Abstract P099: Heart Rate Variability Differences Between Green and Suburban Walking: A Pilot Crossover Study

Abstract

Introduction: Heart rate variability (HRV) assesses the sympathetic and parasympathetic innervation of the heart. Lower HRV indicates greater sympathetic tone and is associated with adverse cardiovascular outcomes. During exercise, factors like activity intensity influence HRV. Yet, an environment’s inherent anxiety-/stress-promoting characteristics may also affect HRV. Hypothesis: Given nature’s calming characteristics, we hypothesized green (i.e., nature-based) walking would promote higher HRV and less HRV reduction than suburban walking. Methods: Twenty-four middle-aged adults (20 female; 49.3 ± 6.7 yrs; BMI: 30.7 ± 7.9 kg/m 2 ) participated in this nine-week crossover study comprised of once-weekly 50-minute moderate-intensity walking sessions. Participants walked for three weeks in each of two treatment conditions: (1) green walking: nature-based walking on trails of local arboretum; and (2) suburban walking: walking on sidewalks of a local suburb’s downtown area. Half were assigned to suburban walking in the first treatment period, and half completed green walking first. A two-week washout separated treatments. The Zephyr BioHarness 3 assessed HRV—reported as standard deviations of normal-to-normal RR intervals in milliseconds (ms) while also tracking activity intensity. For analysis, walks were split into 15-minute intra-walk phases with the first, second, and third phases representing each walk’s beginning, middle, and end. Five-minute pre- and post-walk sitting phases for HRV were also created. Linear mixed models with repeated measures evaluated HRV phase differences between treatments and HRV change between phases by treatment. Results: Lower HRV was observed during the pre-walk sitting phase in the green (56.5 ms) vs. the suburban walking treatment (62.4 ms; p = 0.04); however, a sequence effect was present ( p = 0.02). Due to lower walking intensity during green walking (0.31 g vs. 0.27 g; p < 0.01), activity intensity was a covariate in all intra-walk analyses. Intra-walk phase analyses revealed higher green vs. suburban walking HRV during the second (29.1 ms vs. 24.2 ms, respectively; p < 0.01) and third (33.9 ms vs. 23.8 ms, respectively; p = 0.03) phases. Further, less HRV reduction was seen between the first and second intra-walk phases during green (-7.6 ms) vs. suburban walking (-12.4 ms; p = 0.05). Between-treatment HRV differences were not found for the post-walk sitting phase. No sequence effects were observed for the intra-walk or post-walk phases. Finally, no carryover effects were observed—suggesting adequate washout between treatments. Conclusions: Green walking elicits higher HRV and less HRV reduction vs. suburban walking. Activity intensity did not explain these observations. Fully powered longer-term trials should rigorously evaluate green walking’s potential anxiety-/stress-reducing characteristics.