Exercise HRV (A, SDNN; B, RMSSD; C, LF; D, HF; E, LF/HF) on low-stress and high-stress days within the placebo (PLA) and testosterone (TEST) group participant. Exercise HRV within the placebo (PLA) and testosterone (TEST) group participants on high-stress (HS) and low-stress (LS) days. Mean values based on this model were used to assess differences in HRV between the first and the last LS and HS day across all participants and between treatment groups, as well as between treatment groups irrespective of stress day designation. Although the LF/HF ratio is often reported to indicate the sympathetic-to-parasympathetic balance (52–54), there is controversy with this interpretation due to the conflicting interpretations of LF (55,56).5 LF in nonresting conditions has previously been considered to reflect sympathetic activity (4,45,46); however, sympathetic and parasympathetic influence as well as baroreflex sensitivity are now understood to influence LF (3,47–50). SDNN reflects the influence of both sympathetic and parasympathetic nervous systems on the heart and is sensitive to the effects of changes in workload (3,43). For example, while it generally elicits hypertensive responses via its actions on vascular smooth muscle cells (VSMC) in most tissue beds, it has hyperproliferative effects only on VSMC in the cerebral vasculature, and not on VSMC located peripherally (Hunyady and Catt, 2006). Renal blood flow and vascular response was also unchanged between POTS patients and healthy subjects (Mustafa et al., 2012). They found that POTS patients have a blunted systemic vascular and hypertensive response to Ang-II versus healthy subjects. Subsequent work has also shown that plasma Ang-II levels in some POTS patients are elevated when compared to healthy subjects (Stewart et al., 2006a; Mustafa et al., 2011). Heart rate data from all morning load carriage exercises were downloaded using OmniSense™ 5 software (Zephyr Technology Corporation, Annapolis, MD). Average speed for each participant was calculated by dividing the distance covered in miles during the load carriage exercise by duration of the exercise. Distance and duration of each load carriage exercise were monitored using Garmin Forerunner® 15 (Garmin Ltd., Olathe, KS) GPS watches; one GPS watch was attached to each participant’s loaded backpack, and one was worn by the accompanying staff member for distance and duration verification. Future studies should therefore consider both the separate and interactive effects of estrogen and progesterone on the function of central autonomic nuclei. In a more direct assessment of autonomic function, postmenopausal women exhibited elevated peripheral sympathetic vasoconstrictor nerve activity compared to young women 54, 82. Accordingly, heart rate variability was lower in oophorectomized women compared to age-matched controls , suggesting decreased parasympathetic tone in these women. Sample of the data of the blood volume pulse (BVP) amplitude measured with photoplethysmographs (PPGs) at fingertip. In summary, the participants listened to the sound and provided a saliva sample three times. The participants came to the laboratory for 3 days, and on each day they listened to one of the sound types and a saliva sample was collected once before the listening session. When the listening session started, the participants were presented with one of the experimental sounds, types 1, 2, and 3, for 1 min. Staff at the company were not informed of the sample content or the nature of the study. The finger BVPR indicates the vasoconstriction of the arterioles at the finger mediated by α-adrenergic receptors (Grote et al., 2003).
