Maximizing the Miles: Effects of Outdoor vs. Indoor Exercise on Physiological Responses to Stress
Introduction
With the busyness and chaos of everyday life, psychological stress seems to be inevitable, and people are adamant in the pursuit of reducing stress. Many people turn to journaling, tea and meditation to find relief. Additionally, research has found that spending time in nature has also proven to be effective in reducing stress, by influencing blood pressure and cortisol levels(1). Another common tool for managing stress is exercise in its many forms. While these strategies surely help those who fall victim to the curveballs and challenges that life brings, are people utilizing them in a way that maximizes stress reduction? For my capstone research project, I took a closer look at the benefits of exercise and the outside environment to answer the question of whether or not where people, specifically college students, exercise affects their cortisol levels, heart rate, blood pressure and overall subjective feeling of stress.
Systolic and diastolic blood pressure, heart rate and perceived level of stress were collected before and after exercise performed both outdoors and indoors. Cortisol levels of one participant were collected and also considered. The magnitude of the values before and after exercise of each trial were then compared. I hypothesized that exercises performed in outdoor spaces would lead to a greater reduction of physiological effects of stress as well as the mental perception of stress, compared to exercise done in indoor spaces, specifically gyms. The data collected and analyzed found no significant difference in the magnitudes of diastolic blood pressure and heart rate. However, data analysis did find a statistically significant difference in the magnitudes of systolic blood pressure before and after outdoor and indoor workouts. This research is valuable as it aims to maximize stress reduction among individuals. Chronic stress needs to be minimized as it can contribute to memory disorders as cortisol can yield structural and functional changes in the hippocampus as well as cause the autonomic sympathetic nervous system to increase vasoconstriction which raises blood pressure, ultimately putting stress on the heart, to name a few.(2)
Background
“Green exercise”
As I was researching more on the effects of green spaces and mental health, I came across an article that described outdoor exercise or indoor exercise with a “pleasant rural scene,” as “green exercise” (6). The article addressed the difference between anxiety, negative emotions and feelings of restoration when engaging in green exercise and when exercising indoors with no green scenery. They found that with green exercise, anxiety and negative emotions were reduced and restorative feelings were increased. In a different article, they mentioned similar findings, but additionally noted that people engaging in green exercise reported less anxiety and depression and were also able to have longer, more intense workouts (7). Another article noted that while exercising outdoors yielded positive benefits in increasing energy and reducing anger, frustration and depression, they mentioned these benefits were temporary(8). While these articles did not address the differences in physiological responses that occurred between the green exercise and indoor exercise, it was helpful to find data that revealed positive benefits of green exercise to help support my hypothesis that outdoor exercise leads to a greater decrease in the physiological effects of stress.
I also read up on some articles that discussed the benefits of green space in urban areas. I found that urban and indoor environments can provide an excess amount of stimulation which yields cognitive fatigue among individuals. Contrasting that with green spaces, the green spaces are less stimulating and more relaxing which contributes to a reduction and stimulation and ultimately stress, which in turn can alter one’s physiology(9). In another study, they looked at the benefits of urban green space and how they affected different senses in reducing physiological stress. Surprisingly, they found that the olfactory stimuli yielded a greater contribution of stress reduction compared to visual stimuli(10). They reported this was interesting as the construction of green spaces in urban environments typically has visual aesthetics at the forefront of how they design the spaces. This was helpful in thinking about potential reasons why performing exercises in outdoor spaces and indoor spaces may result in differing reductions of stress. Perhaps, the mustiness of the gym compared to the crisp air of the outdoors plays more of a factor than when initially considered.
It is also important to note that each of the articles I read that contrasted the effects of indoor compared to outdoor exercise mentioned there were significant limitations to their methods of research due to confounding variables that come along with comparing data from participants on different days. These external factors include stress varying depending on the day. One study was intentional about making sure the weather was clear and warm while subjects performed the outdoor exercise and required that all subjects perform their tests in the afternoon, after having at least one hour of class to ensure that all subjects were exposed to at least some fatigue(6). Seeing how others formulated their methods to yield in accurate data collection was helpful and I was able to consider similar methods when designing my study.
Diurnal Slope cortisol kits
I wanted to measure the wake-sleep slope of cortisol levels in the body, as cortisol is the primary hormone released in response to stress. Cortisol levels fluctuate throughout the day, peaking when one first wakes up and then lowering as the day goes on. When individuals are in high stress, more cortisol will remain in their body and their diurnal slope will be flatter. Finding an at-home cortisol test that collects the data I waslooking for, while also staying within a reasonable price range proved to be no small task. Because I was interested in the wake-sleep slope of cortisol levels, I was hoping to record levels when participants woke up in the morning and right before they went to sleep to then find the slope of the line of the two points. During my research, I was unable to find a test that provided this option with an at home kit. Instead, I found kits that took one saliva sample and evaluated the amount of cortisol in it and compared it to “normal values” and others that collected four samples throughout one day to evaluate the fluctuation in the level of the hormone. While the latter would provide useful data, the kits are expensive, priced at about $130 a kit. Finding detailed and reliable information about each of the kits was also challenging. Many of the websites listed vague descriptions simply stating that they measure cortisol levels. Hoping to gain more information, I contacted a company, Verisana, who sells the kits and analyzes the data from them and purchased two kits for my research. While I was only going to be able to purchase enough kits to conduct an analysis for one of the participants, I believed that it would still be valuable to see how this hormone, which plays a key part in physiological stress response, could be affected by differences in location of exercise.
Image 1: Cortisol Saliva Test from Verisana
Image Source: Verisana Laboratories 2021
Effects of chronic stress
Chronic stress is dangerous for the body as it can cause hormone imbalances that can lead to malignant cancer. One hormone that is primarily affected by stress is cortisol. Cortisol levels regulate on a diurnal slope, with levels highest at the start of the day and decreasing in value as the day goes on. In response to stress, cortisol remains in the body which can prolong stress responses in the body. For example stress affects the cardiovascular system by increasing contractility, vasoconstricting veins and increasing blood pressure (4). In the GI tract, stress can lead to inflammatory diseases like Crohn’s disease as well as ulcerative based diseases(2).
It is clear that stress, especially chronic stress, is an issue that needs to be taken seriously. It is necessary that humans find a way to maximize their stress reduction in healthy ways to not only benefit their mental health, but their physical health as well.
Methods
In order to answer the question of whether there is a difference between the reduction of physiological responses to stress when exercising indoors and outdoors, I decided to construct a study where participants measured and self-reported the results of each trial. Each participant selected a workout of their choice and performed it in the indoor exercise facility. 30 minutes prior to their workout, they recorded their heart rate and blood pressure using a wrist blood pressure monitor. They also recorded the exact date and time of their workout as well as a brief description of the activities of their day and their perceived level of stress. After their workout, they recorded their heart rate and blood pressure for 60 minutes, in 15 minute intervals. Once 60 minutes had been reached, they recorded their perceived level of stress. This exact process was repeated on the day of their outdoor data collection, where they performed the exact same workout on the same day of the week at the same time. The order in which the outdoor and indoor trials are completed was not significant and was decided by the participant. I created a document that I sent to each participant to fill out before they began their trials, as described below:
Age
Gender
Major
Date and time of Trial
A brief description of the events of your day
Description of Exercise
Location of Exercise
Pre-Exercise data collection – record 30 minutes before exercise
Heart Rate
Blood Pressure
How would you rate your stress level? Choose below
None light moderate heavy
Post-exercise data collection – collect data for 60 minutes following the workout, recording in 15 minute intervals
Heart Rate
15 min
30 min
45 min
60 min
Blood Pressure
15 min
30 min
45 min
60 min
1 hour post exercise:
How would you rate your stress level? Choose below
None light moderate heavy
Salivary Cortisol Tests
Although I was not able to obtain diurnal cortisol kits for each participant due to the high cost, I did obtain two kits. I collected cortisol measurements from myself on the days that I performed my indoor and outdoor workouts. I collected samples throughout the duration of the day; when I woke up in the morning, early afternoon, in the evening, and before I went to bed. I sent my saliva samples back to Versiana, the company that analyzes the cortisol levels, with the intention of comparing the slopes of each day. This was done in addition to evaluations of heart rate, blood pressure and perceived level of stress collected from the day of each trial.
Image 2: Salivary Cortisol Collections from day of Outdoor Workout
Evidence for Appropriate Methods
I am confident the methods I used to conduct my research were appropriate as I identified several other studies that have utilized similar practices. I know it is reasonable to analyze differences in heart rate before and after exercise as a meta-analysis I read found consistent decreases in resting heart rate upon completion of exercise compared to initial measures (16). Another study, which was looking at the post exercise hypotension phenomena in men with high blood pressure, consistently found blood pressure to be lower after exercise, which justifies using this measurement as a part of my methods (17). Additionally, their methods involved measuring the blood pressure of participants 10 minutes before and 60 minutes after exercise. Although these time periods are not exactly the ones I used in my study, they are similar, providing evidence of appropriate timing of my data collection. Moreover, it is reasonable to hypothesize that circulating cortisol levels will decrease after light to moderate exercise as several studies concluded these levels of exercise were beneficial in the reduction of cortisol levels (18 and 19).
In addition to these quantitative measures of heart rate, blood pressure, and cortisol levels, overall perception of stress level is appropriate to collect in order to identify the differences in the benefits of stress reduction when exercising indoors compared to outdoors. Although it is not measurable data, these subjective observations can help affirm or deny differences in the benefits of exercising indoors compared to outdoors. In the studies I read to research for my data collection, many also used this self reporting technique along with more tangible data to help make conclusions about their collected data (19 and 20).
Results of Data Analysis
Blood Pressure and Heart Rate
In order to analyze the data I collected, I used a Paired T-test to find the mean difference between the changes in physiological responses to stress when an individual exercises outside compared to inside. This statistical test was reasonable to use for my study as I was comparing the results of data that are in matched pairs (22). These matched pairs were the participant’s outdoor exercise trial and their indoor exercise trial. When analyzing the data I collected, I found the magnitude of the difference between pre and post workout levels of heart rate and both systolic and diastolic blood pressure for each trial (both indoors and outdoors). I chose to compare the values collected from 45 minutes after the workout with the initial values as the blood pressure values specifically started to return back to normal values by this time, while the outdoor trials remained low. It appeared that the post-exercise hypotension phenomena was prolonged in the outdoor trials. By the time 60 minutes post workout was reached, blood pressure values from both the indoor and outdoor trials were closer to the recorded values prior to the exercise trials.
Next, I calculated the difference between the magnitudes of each variable of each trial for each individual participant. Finally, I calculated the mean of those samples as well as the sample standard deviation. The null hypothesis of my study was there is no difference in the magnitude of pre and post workout heart rate, systolic blood pressure, and diastolic blood pressure measurements for outdoor and indoor exercise. The alternative hypothesis was that the magnitude of pre and post workout heart rate, systolic blood pressure, and diastolic blood pressure measurements for outdoor exercise are greater than those recorded before and after indoor exercise.
With these null and alternative hypotheses in mind, I calculated the t-statistic for a paired t-test. I did this by subtracting the mean difference by the hypothesized value of difference, which in this case was zero. Then, I divided this value by the quantity of the standard deviation divided by the sample size, which was twelve. For the systolic blood pressure data, the t-statistic was -1.9078. For the diastolic blood pressure data, the t-statistic was -0.67171. Finally, for the heart rate data, the t-statistic was 0.542901.
After the t-statistics for each variable were calculated, I calculated the p-value to determine if the data I collected was statistically significant and provided reasonable evidence to reject the null hypothesis. The p-values were calculated by using the T.DIST feature on excel where I listed the t-statistic, the degrees of freedom, which were 11, and indicated that it was a one-sided t-test. The t-sample contained one side as I was only concerned if the outdoor results would possess a larger before and after magnitude of each of the variables collected.
I set the alpha, or the level of significance to 0.05, meaning that if the p-value was lower than 0.05, the null hypothesis could be concluded and there is statistically significant evidence to conclude that the magnitude of the outdoor trial variables were greater than those of the indoor trials. The p-value for the systolic blood pressure data was 0.041424. The p-value for the diastolic blood pressure was 0.257815. The p-value for the heart rate data was 0.542901. Based on these values, the systolic blood pressure data was statistically significant. There was failure to reject the null hypothesis and it could be concluded that the outdoor exercise trials were able to produce a greater magnitude for before and after systolic blood pressure values compared to indoor workout trials.
Cortisol
After sending off my cortisol samples to the Verisana Lab upon completion of my indoor and outdoor workout trials, I unfortunately was told that the first sample I sent in for my outdoor trial had been lost in the mail and had not been received by the company. While Verisana was extremely accommodating, apologetic, and even offered to send me a new kit so that I could recollect samples, I did not have enough time to gather my samples and ship them back to be analyzed and ultimately use those results to compare with the results from the indoor trial. Instead, I took the cortisol results from the indoor trial and was able to analyze them against the ZRT Lab’s given reference range, the normal range listed from Precision Analytical Inc, as well as the reference range from Verisana. All of these are companies that offer kits for at-home cortisol collection.
ZRT Lab listed the morning range to be 3.7—9.5 ng/mL in the morning, 1.2—3.0 ng/mL in the afternoon, 0.6—1.9 ng/mL in the evening and 0.4—1.0 ng/mL at bedtime (28). Precision Analytical Inc. listed the morning range to be 1.6-4.6 ng/mL, 0.4-1.5 ng/mL in the afternoon and between 0-0.9 ng/mL at night (27). Versiana listed their normal ranges as 1.5-9.6 ng/mL in the morning, 0.6-4.1 ng/mL in the afternoon, 0.2- 2.3 ng/mL in the evening and 0.1-1.0 ng/mL at bedtime (29). I wanted to compare the values measured from my samples with more than one source as I assumed the reference ranges may vary depending on the source.
All of my samples throughout the day fell within the reference ranges I found. I was pleasantly surprised by this because as a college student, it feels like I am stressed more often than not and was worried that this may have been impacting the amount of free cortisol left in my body at the remainder of each day. The largest reduction in my cortisol levels occurred between the samples collected in the morning, right when I woke up in the morning, and in the afternoon. One possible reason for this could be the fact that I performed my workout in between these collection times as cortisol has been found to reduce in amount after exercise is performed. However, free cortisol does decline at a seemingly faster rate earlier in the day whereas the slope is less steep as one is approaching bedtime (25).
Image 3: Graph of Diurnal Cortisol Slope
Perceived Levels of Stress
Upon analyzing the perceived levels of stress from before and after the indoor and outdoor workouts, there were differences in the number of people who reported improvements in their stress level before and after exercise depending on where they completed their exercise. For the outdoor trials, 58% of participants reported improvements in their perceived level of stress upon completion of their workout. However, when participants finished their indoor workout, only 33% of them reported they felt less stressed than before starting their exercise. Although this data is solely subjective, it is still interesting to see how individuals reported feeling better after their outdoor workouts. These findings are supported by other studies, including the ones I mentioned in the background section, where participants reported greater improvements of anxiety and depression while participating in outdoor activities.
Limitations and Challenges
With every study comes potential limitations and sources of error which affect the accuracy of data collection. A limitation of my study could be my small sample size. I realized very quickly that finding college students who would be willing to not only exercise in different locations, but also take the time to collect the data I needed, is a difficult task. I was only able to recruit 12 participants to participate in my research study. I had several people who agreed to the study, performed the first trial and then when I reached back out to them about getting them the sphygmomanometer for the second trial, they never replied. It was tricky planning out who was going to do their trials when, as I only had one blood pressure cuff. In hindsight, I would have checked out multiple cuffs so that multiple participants could have collected their data at the same time. Moreover, the blood pressure cuff itself did not seem to provide consistent feedback, with blood pressure readings varying significantly in back to back recordings. I do not think my data are going to yield statistically significant results and this could potentially be a reason why.
Additionally, a confounding variable that could contribute to differing physiological responses post exercise could be the weather. With the temperatures getting colder and daylight becoming more limited, there is potential for the difference in pre and post exercise measurements to be less significant as the lack of sunlight and colder temperatures may result in a lower reduction of stress compared to a nice, warm day. I was required to take these factors into consideration when analyzing my data. Moreover, another study I referred to while researching mentioned the potential differences in heart rate and blood pressure as a result of differences in the stress factors of each collection day (6). In an effort to prevent this discrepancy, participants in my study performed their exercises on the same day of the week (ie: if the first trial was performed on a Monday, the second trial would be performed on a consequent Monday). My hope was that participants would have similar schedules on the same day of the week which would limit differences in stress level on these days.
Finally, the cortisol samples I collected on the day of my outdoor exercise trial got lost in the mail. The Verisana website has a portal where you can register your kits and track when they are registered and when the company actually receives the test back to their labs. I completed my outdoor workout the week before my indoor and the outdoor sample is not logged in their system while I received the indoor results. I reached out to the company and they said they never received the first samples. I was really disappointed by this as I was curious if the cortisol levels would yield more accurate data compared to the seemingly faulty data from the blood pressure cuff.
Implications and Suggestions for Future Research
There is extreme significance in finding that there is a greater reduction in systolic blood pressure when one exercises outdoors compared to indoors. When someone has low systolic blood pressure, they have lower risk of heart attack, stroke and even heart failure (24). Conversely, high systolic blood pressure is the most common form of hypertension and has the ability to predict the risk of cardiovascular issues such as those listed above (26). Knowing that outdoor exercise has the ability to increase the magnitude in which systolic blood pressure is lowered, individuals can have the tool that is outdoor exercise, to not only maximally decrease the stress they feel mentally, but the stress which impacts their physical body and long term health as well.
This research is also valuable as it looks to dive further into finding a practical solution to the “stress problem” we have through outdoor exercise, or “green exercise.” It aims to maximize stress reduction among individuals. Chronic stress needs to be minimized as it can contribute to memory disorders as cortisol can yield structural and functional changes in the hippocampus as well as cause the autonomic sympathetic nervous system to increase vasoconstriction which raises blood pressure, ultimately putting stress on the heart, to name a few (2). This is beneficial to not only the city of Bloomington, IN, but the human population as a whole as it will provide individuals with practical tools to reduce their stress most effectively.
Due to the limited ability to utilize cortisol samples within my study, future studies on this topic might benefit from taking a deeper look at how this hormone is affected by outdoor and indoor workouts. In doing so, they would be able to see if there is a difference in the presence of free cortisol after completion of each workout. Additionally future studies may benefit from having a more reliable device to obtain blood pressure and heart rate in order to obtain more meaningful results. I would also suggest that the data collection of participants be monitored to ensure they are properly using equipment and collecting data correctly.
References
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