Heart Rate Variability Mediates Fatigue And Motivation Throughout A High-intensity Exercise Program.

Abstract

PURPOSE: High-intensity exercise programs are often promoted as a time-efficient public health intervention to combat chronic disease. Increased physical effort, and subsequent fatigue, can be barriers to long-term maintenance of these exercise programs. The purpose of the present study was to determine if heart rate variability (HRV) mediates state traits related to exercise program adherence. We hypothesized that exercise-induced, temporary shifts in resting HRV would significantly affect daily fatigue and motivation. METHODS: Fifty-five healthy men and women (ages 19-35 years) used a commercially-available smartphone application to monitor daily HRV status throughout a six-week high-intensity exercise intervention. Participants were randomly assigned to either control (CON) (n = 29, 24.1 ± 4.1 years, 41.4% male) or treatment (TREAT) (n = 26, 23.7 ± 4.5 years, 53.8% male) groups. Within CON, exercise intensity was completed as prescribed while intensity within TREAT was modulated in response to observed shifts in daily HRV. Participants reported state motivation to exercise and global physical fatigue immediately prior to each exercise session. RESULTS: Prevalence of temporary shifts in resting HRV were 37.6 and 38.7% for the CON and TREAT conditions, respectively. Within CON, shifts in HRV resulted in less motivation (mean diff. = -4.00%; 95%CI = -7.56, -0.44; F = 4.86, p = 0.028) and more physical fatigue (mean diff. = 4.79%; 95%CI = 1.85, 7.74; F = 10.24, p < .001). Spectral domain metrics (i.e., LF:HR ratio) were significantly lower (mean diff. = -0.14 au; 95%CI = -0.27, -0.01; F = 4.715, p = .030) during HRV shifts. Within TREAT, shifts in HRV resulted in no change in motivation (mean diff. = 2.58%; 95%CI = -6.54, 1.38; F = 1.63, p = .202) with reduced physical fatigue (mean diff. = -5.94%; 95%CI = -9.56, -2.32; F = 10.40, p = .001). Within TREAT, LF:HF ratio was higher (mean diff. = 0.13 au; 95%CI = 0.23, 0.24; F = 5.59, p = .018) during HRV shifts. CONCLUSIONS: These data establish a link between expected shifts in heart rate variability throughout high-intensity exercise programs with motivation to participate and physical fatigue. Additionally, modulation of training volume, in response to these shifts, can optimize adherence-related behavioral responses during high-exercise programs.