour. We assessed risk of bias using the Revised Cochrane risk‐of‐bias tool for randomized trials. At least two authors screened articles for eligibility, extracted data, and evaluated risk of bias, with discrepancies resolved by discussion with at least one other author. Due to outcome heterogeneity and a paucity of studies, we performed a narrative synthesis. Results We identified 10 eligible trials from 12 papers, including seven randomized controlled trials. Participants’ (n = 496 total) demographics varied widely (mean age ranging from 11‐80 years). Most exercise protocols involved 30‐60 minutes of aerobic training (walking or running) at moderate to high intensity. Other forms of training included mixed aerobic and resistance training, agility, balance, and strength training, and dual‐task training. Exercise sessions took place one to 5 times per week for one to 18 weeks. Outcomes overlapped minimally between trials, with psychological outcomes assessed in seven trials, physical outcomes assessed in eight trials, and behavioural outcomes assessed in three trials. In total, 123 comparisons were made between outdoor and indoor exercise, including 33 significant comparisons, all of which favoured outdoor exercise. Interpretation of findings was hindered by an overall high risk of bias, unclear reporting, and high heterogeneity in reported outcomes. Conclusion There is limited evidence for added health or behaviour benefits of chronic exercise in outdoor environments, as compared with indoors, and a need for future, high‐quality longitudinal trials with adequate power and clear reporting.