The Effects of Green Exercise on Physical and Mental Wellbeing: A Systematic Review

We aimed to examine the evidence for the proposed additive effect of exercise in the presence of nature (green exercise) by systematically reviewing studies that investigated the effects of outdoor or virtual green exercise compared with indoor exercise. Our review updates an earlier review, whose searches were conducted in April 2010. Trials were eligible if: (a) participants in an outdoor or virtual exercise condition were exposed to views of nature (green exercise); (b) green exercise was compared with indoor exercise with no exposure to nature; (c) included an outcome related to physical or mental health; (d) used comparative or crossover trial design. We searched the following databases from 1st January 2010 to 28th June 2018: PubMed, CENTRAL, EMBASE, PsycINFO, GreenFile, and Sports DISCUS. We assessed risk of bias using the Cochrane “risk of bias” tool. Where possible we conducted a meta-analysis using the inverse variance random-effects method, and where this approach was not possible we presented the results qualitatively and in harvest plots. We identified 28 eligible trials. In a meta-analysis of just three longitudinal trials, the only statistical finding was slightly lower post-intervention perceived exertion with green versus indoor exercise (mean difference: −1.02; 95% confidence intervals: −1.88, −0.16). Compared with indoor exercise, acute bouts of outdoor green exercise may favorably influence affective valence and enjoyment, but not emotion, perceived exertion, exercise intensity, and biological markers. No other consistent statistical differences were observed, apart from a higher enjoyment of outdoor green versus virtual green exercise. We found a high risk of bias across trials and an overall low quality of evidence. In conclusion, there was limited evidence to support the view that green exercise offers superior benefits to exercise without exposure to nature. The low quality of evidence prohibits clear interpretation of trial findings. Future robust and rigorously designed trials are needed to evaluate the effects of long-term and multiple-bout exposure to nature during exercise compared with exercise indoors.

1) 10-min self-paced walk outdoors in a natural environment. Description: "a paved trail by a river, with view of grassland and some trees" Month: May Temperature: 7-17°C 2) 10-min self-paced walk on a manually driven treadmill, with VR goggles and headphones reproducing a 360-video and sound of the same walk as above (1). Temperature: 21°C.
Outdoor 6MWT at self-selected on a 30 m course and 30 m walk test at max speeds. Description: "an outdoor walkway in a calm garden and quiet neighborhood" Temperature: NR ("the walkway was free of snow and ice and was treated with sand") Indoor 6MWT at self-selected and max speeds on a 30-m course in basement or clinical setting Description: A) basement: "empty corridor" B) clinical: "corridor in a clinic rehabilitation unit". "The two indoor corridors had the same regular wall-to-wall plastic carpet with an even surface, well kept, and well lit." Temperature: NR CRF (6MWT distance); Walking performance (selfselected and max speed over 30 m).

Kerr 2006 (Japan)
Design: Non-RXT, counterbalanced Acute, 2 single visits 1 week apart Sample: 44 male students, recreational (50%) and competitive (50%) runners Age (mean ± SD): recreational runners, 5 km run at 60% HRR in natural environment Description: "participants ran 5 km on a tree-lined footpath… outdoor running pathway ran alongside two 5 km indoor run at 60% HRR on a treadmill Description: "treadmill located in the university sports medicine laboratory" Emotional response (TESI).

Nisbet 2011 (Canada)
Design: RCT x 2 Acute, single session in both study 1 and 2. Sample: Study 1) 150 University students (57% female, 37% male, and 6% unspecified) Age (mean ± SD): 20.8 ± 5.0 y Study 2) 80 participants (no participant characteristics were reported) 1) 17-min walk in nature in groups of 1-11 students. Description: "walking and biking path along the Rideau Canal… a green corridor… 8 km through the heart of Ottawa... relatively picturesque… urban nature." 2) Participants received description of walk and rated their anticipated effect before walking in a different outdoor environment to above Description: "on-campus… walking path between a road and a river that borders the campus." Temperature: 2.5-14.6°C ("Walks conducted in Fall on days with no rain.") 1) 17-min walk in indoor environment in groups of 1-11 students Description: "proceeded to their destination, the athletics building, via tunnels" 2) Participants received description of walk & rated their anticipated effect before they walked in an indoor environment Description: "different building than the indoor route in Study 1, but also used parts of the university tunnel

Peacock 2007 (UK)
Design: Non-RXT Acute 2 single visits 1 week apart Sample: 20 participants (65% female and 35% male) Age range: 31-70 y (aged 31-50 y: 47%; aged 51-70 y: 53%) 30-min outdoor walk. No intensity provided but "continuous walking was preferred... participants were allowed to stop briefly to admire the scenery… social interaction was also encouraged" Description: "Belhus Woods Country Park, which has a diverse landscape of woodlands, grasslands and lakes." Temperature: NR 30-min indoor walk. No intensity provided but "continuous walking was preferred... participants were allowed to stop briefly to admire… shop windows, and a certain level of social interaction was also encouraged" Description: "walking around Lakeside shopping centre." Temperature: NR Enjoyment (via 1-5 Likert scale); Mood (POMS); Selfesteem (RSE scale).
20-min of walking on a treadmill (speed: 2.7-3.5 mph) without any VR technology Description: "No visual stimulus was presented before the walk to the participants" Temperature: NR Mood (AD-ACL); Social Desirability or defensiveness (MC-SDS) laboratory, women 23.5 ± 3.3 kg/m 2 and men 24.1 ± 2.3 kg/m 2 Description: "same route around campus that walkers used in condition 1 (see above)" (sunny conditions) Temperature: NR ("no participant was made to walk in rainy weather or at night.")

Plante 2006 (USA)
Design: RCT, parallel groups Acute, 2 single visits on separate days Sample: 112 undergraduate psychology students (58% female and 42% male) (no participant characteristics were reported) 1) 20-min brisk outdoor walk (speed ~4.8 km/h) Description: "Garden at the campus of Santa Clara University"* 2) 20-min walking (speed: 4.3-5.6 km/h) on a laboratory treadmill while watching a video projected on a screen of the same route around campus that walkers in condition 1 (see above) completed (sunny day) Temperature: NR ("northern California during the fall… no participant was made to walk in rainy weather or at night.") No non-green exercise intervention included.

Plante 2007 (USA)
Design: RCT, parallel groups Acute, single visit Sample: 88 female undergraduate students Age (mean ± SD): 19 ± 1 y 1) 20-min moderate intensity walk (60-70% HRmax) along a prescribed route on the university campus. 2) Same walk as above but accompanied by a friend All conditions performed "same exercise task in terms of type and intensity of exercise." Description: "Garden at the campus of Santa Clara University"* Temperature: NR 1) 20-min moderate intensity walk (60-70% HRmax) alone on a treadmill. 2) Completed the same treadmill walk as above but did so alongside a friend who walked on an adjacent treadmill. Memory (Free recall test, Forced-choice recognition Study 1) participants studied a word list, then walked; Study 2) participants walked, then studied a word list. Sample: Study 1) 24 undergraduate students (83% female and 17% male) Mean age: 22 y Study 2) 24 undergraduate students (79% female and 21% male) Mean age: 22 y Description: "walk through a number of stands of trees, bushes, and grassy areas along a relatively quiet asphalt path on University campus." (image provided) Month: Sept-Oct 2014 Temperature: -5°C to 21°C (weekday mornings or afternoons) "hallways on three floors of the University building and did not provide much exposure to natural or urban outdoor elements" Temperature: -5°C to 21°C test); Mood (11-point scale, -5 = very negative, 0 = neutral, +5 = very positive); Rating of experience (asked to indicate which walk was most enjoyable, most beautiful, least distracting).

Rogerson 2016 (UK)
Design: RXT, counterbalanced Acute, 3 visits -baseline, indoors, outdoors (baseline to first condition separated by 6 ± 3 d; condition 1 and 2 separated by 9 ± 8 d). Participants took part in pairs (who already knew each other) Sample: 24 participants (79% female and 21% male) (1 staff member, 10 students, 13 public) Age (mean ± SD): 35.1 ± 20.1 y Weight (mean ± SD): 70 ± 15 kg 15-min on cycle ergometer at 50% HRR. Participants advised that they were free to talk as much or as little as they liked. Description: "Exercise was performed outside, on the University sports fields, a large area of largely level gradient, maintained grassland, lined and partly interspersed with trees" (Image provided) Temperature: NR 15-min on cycle ergometer at 50% HRR. Participants advised that they were free to talk as much or as little as they liked. Description: Exercise was performed in a laboratory (8.3 x 4.9 m). with a view of a white painted brick wall (Image provided) Temperature: NR Directed attention (backward digit span task); Effort (Borg RPE); Mood (30-item POMS); Intention for future (VAS [0-100] to rate enjoyment of session, intention to attend a session in the future-if offered free); Social interaction time.

Rogerson 2015 (UK)
Design: RXT, counterbalanced Acute, 4 visits separated by 7-25 days (average: 13 days) Sample: 12 healthy adult participants (50% female and 50% male) Age (mean ± SD): 27.8 ± 5.5 y Weight (mean ± SD): 65.4 ± 10.5 kg 1) 15-min bout of exercise on the treadmill, at 60% VO2 peak (via gas analysis) while watching nature video 2) run at 85% VO2 peak to voluntary exhaustion while watching nature video Description: video consisted of scenes extracted from "Evening Run through Endless Forest". (all videos played at the same speed without auditory sound) Temperature: NR (time of day was consistent between occasions-within 2 hours) 1) 15-min bout of exercise on the treadmill, at 60% VO2 peak (via gas analysis) while watching 1) blank screen and 2) watching built environment video 2) run at 85% VO2 peak to voluntary exhaustion while watching 1) "blank white screen" and 2) watching built environment video Temperature: NR Directed Attention (subtraction of serial sevens test, spelling words backwards, and backwards digit span test); Effort (Borg 6-20 RPE scale); Energy expenditure ((gas analysis method not reported)); HR average; Respiratory exchange ratio (gas analysis method not reported); TTE at 85% VO2 peak.

Ryan 2010 (USA)
Design: RCT, parallel groups (study 2) Acute, single visit Sample: 80 undergraduate university students (82% female and 18% male) Mean (range) age: 20 (18-22) y 15-min walk in outdoor setting (no intensity reported) Description: "Participants in the outdoor condition walked on a largely tree-lined footpath along a river that runs parallel to the university campus." Month: Sept-Oct Temperature: NR (walks completed 11am-4pm) 15-min walk in indoor setting (no intensity reported) Description: "series of underground hallways and tunnels that were devoid of living things" Temperature: NR (walks completed 11am-4pm)

Teas 2007 (USA)
Design: Non-RXT Acute 2 single visits 1 week apart Sample: 19 healthy, non-smoking, postmenopausal women Ethnicity (n): European American, 13; African American, 6 Age (mean ± SD): 58 ± 4 y BMI (mean ± SD): 27 ± 6 kg/m 2 1-hour walk at a "comfortable" selfchosen speed (mean: 5.1 km/h) outdoors Description: "campus horseshoe (grassy area lined with brick paths, old trees, and flowerbeds)." Month: May Temperature: 22°C (women walked together on same day at 6:20-7:20 pm) Humidity: 45% (Light: 220 lux; Noise: 65 decibels) 1-hour walk at a "comfortable" selfchosen speed (mean: 4.3 km/h) on a treadmill Description: "gym lab was located in the university gym, and provided an environment similar to that found in a commercial gym" Temperature: 23°C (walk completed 5:00-6:45pm) Humidity: 46% Relatedness to nature (n): high, 19; low: 3 6 km run -first 3 km at steady-state pace, second 3 km at maximum intensity (fast as possible) with the second half completed at maximum effort, followed by 10-min recovery Description: "Large woodland area, with walking/running trails lined with trees and bushes... Dog walkers and other runners were present." Temperature: 17.5°C (typically cloudy with sunny intervals) 6 km run -first 3 km at steady-state pace, second 3 km at maximum intensity (fast as possible) with the second half completed at maximum effort, followed by 10-min recovery Description: Treadmill in a large fitness suite. "Digital screens on the treadmill displaying feedback data remained visible, to allow for selfpacing. Other users of the fitness suite were present". Temperature: 19°C Affect (FS, FAS); Effort (Borg 6-20 RPE scale); State vitality (7-item SVS).
Cycle ergometer for 15-min while watching a 1) green video and 2) blue video. Description: Green video: 3 x 5-min scenes of fields with sheep, hedgerows and a small wood. Blue video: 3 x 5min clips from a headland overlooking a beach and of views from beach height across rocks and the sea Temperature: NR Cycle ergometer for 15-min while facing 1) a blank wall (control) or 2) watching an 'urban video'. Temperature: NR Affect (FS, FAS); BP; Effort (Borg RPE scale); Experience (Enjoyment of session); HR; Time perception (asked participants for how long they felt they had been cycling).

Yeh 2017 (UK)
Design: Non-RXT, counterbalanced Acute, 3 20-min treadmill runs with minimum 7 day gap between conditions Sample: 30 adults (40% female and 60% male) Age (mean ± SD): 28 ± 9 y BMI (mean ± SD): 22.2 ± 2.1 kg/m 2 1) 20 minute treadmill run at selfselected pace looking at static image of nature (visual only) 2) 20 minute treadmill run at selfselected pace looking at dynamic image of nature (visual only) Description: Dynamic image condition: video of the "Sheffield Botanical Gardens… series of paths within the gardens, capturing the trail through lawns, trees and flower beds… sunny spring afternoon" Temperature: NR 20 min treadmill at self-selected pace focusing on self-selected entertainment (music = 23; movies with sound = 6; viewing a picture = 1) Description: "self-selected, preferred entertainment where participants were able to choose preferences that included visual and/or auditory information" Temperature: NR