Physical activity is known to provide a wide range of health benefits that can protect individuals from diseases and enhance their mental and physical health [1
]. Regular physical activity can prevent and manage a range of chronic conditions, including cardiovascular disease, type 2 diabetes, and certain cancers, and improve musculoskeletal health, weight management, motor skill development in children, and mental health problems [1
]. However, global estimates show that around one-quarter of adults aged 18 years and over are insufficiently active worldwide [1
]. As a result, physical inactivity is one of the leading risk factors for global premature mortality, responsible for 9% of early deaths worldwide [3
]. Recommendations for health-enhancing physical activity targeted to different population sub-groups, often make explicit reference to the activity mode, duration, intensity, and frequency. In the past decades, however, the environment in which physical activity take places has emerged as an additional element that can determine the activity’s health benefit. In particular, it has been postulated that physical activity in the presence of nature, a practice also known as green exercise, can provide additional health benefits and, thus, have greater value for preventing disease and enhancing population health in the population [4
There are several ways in which the added health benefits of green exercise might arise. Simply having better access to natural environments, such as parks, playing fields, or woodlands, provides the space and facilities for physical activity, which may in turn foster a more active lifestyle. Though intuitive, evidence that having good access to natural environments (green and blue space) can promote physical activity is equivocal [5
]. A review of 50 epidemiological studies of objectively measured access to greenspace and physical activity found positive associations in 20 studies, whereas 28 studies offered little support and two reported negative associations [6
]. However, this field of research is dominated by studies with cross-sectional design that often prevent the identification of causal relationships between availability of natural environments and increased physical activity in the local population [8
]. Therefore, a question remains about the possibility of a ‘self-selection’ phenomenon: do natural environments elicit increased physical activity and well-being, or do physically active individuals choose to live in areas with more opportunities for physical activity?
Secondly, there is evidence that people tend to engage in physical activity when in green space and might be active for longer and or at higher intensities in natural environments [10
]. For example, the activities that might be well supported by outdoor environments, such as running, hiking, mountain biking, or horse riding, are those that might be undertaken for longer periods of time (compared with indoor activities) [11
]. Other studies in trained athletes have indicated that they might be able to exercise at higher intensities in natural environments as they are more distracted from internal signs of fatigue [12
] or have lower perceived effort [13
]. These two effects can interact, resulting in people being more active than they would be in other settings, thus gaining greater health benefits [15
Finally, being physically active in natural environments may confer additional health benefits, compared with those that would result from the equivalent activity in an urban/built or indoor environment [16
]. This is associated with the notion that exposure to scenes of nature can elicit positive psychological states such as increased positive affect and reduction of psychophysiological stress [5
]. The underlying mechanism linking nature exposure to such psychological outcomes is not yet clear. Possible explanations have included evolutionary perspectives [19
], elicited feelings of connectedness with nature [21
], and visual recognition of characteristic features such as the color green [23
], and geometrical fractals [24
]. Irrespective of the underlying processes, a 2010 review of 25 studies comparing responses to activities (mostly walking or running) in natural versus non-green outdoor built environments or indoor environments found that the former were associated with greater energy and reduced anxiety, anger, fatigue, and sadness [16
]. However, by conflating non-green outdoor built environments and indoor environments, as well as exercise and non-exercise conditions, this review might have not taken into account possible confounders such as factors eliciting negative emotional responses (e.g., street traffic) or the acute psychophysiological responses to physical exercise. Thompson Coon et al. [18
] reviewed the effects of physical activity in natural environments compared with physical activity indoors on mental and physical wellbeing, health-related quality of life, and long-term adherence to physical activity in 11 studies. The authors reported beneficial effects of natural environments for a range of psychological outcomes, such as revitalization, positive engagement, tension, confusion, anger, depression, and energy. There was also evidence of greater enjoyment and satisfaction with outdoor activity, with indications of greater intent to repeat the activity. However, the review was limited by the small number of papers included, poor methodological quality of the available evidence, and the heterogeneity of outcome measures employed. This made interpretation and extrapolation of the findings difficult.
Given the recent proliferation of work in this area, we have updated Thompson Coon et al.’s [18
] systematic review. In line with this previous review, and to explore the potential causal relationship between green exercise and different health outcomes, we restricted our search to studies with experimental or quasi-experimental designs. Specifically, we sought to address the following research questions: (1) Are the longitudinal effects of exercising in an outdoor natural environment (green exercise) different to exercising indoors without exposure to nature (indoor exercise)? (2) Are the acute effects of outdoor green exercise different to exercising indoor exercise? (3) Are the acute effects of exercising indoors with virtual exposure to nature (‘virtual green’ exercise) different to indoor exercise? (4) Are the acute effects of outdoor green exercise different to virtual green exercise?
4.1. Summary of Findings
This current review updates and expands upon Thompson Coon et al.’s [18
] systematic review that investigated the potential added health benefits of green exercise compared with indoor exercise. We identified 28 eligible trials—18 trials were found from the updated search and 10 were retained from Thompson Coon et al. [18
]. Across the 28 eligible studies, we found largely inconclusive evidence for the benefits of outdoor and virtual green exercise over indoor non-green exercise. In our meta-analysis of three RCTs investigating the longitudinal effects of green versus indoor exercise, the only statistical effect was slightly lower post-intervention perceived exertion scores with green exercise. It was difficult to interpret the outcomes assessed in the longitudinal trials because most outcomes were assessed by single studies.
Of the affect and emotion outcomes assessed across 17 acute trials that compared outdoor green exercise with indoor exercise (i.e., without exposure to nature), only affective valence appeared to be more favorably affected by green exercise—although the number of trials with this outcome was small. More studies (3 out of 4) reported greater enjoyment or satisfaction after green versus indoor exercise. There were, however, consistent null findings (6 out of 7 trials) for the effect of green versus indoor exercise on perceived exertion, and equivocal findings for the effects on energy, calmness, tension, anger, depressed mood, fatigue, attention and memory, intention for future exercise behavior, biological markers, and exercise intensity and performance (walking/running speed, and heart rate). Compared with indoor exercise without exposure to nature, we found no consistent statistical effects on general affect, energy, tension, fatigue, perceived exertion, heart rate, or blood pressure with virtual green exercise. Across the five studies that compared the acute effects of outdoor green exercise with indoor virtual green exercise, no consistent differences were found between conditions for energy, calmness, tension, fatigue, attention, and heart rate response, however the two studies that included enjoyment as a measure reported statistically higher enjoyment scores with outdoor versus indoor green exercise.
4.2. Overall Completeness and Applicability of Evidence
There is a dearth of well-designed studies investigating the long-term effects of exercising with exposure to nature—we found only three that met our eligibility criteria and one of these was just two weeks duration. Whereas the majority of studies were conducted in North America and the UK, there was also representation from Scandinavia, Japan, Iran, Australia, and Central and Eastern Europe. There was an overrepresentation of young University-aged participants, but a fairly even gender split, in the acute studies reviewed. Most of the studies consisted of small sample sizes, with a median size of 33 (minimum-maximum = 8–181).
All acute studies used only one single episode of exercise per condition. Therefore, repeated bout effects are unknown. Walking and running were the most common exercise type, and woodland trails and footpaths through gardens and parks were the most common green exercise settings. However, the green exercise setting was not always well described. Similarly, authors did not report the environmental conditions consistently across the studies, which limits the generalizability of findings to different climates. Almost all studies consisted of moderate-intensity or “comfortable” self-paced exercise performed for short durations (most were ≤ 20 min). Most of the indoor comparison groups were performed on a treadmill, with only five studies including an indoor walking condition (i.e., walking through indoor hallways/tunnels/shopping center).
4.3. Quality of the Evidence
The current review includes evidence from 28 trials consisting of 1344 participants. However, we identified a number of methodological issues that resulted in an overall low quality of evidence. Imprecision resulted from small sample sizes and too few trials measuring the same outcomes with the same measurement tools. There was a high risk of bias across studies through: unclear randomization procedures; lack of allocation concealment; non-blinding of outcome assessors; scarcity of preregistration to rule out selective reporting; insufficient, unclear, or different washout periods in trials with crossover design (the effects of one environmental exposure might have influenced responses to the subsequent environment); and potential contamination in control conditions (e.g., possible exposure to green environments on way to indoor facilities). Most trials had insufficient statistical power (only 4 trials performed a priori power calculation), and a combination of low power, small samples, and lack of controlling for multiple comparisons (only three trials corrected for this) means the prevalence of false positives and false negatives is likely to be high. Inconsistency was evident in the diversity of ‘green’ setting, environmental conditions, exercise dose, control (or lack of) of exercise intensity, and outcome measures used. Poor reporting hampered both the extraction of methods information, outcome data, and risk of bias assessments.
4.4. Recommendations for Future Research
The fact that this review found largely inconclusive evidence for the benefits of outdoor and virtual green exercise over indoor exercise should not be interpreted as an appeal to researchers to stop conducting investigations in this area. We rather intend to set a challenge to researchers in this area to conduct more robust and rigorously designed trials to evaluate the effects of exposure to nature during exercise compared with exercise indoors. In particular, there is a clear need for trials with large enough samples to achieve sufficient statistical power, adequately report trials through the use of reporting guidelines (e.g., CONSORT), and to improve methodological transparency and rigor via preregistration of study designs and statistical plans, providing open data, appropriate randomization, allocation concealment, blinding procedures, and sufficient wash-out periods. Moreover, future robustly designed studies are needed to evaluate the effects of long-term exposure to nature during exercise and multiple bouts of longer duration exercise in nature in samples that are representative of the general population or in specific groups (e.g., people with depression or anxiety).
4.5. Biases in Review Process
We attempted to avoid bias by identifying all relevant studies through a comprehensive systematic search of seven major databases, screening all eligible studies for potential papers, and not excluding based on language. However, given the preponderance of small sample studies in this area there is a risk of publication bias—there are potentially “file drawers” with more null-findings from green exercise studies. We did not include or search the grey literature for unpublished studies or studies only published in an abstract form as these tend be of poor methodological quality, would not have been peer reviewed, and are often not reported well enough to extract sufficient information from [66
Due to the small number of trials reporting on the same outcome, meta-analysis was possible for few outcomes. Instead for most outcomes, we used harvest plots to summarize and present data. Harvest plots are an extension of vote counting, whereby positive and null findings for each outcome using a predefined p-value threshold are counted [68
]. Vote counting methods have been criticized because they do not provide measures of effect magnitude or take into account the size of the studies—a study with low power is less likely to find a statistically “significant” result even if there is a true effect. Therefore, by counting statistical and null-findings, we are potentially counting both false positives and negatives [69
]. However, in the absence of a suitable alternative, the harvest plots allowed us to graphically display complex and diverse data (i.e., trial design, exercise bout duration, sample size, and p-values and effects sizes) within an easily interpreted format.
4.6. Agreements and Disagreements with Other Studies or Reviews
The equivocal findings in the current review is in slight contrast with a previous meta-analysis [16
], which found more statistically favorable feelings of energy, anxiety, anger, fatigue, and sadness after direct exposure to a natural versus synthetic environment. In addition to the inclusion of a greater number of more recent trials, there are three important differences between the current review and Bowler and colleagues’ review [16
], which makes comparisons between the two reviews difficult. Firstly, in addition to green exercise conditions, Bowler et al. [16
] included non-exercise conditions that involved exposure to a natural environment while remaining passive or sedentary. Secondly, Bowler and colleagues’ [16
] ‘synthetic environment’ conditions included built outdoor environments as well as indoor environments. Finally, Bowler et al. [16
] performed a meta-analysis of outcomes, whereas, in the current review, in agreement with Thompson Coon et al. [18
], this approach was deemed inappropriate due to a paucity of reported paired analysis. In their meta-analysis, Bowler et al. [16
] did not take trial design into consideration, treating crossover trials as comparative trials (i.e., assuming treatment arms were independent), and combined both trial designs in their meta-analysis. This is the least desirable approach to combining crossover trials, and can overestimate the variability of the within-study treatment effect [70
In their 2011 systematic review, Thompson Coon et al. [18
] found favorable effects on feelings of anger, confusion, depression, energy, enjoyment and satisfaction, positive engagement, revitalization, and tension, but negative effects on feelings of calmness, with green exercise compared with exercising indoors. Similar to the current review, Thompson Coon and colleagues [18
] also identified a number of issues with the available evidence, and concluded that the interpretation and generalization of findings was impeded by poor methodological quality and the diversity of outcome measures assessed in the eligible studies.