1. Introduction
Green exercise is defined as engaging in ‘physical activities whilst being directly exposed to nature’ [
1]. Green exercise research has adopted three different approaches for comparing both physiological and psychological exercise outcomes. Some research compares outcomes of built-
vs. nature-based outdoor exercise [
2,
3,
4,
5,
6]. Other research compares outcomes of indoor exercise to those of outdoor exercise [
7,
8,
9]. A third approach uses ergometers within laboratory settings to control the exercise performed and examine the importance of the visual exercise-environment [
1,
10,
11]. Collectively, compared to non-green exercise, green exercise improves psychological measures such as mood, self-esteem and vitality [
7,
8,
9,
10,
11,
12], and physiological markers such as blood pressure and heart rate variability (HRV) [
1,
2,
3,
4]. However, some conflicting findings suggest that influences of exercise-environments on psychological outcomes may be overstated [
13], and findings in adult samples have not been mirrored in adolescent samples [
11]. Additionally, often convenience methodologies used do not decipher relative contributions of the respective exercise and environment components [
14,
15].
“Directed attention” is another psychological outcome that can be influenced by exercise-environment selection [
5,
6]. Directed attention is the effortful cognitive ability to avoid being distracted by competing stimuli [
16,
17]. Brain regions associated with processes of mental effort, attention and mediation of cognitive control can fatigue over time [
16]. This depletion of directed attention is termed directed attention fatigue. Using involuntary attention—that which does not involve mental effort [
5,
18], decreases use of directed attention. This provides opportunity for restoration of depleted attentional resources [
16].
Transient hypofrontality [
19,
20,
21] posits a pathway linking exercise with attention restoration. Prefrontal cortex activation is associated with processes of directed attention [
22,
23,
24,
25]. Exercise facilitates prefrontal cortex restoration via transient hypofrontality [
19,
20,
21], that is, decreases in prefrontal cortex activity that occur in conjunction with increased motor cortex activity.
During exercise, this decreased prefrontal cortex activity may be detrimental to cognitive performance [
20,
26]. However,
following prolonged opportunity for prefrontal cortex restoration, transient hypofrontality improves post-exercise executive functioning and cognitive performance [
27,
28].
Disparate to exercise-related mechanisms, attention restoration theory proposes that visual characteristics of natural environments promote involuntary attention and facilitate restoration of directed attention capacity and affective states [
18,
29,
30]. Spending an hour at rest in an outdoor garden facilitates directed attention improvements in elderly individuals, compared to equivalent rest in a favorite indoor room [
31]. Viewing photographs of nature environments improves directed attention-related task performance, whereas viewing urban environments does not [
32]. Within workplace settings, “micro-restorative experiences” provided by views of nature through a window or the presence of plants indoors can reduce directed attention fatigue [
33,
34].
Green exercise promotes a greater psychological engagement with nature than does viewing nature [
35]. Greater immersion may elicit a fuller experience and greater responses to nature environments [
36]. Such a role of immersion suggests that green exercise might provide greater scope for attention restoration than either micro-restorative experiences or time spent at rest in nature environments. Additionally, the disparate influences of exercise and of nature environments may positively interact in a manner that further promotes attention restoration. Directed attention improves following a walk in nature environments but not after an equivalent walk around more built routes [
5,
6]. However, the variable of social presence of others was not controlled in this research. Additionally, although Hartig
et al. [
6] attempted to control exercise intensity via researchers leading participants in order to maintain a slow walking pace with stops at specified locations on route, exercise intensity (defined as the total or rate of energy expenditure during exercise [
37,
38]) was not rigorously controlled in these studies. This is important because intensity influences psychological outcomes of exercise including cognitive performance [
39,
40,
41,
42].
Exercise environment also influences perceived exertion (how hard one feels that they are physically working during activity; measured by Rated Perceived Exertion scale [
43,
44,
45]). This is one possible pathway via which a lack of control of the exercise component can promote problematic differences in exercise intensity between experimental conditions. When exercising at an instructed perceived exertion, individuals work harder (measured by speed, heart rate and blood lactate concentration) during outdoor exercise than during indoor treadmill exercise [
46]. Concurrently, during self-paced exercise individuals walk faster and work harder (measured by heart rate) yet report lower perceived exertion during outdoor walking compared to indoor treadmill walking [
7]. These findings are concurrent with the notion that synthetic environments demand greater directed attention processing [
18,
29,
30,
47], as cognitive fatigue promotes greater perceived exertion and impairs performance of exhaustive exercise [
48]. However, biomechanical and climatic differences between indoor and outdoor exercise are not controlled within these research designs, therefore origins of reported effects are unclear [
7,
46]. The color of visual environment is important to perceived exertion during exercise. Individuals report significantly lower perceived exertion during cycling exercise whilst viewing a nature-scene video, compared to cycling whilst viewing either achromatic or red-filter versions of the same video [
10]. However, it is not known how perceived exertion varies with different visual exercise-environment
types during controlled exercise.
Lacking control of the exercise component in these studies is also problematic for understanding the origins of reported influences on
physiological outcomes. It is reported that walking in nature environments promotes lower post-exercise heart rate, greater HRV, lower blood pressure and lower sympathetic nerve activity than equivalent walks in built environments [
4,
49,
50]. Environment-associated differences in physiology as measured during exposure have also been reported [
4]. However, uncontrolled exercise intensity means that findings from these studies cannot be confidently attributed to environment alone. Physiological effects of environment are yet to be demonstrated during controlled exercise. Heart rate, VO
2 and respiratory exchange ratio are not significantly influenced by color properties of visual exercise environment during controlled cycling exercise [
10]. However, influences of holistic visual exercise-environments on directed attention have not been examined during controlled exercise.
The design of the current study enables examination of the reported influence of exercise-environment on perceived exertion [
7,
10,
46]. Biological links between subjective sensations of effort and exercise-related physiology comprise a mechanism by which perceived exertion contributes to limiting of exercise performance [
51,
52]. Perceived exertion is strongly related to individuals’ time to exhaustion during high-intensity exercise [
53]. The current study examines whether visual exercise-environment influences perceived exertion and time to voluntary exhaustion during high-intensity exercise.
The current study measured pre- to post-exercise changes in directed attention in three conditions: a nature condition, a built condition and a control condition. Energy expenditure, respiratory exchange ratio and heart rate were measured to ensure: (i) comparability of the exercise component between conditions; (ii) identification of physiological effects of visual environments during exercise, and (iii) impacts of physiological differences on changes in cognitive function. The primary hypothesis is that (i) directed attention will improve more after a 15-mins exercise bout whilst viewing video footage of a nature environment, compared to footage of a built environment or viewing a blank screen (control). Secondary hypotheses include (ii) during a steady-state 15-mins exercise bout there will be no significant differences in participants’ energy expenditure, respiratory exchange ratio and heart rate between conditions; (iii) perceived exertion will be lower and time to exhaustion will be longer in the nature condition than in a built condition or control condition.