Box breathing and prolonged exhalation reduces markers of physiological stress reactivity in response to a virtual trier social stress test

Acute exposure to psychological stress activates the sympathetic "fight or flight" response, increasing heart rate, respiratory rate, and metabolic activity. High-stress occupations such as law enforcement, firefighting, and military service regularly involve exposure to intense stress and may benefit from interventions that mitigate physiological and psychological stress responses. Slow-breathing techniques such as box breathing and prolonged exhalation have been shown to reduce perceived stress and certain physiological markers; however, their relative effectiveness under acute stress conditions remains unclear. In this randomized controlled trial, sixty-six participants (n = 66) were randomly assigned to normal breathing (NB; n = 22), prolonged exhalation (PE; n = 22), or box breathing (BOX; n = 22) prior to completing a modified, virtual Trier Social Stress Test (TSST). Markers of subjective and physiological stress were collected before and after the TSST, including state anxiety (SAI), heart rate (HR), heart rate variability (HRV; RMSSD, SDNN), and salivary concentrations of alpha-amylase (sAA), secretory immunoglobulin A (SIgA), and cortisol, as well as cognitive performance (Stroop color-word task and mental arithmetic). Utilizing a linear mixed model with a random subject intercept, the TSST elicited significant increases in HR, SAI, sAA, and SIgA. Both PE and BOX breathing attenuated post-stressor increases in HR, SAI, and sAA compared to NB, indicating that brief breathing practice can buffer acute physiological stress responses. HRV, Cortisol, and cognitive performance were statistically unchanged across the three conditions. These findings support the short-term effectiveness of both box breathing and prolonged exhalation for reducing subjective and peripheral physiological markers of stress.