Many experts and running enthusiasts have touted outside running as a superior activity when compared to its indoor counterpart, the treadmill; they use a plethora of arguments, all of which generally revolve around what happens in working muscles and joints. Some attempt to argue on the grounds of biomechanics, claiming that treadmill running results in form and movement that is less natural than forward running. Others argue that physiological and energy requirements of running differ between treadmill and outdoor running, since wind resistance and other factors come into play. While each of these arguments have merit, all these parameters can be corrected in a way to make treadmill running virtually identical to road running.1,2,3 This ultimately has led the treadmill debate to stagnate and primarily be biased by opinion.
Speaking from personal experience, I choose to run outdoors whenever possible simply because I feel better, mentally and emotionally, during and after the run when compared to treadmill workouts. I have heard this sentiment echoed by many friends and colleagues; while it is just anecdotal information, the mental and emotional benefits of running in general are clear, but still being explored. Studies like the Mood treatment with antidepressants or running (MOTAR) trials recently finished data collection to assess the comparison of running and antidepressants for management of depression and anxiety.4 Other research groups have explored the current state on mechanisms of fear reduction from running, but results are conflicting. Some studies hypothesize a role for dopamine (a brain chemical involved in motivation and reward), while others claim brain-derived neurotrophic factor (a protein that promotes neuron growth and differentiation) plays a role.5 For those who run for stress management, this information could be valuable in structuring an exercise program.
The current evidence suggests that there are likely many factors that explain the mental and emotional benefits of running as a whole, but no human studies have explored how the cognitive effects of this exercise differ between stationary and forward-movement running. Recently, however, there was an article published in Nature that explores a new potential mechanism for the differences many feel between treadmills and outdoor running.
This study by Baek et al.6 used mice models to test a current psychological treatment method already used in humans to treat PTSD. Eye-movement desensitization and reprocessing (EMDR) therapy is a technique that uses bilateral repetitive eye movements, along with traditional talk therapy to reduce fear responses to a stimulus. Studies have shown its effectiveness when compared to therapy that doesn’t use these eye movements,6 but without a known mechanism. To investigate this, the research group used mice that were conditioned to elicit a fear response from sounds by electric shock, and then used ‘freeze-responses’ as a metric for fear to gauge response. Amazingly, mice shown alternating LED sequences that move back and forth (to simulate EMDR techniques) observed a significant reduction in fear response to the conditioned sounds.6
One might think that these results are caused by simple distraction from the fear-inducing sound, but that is not the case. After simulated EMDR therapy, these mice have continued suppression of fear responses. One might also think that the repetitive back and forth eye movements might not play a role, and the results are simply due to the LED flashing, but this is also not the case. Mice shown a random LED sequence showed no fear reduction.6 After seeing these results, the researchers decided to go deeper to see what is happening in the brain to cause these results.
The area of the brain that the researchers were first interested in was the superior colliculus, an area that integrates visual information and attention. They first found that EMDR-like techniques on these mice elicited greater activity in this area of the brain. They then investigated further, to the mediodorsal thalamus, an area that receives and sends information from the superior colliculus. The researchers also noticed activity in this area of the brain associated with fear reduction in the mice.6
What happens after the mediodorsal thalamus in this fear-reduction response to EMDR-like therapy? The researchers found that the basolateral amygdala, the area of the brain that ‘calibrates’ fear responses, was next in line in this pathway. As anticipated, the researchers observed a decreased level of activity of these ‘fear cells’ of the brain,6indicating that an inhibitory communication between the mediodorsal thalamus and basolateral amygdala occurred, resulting in decreased fear responses in the mice.
In summary, after discussion of the data the researchers hypothesized that EMDR-like eye movements shifted the balance between brain circuits in the mice to the path described above, which favors persistent fear extinction over fear promotion.6 What they didn’t explain, however, is how this data could translate to running. That requires some slight extrapolation, and some broader thinking about what happens during running.
When we run, walk, drive, or do any activity with forward movement, our eyes do not focus on one location. The eyes scan our environment during forward movement to perceive objects moving past on either side. In addition to this, peripheral eyesight perceives objects bilaterally, eliciting a state of visual hyper-alertness. Moving backwards to EMDR therapy, some scientists believe that the back-and forth eye movements of EMDR simulate forward movement (and alert the neural pathways described above) while allowing a patient to be able to sit in a therapists’ chair, priming neural reprogramming to be enhanced with traditional therapy.
As we slowly start to move towards forward-movement running as opposed to stationary running, one can start to see the connection of enhanced stress relief. Activating these neural pathways while running forward in space perhaps allows the brain to better reduce fear, anxiety, and stress by adding this visuospatial component. In addition to this, other research groups have tested other brain circuits associated with increased dopamine release as a result of forward movement,7 which could increase motivation and reward from the forward running when compared to stationary running.
Why not just go for a drive or walk to relieve stress and anxiety? Although these are great methods of stress reduction, and operate through the same or similar mechanisms, one would be missing all of the other cognitive benefits of running that was discussed earlier! Increased perfusion, dopamine, BDNF, and others only add to the mechanism established by Baek et al.
Although it requires extrapolation that is not scientifically concrete, the animal data from the study described can finally provide some more evidence to the individuals that love running outside and loathe the treadmill. There are likely other factors involved in this debate, like sunlight, terrain, and environmental differences, but more research will need to be conducted to further parse out the differences. Moving further away from the scientific aspect of forward running, one can also examine the metaphorical aspects of forward running’s effect on stress and fear. When an individual moves forward, they are consciously leaving things behind them, and exploring what is present and ahead. Hopefully, the future of exercise and mental health research will extrapolate these findings by Baek et al. and apply these techniques to the masses. As sedentary behavior and stagnation becomes increasingly prominent, mental health will only continue to decline unless interventions are in place.
References
Van Hooren B, Fuller JT, Buckley JD, et al. Is Motorized Treadmill Running Biomechanically Comparable to Overground Running? A Systematic Review and Meta-Analysis of Cross-Over Studies. Sports Med. 2020;50(4):785-813. doi:10.1007/s40279-019-01237-z
2. Panascì M, Lepers R, La Torre A, Bonato M, Assadi H. Physiological responses during intermittent running exercise differ between outdoor and treadmill running. Appl Physiol Nutr Metab. 2017;42(9):973-977. doi:10.1139/apnm-2017-0132
3. Jones AM, Doust JH. A 1% treadmill grade most accurately reflects the energetic cost of outdoor running. J Sports Sci. 1996;14(4):321-327. doi:10.1080/02640419608727717
4. Lever-van Milligen BA, Verhoeven JE, Schmaal L, et al. The impact of depression and anxiety treatment on biological aging and metabolic stress: study protocol of the MOod treatment with antidepressants or running (MOTAR) study. BMC Psychiatry. 2019;19(1):425. Published 2019 Dec 30. doi:10.1186/s12888-019-2404-0
5. Tanner MK, Hake HS, Bouchet CA, Greenwood BN. Running from fear: Exercise modulation of fear extinction. Neurobiol Learn Mem. 2018;151:28-34. doi:10.1016/j.nlm.2018.03.021 6. Baek J, Lee S, Cho T, et al. Neural circuits underlying a psychotherapeutic regimen for fear disorders. Nature. 2019;566(7744):339-343. doi:10.1038/s41586-019-0931-y
7. Salay, L. D., Ishiko, N., & Huberman, A. D. (2018). A midline thalamic circuit determines reactions to visual threat. Nature, 557, 183-189. https://doi.org/10.1038/s41586-018-0078-2
Benson Lagusis is a member of the University of Arizona College of Medicine - Phoenix Class of 2026. He graduated with a B.S. in Exercise Science from Northern Arizona University before earning a Master's degree in Biomedical Sciences from Midwestern University. Benson played baseball in college, and is interested in sports medicine and health promotion/education. During his free time, Benson is an avid hiker, reader, and Netflix binge-watcher.