Effects of Resistance Training Intensity on Heart Rate Variability at Rest and in Response to Orthostasis in Middle-Aged and Older Adults

Objective: Aging and deficits related to decreased physical activity can lead to higher risks of autonomic nervous system (ANS) dysfunction. The aim of this study was to evaluate the effects of 24 weeks of resistance training (RT) at various intensities on hemodynamics as well as heart rate variability (HRV) at rest and in response to orthostatic tests in middle-aged and older adults. Methods: Forty adults were randomized into three groups: high-intensity (HEX) (80% 1-RM) (11 female, 4 male; 60 ± 4 years); low–moderate-intensity (LEX) (50% 1-RM) (nine female, four male; 61 ± 5 years); and a control group (CON) (eight female, four male; 60 ± 4 years). The RT program consisted of nine exercises, with two sets performed of each exercise two times per week for 24 weeks. Data collected included 1-RM, heart rate, and blood pressure and HRV at rest and in response to orthostasis. Results: Both the HEX (42–94%) and LEX (31.3–51.7%) groups showed increases in 1-RM (p < 0.01). The HEX group showed decreases in resting heart rate (−4.0%), diastolic blood pressure (−3.2 mmHg (−4.2%)), and low frequency/high frequency (LF/HF) (Ln ratio) (p < 0.05). Post-study, the HEX group had higher HF (Ln ms²) than the CON, adjusted for pre-study value and age (p < 0.05). Post-study, the supine–standing ratio (SSR) of LFn (normalized unit) in the HEX group was greater than that in the LEX and CON groups, while the SSR of LF/HF in the HEX group was greater than the CON (p < 0.05). In conclusion, high-intensity RT can improve resting heart rate and HRV by enhancing cardiac vagal control. High-intensity RT might also improve the orthostatic response in terms of HRV. High intensity RT might assist ANS modification and could perhaps decrease the risks of cardiovascular disease and orthostatic intolerance.