Exercise training activates the sympathetic nervous system (SNS) in direct proportion to intensity and duration (1). The SNS facilitates exercise performance through physiological responses like increases in skeletal muscle blood flow, blood pressure, cardiac output, heart rate, stroke volume, and respiration (2, 3). Post exercise, the SNS continues its dominance, supporting physiological recovery processes like replenishing myoglobin O₂ stores, resynthesizing phosphocreatine, degrading hormones, increased glyconeogenesis from lactate accumulation, and removal of CO₂ (4, 5). Eventually, the parasympathetic nervous system (PNS) reactivates, regaining homeostatic control (6). It is, in this state, an athlete, when provided sufficient time, may fully recover, progressively adapt, and subsequently, enhance their sport performance (7). Delays in reactivating the PNS, however, may prolong recovery as the sustained catabolic state inhibits processes such as full repletion of energy stores and repair of musculoskeletal damage (8–11). Thus, athletes may benefit from utilizing methods that reactivate the PNS immediately following training sessions, especially sessions of high volume and/or include high intensity exercise.

Substantial evidence shows that relaxation techniques such as mindfulness, progressive relaxation, autogenic training, and deep breathing positively influence the PNS (12, 13). Individuals implementing these techniques following various physiological stressors elicited reductions in heart rate (HR), respiration rates (RR), and increases in HR variability (HRV), indicating PNS reactivation (13). HR, RR, and HRV are indicators of the interplay between the PNS and SNS (i.e., sympathovagal balance). During parasympathetic dominance, acetylcholine released from cardiac nerve fibers slows the sinoatrial (SA) node, reducing HR. The controlled and slower inspirations and expirations observed in this state contribute in optimizing pulmonary gas exchange, venous return, and stroke volume (14, 15). Many studies previously demonstrated that deep and slowly controlled breathing, a primary component to relaxation techniques, regains PNS activation through these physiological mechanisms (16). Specifically, the activity of the efferent cardiac nerves fibers is at its peak throughout each prolonged expiration, progressively slowing HR and increasing HRV (12). During the lengthened inspiration, the expanding lung volume and intrathoracic pressure increases alveolar ventilation and perfusion, allowing for optimal loading and unloading of oxygen and carbon dioxide, respectfully, in addition enhanced cardiac blood flow (17, 18).

In the sports realm, current research shows that the anxiolytic effects of relaxation techniques are a powerful tool for elite athletes to use in reducing psychophysiological stressors that often precede competition (19). Athletes often experience high state anxiety, self-doubt, decreased confidence, and exhibit heightened activity of the SNS, all of which may negatively affect sport performance (19–21). Studies show that athletes implementing specific relaxation techniques like auditory and visual imagery, self-hypnosis, deep breathing, and progressive relaxation report reduced anxiety, positive self-image, increased self-efficacy and improved performance (22–24). Interestingly, however, previous studies showed that relaxation exercises following training sessions equivocally affected HR and HRV (25–27). Some factors potentially explaining these inconsistencies could be that studies relied on participants self-administering the intervention outside of training, resulting in self-reported compliance and its' overall effects on feelings of relaxation, stress, anxiety, and perceived performance (25, 27). As such, among athletes, the effects of relaxation techniques on physiological markers of PNS reactivation like HR, RR and HRV are less clear. Moreover, a dearth of literature exists on using relaxation techniques following acute bout of exercise training among athletes competing in high-intensity team sports like American football (28) and the effects on PNS reactivation.

Utilizing relaxation techniques following training sessions may greatly benefit collegiate football athletes for several reasons. Football athletes train with high frequency (5–6 sessions per week), longer duration (2–4 h) and at high intensities (29, 30). Consequently, these athletes may experience prolonged, elevated SNS activity, potentially delaying recovery (26). To our knowledge, no previous studies examined the influence of a brief relaxation intervention following an acute bout of training on PNS reactivation in collegiate, male football athletes. Therefore, the purpose of this study was to determine the effects of two, 15-minute relaxation exercises, mindfulness, or rest, on post-exercise HR, RR and HRV. We hypothesized that the athletes engaging in either mindfulness or rest techniques would elicit lower and larger reductions post-exercise HR and RR and higher increases in HRV compared to their no-intervention counterparts.

Table 2 presents the pre-intervention, post-intervention and change in HR, RR, rMSSD and SDNN across the three groups. At baseline (pre-intervention and post-exercise training), no statistically significant between-group differences in either HR, RR, or HRV metrics were reported. For HR, statistically significant reductions were observed for the mindfulness, rest, and no-intervention groups: −81.6, −66.4, and −40.9 bpm, respectively. Similarly, for RR, significant reductions were found for the mindfulness, rest, and no-intervention groups: −31.7, −26.9, and −19.0 breaths⋅min⁻¹, respectively. Moreover, both the mindfulness and rest groups exhibited a larger reduction in both HR and RR compared to the no-intervention group, p < 0.0000. Additionally, the mindfulness group showed a larger reduction in HR and RR compared to the rest group, p < 0.05. For HRV, statistically significant increases in rMSSD and SDNN were observed for the mindfulness, rest, and no-intervention groups: + 8.6, + 7.3, + 4.8 ms and +27.4, + 22.7 and 14.4 ms, respectively. Like HR and RR responses, the mindfulness and rest groups exhibited larger increases in both HRV metrics compared to the no-intervention group, p < 0.0000. Also, the mindfulness group showed a larger increase in HRV compared to the rest group, p < 0.05.

Between-group differences in post-intervention HR, RR and HRV are illustrated in Figures 2–4, respectively. Both intervention groups elicited lower HR and RR post-intervention. Specifically, athletes in the mindfulness group exhibited a post-intervention HR, 43.1 bpm below the no-intervention group (77.0 vs. 120.1 bpm, respectively) and showed a significantly lower respiration rate following the intervention (15.0 vs. 23.6 breaths⋅min⁻¹, respectively). Similarly, athletes in the rest group showed significantly lower post-intervention HR (−30.2 bpm, 89.9 vs. 120.1 bpm, respectively) and RR (−4.3 breaths⋅min⁻¹, 19.3 vs. 23.6 breaths⋅min⁻¹, respectively) compared athletes in the no-intervention group. Lastly, athletes using mindfulness following their training session elicited lower HR (−12.9 bpm, 77.0 vs. 89.9 bpm, respectively) and RRs (−4.2 breaths⋅min⁻¹, 15.0 vs. 19.3 breaths⋅min⁻¹, respectively) compared to athletes using the rest intervention. For HRV, athletes in the mindfulness group elicited significantly higher post-intervention rMSSD and SDNN values compared to the rest (+4.0 ms and +1.2 ms) and no-intervention (+13.8 ms and + 4.1 ms) groups. Additionally, athletes in the rest group exhibited significantly higher post-intervention rMSSD and SDNN values compared to the athletes in the no-intervention group (+9.8 ms and + 2.9 ms).

The purpose of this study was to evaluate the impact of two relaxation interventions on post-exercise training parasympathetic reactivation in a sample of Division-I, male football athletes. The main findings of this study were (1) both mindfulness and rest interventions elicited a greater parasympathetic reactivation compared to the no-intervention group, reflected by lower post-exercise HRs and RRs and higher HRV, (2) the two intervention groups showed the greatest within-group reduction in HRs and RRs and increases in HRV relative to the no-intervention group and, (3) at a smaller magnitude, the mindfulness group exhibited a stronger parasympathetic reactivation following exercise training compared to the rest group.

A novel aspect of this study was the observed parasympathetic reactivation during a 15-minute relaxation intervention that either included mindfulness or unguided rest following a routine training session. Specifically, athletes participating in mindfulness or resting practices, immediately after training, exhibited lower HRs and RRs and higher HRV compared to their no-intervention counterparts that resumed usual post-training activities. Importantly, at the start of the intervention, these athletes elicited post-exercise HRs associated with exercise performance at high intensity, which augments the increase in sympathetic activity, often requiring a longer period of recovery (5). Remarkably, following the mindfulness and resting interventions, the post-exercise HRs (77.0 ± 12.0 bpm and 89.9 ± 19.7 bpm, respectively) and RRs (15.0 vs. 19.3 breaths⋅min⁻¹, respectively) exhibited were similar to values at rest and reached while performing activities of daily living (41, 42).Conversely, athletes in the no-intervention group, at the end of the 15-min trial, exhibited significantly higher HRs and RRs (120.1 ± 19.7 bpm and 23.6 ± 17.5 breaths⋅min⁻¹, respectively) which are similar to values elicited during moderate intensity exercise (34, 35, 43). This finding contributes to the current literature as most studies focused on evaluating the effects of mindfulness practices on pre-competition psychological stressors like performance anxiety (22, 24, 44). Thus, the effects on physiological factors, like parasympathetic reactivation, were unclear. The observed reactivation of the PNS following mindfulness or rest post-training possibly suggests that athletes begin at least one aspect of their physiological recovery earlier. Consequently, these athletes may experience less fatigue during post-training activities, improved sleep, and readiness for subsequent training sessions.

Expectedly, all athletes elicited reductions in HR and RR and increases in HRV within 15 min following the cessation of training (45, 46). However, the magnitude of the reductions was markedly different amongst the groups, with the mindfulness and rest groups exhibiting the largest decreases in HR and RR compared to the no-intervention group. Specifically, the reductions in HRs and RRs among athletes in the mindfulness and rest groups were 200% and 160% and 168% and 140% larger relative to the no-intervention group. Similarly, the mindfulness and resting group exhibited higher increases for SDNN (190% and 160%) and rMSSD (179% and 150%), respectively, relative to the no-intervention group. These findings indicates that the mindfulness and resting techniques implemented post training were more effective in reactivating the PNS via lowering HR and RR and increasing HRV. This is further supported by larger proportion of recovery achieved among the mindfulness and rest groups relative to the no-intervention group. The post-intervention HR values were considerably closer to their respective baseline values for athletes in the mindfulness and rest groups, suggesting an accelerated recovery. Specifically, regarding HR, the mindfulness and rest groups were 78% and 53% recovered with the former 14 bpm above and latter above 28.6 bpm their baseline HRs. Comparatively, the no-intervention group appeared only 4.3% recovered with athletes, on average nearly 60 bpm above their baseline HR. Similar trends were shown for the HRV indices, however the proportion of recovery was considerably less: SDNN (40%–58%) and rMSSD (27%–37%), expectedly. Importantly, the extent of PNS reactivation was achieved in a small timeframe, only 15 min. The minimum time commitment required for effective PNS reactivation using these techniques may reduce the implementation burden making it a more efficacious and sustainable practice. Research shows that interventions of increased complexity demanding many resources and time, are prone to reduced compliance, minimized effects, and eventual withdrawal (47, 48).

Lastly, this study also observed significant differences in HR, RR and HRV between the two relaxation techniques. Specifically, athletes in the mindfulness group exhibited larger within-group reductions and lower post-intervention HRs and RRs (−81.6 vs. −66.4 bpm and −31.7 vs. −26.9 breaths⋅min⁻¹, respectively) and higher SDNN and rMSSD values (+27.4 vs. +22.7 ms and +8.6 vs. +7.3, respectively) compared to the rest group. This finding is rather intuitive as the athletes in the resting group were exposed to more stimuli like lights, conversation, eating and drinking (49, 50). Moreover, the athletes were sitting upright resulting in increased skeletal muscle activation in addition to consuming foods yielding higher HRs, RRs and lower HRV. Despite the greater effectiveness of the mindfulness intervention, the rest group still elicited a significant effect on parasympathetic reactivation. This is encouraging as the resources (e.g., lighted room and seating) are minimal and specialized equipment is not required (e.g., mindfulness instructor or recording), lowering the burden of implementation (47, 48). Furthermore, providing a 15-minute resting period following training sessions may offer athletes an opportunity to bond with team members, positively influencing team cohesion (26).

The observations of the current study demonstrate that implementing brief relaxation techniques like guided mindfulness and low-key resting period following exercise training elicits a powerful effect on parasympathetic reactivation. This effect possibly facilitates an early onset of recovery, which is critical for athletes chronically exercising at high frequency, volume, and intensity, like collegiate football players. Importantly, this effect may reduce the physical, mental, and emotional demands occurring throughout the remainder of their day, which could also aid in a faster physiological recovery. Lastly, the brevity and low burden of this intervention increase its efficacy, especially as large effects were also observed for the resting group. As such, coaches may be more willing to implement and sustain these types of interventions. For future investigations, we strongly recommend researchers conduct longitudinal studies to evaluate longer-term effects on parasympathetic reactivation, sports performance, and other physiological recovery variables. Additionally, we suggest that forthcoming studies include a larger and more representative sample of collegiate athletes including diverse sports and females.