Muscle strengthening activities, including resistance training, are encouraged as a beneficial form of exercise that builds and maintains muscle mass (1), improves body composition (2), strengthens bone density (3), increases cognitive function (4), and lowers blood pressure (5, 6). The Canadian 24-h Movement Guidelines for adults (7) and World Health Organization (8) recommend engaging in muscle strengthening activities (e.g., resistance training, very heavy gardening, etc.) at least 2 times per week. Compared to aerobic exercise, there is limited research on the cardiovascular impacts of resistance training, with early work contributing to the dogma that it may promote arterial stiffness (9, 10). However, recent scientific statements made by the American Heart Association concluded that resistance training can lead to improvements in both traditional (e.g., blood pressure, glycemia, and body composition) and non-traditional (e.g., arterial stiffness, endothelial function, cardiorespiratory fitness) risk factors for cardiovascular disease (11). The American Heart Association (11) reported that resistance and aerobic training improve blood pressure to a similar degree (12), and that adding resistance training in combination with aerobic exercise may provide more benefits to reducing the risk of cardiovascular disease than aerobic training alone. It is important to consider the impact that resistance training has on specific markers of cardiovascular health, including arterial blood pressure (13). A meta-analysis demonstrated resistance training effectively lowered blood pressure in healthy adults (6). However, the magnitude of improvement was dependent upon the mode of resistance training, with greater reductions in blood pressure resulting from isometric handgrip exercises when compared to dynamic resistance training (6). Overall, additional evidence to support or refute the cardiovascular impacts of muscle strengthening exercises are needed.

Blood pressure is regulated on a beat-by-beat basis by the autonomic nervous system whereby the arterial baroreflex modulates sympathetic and cardiovagal activity to maintain blood pressure around an operating point. Cardiovagal baroreflex sensitivity (cvBRS) refers to the sensitivity of the baroreflex in modulating R-R interval (RRI) caused by changes in systolic blood pressure (SBP) (14, 15). Poor cvBRS is associated with a greater risk of hypertension and the occurrence of a major negative cardiovascular event (16). Understanding the impact of the frequency of weekly muscle strengthening exercise on cvBRS may provide important information on the cardiovascular benefits of resistance training.

Although there are complexities in the relationship between resistance training and blood pressure regulation (17, 18), there appears to be a consensus on the favorable impact of resistance training on blood pressure (6). However, current literature has mixed evidence as to whether it improves (19), does not change (20, 21), or decreases cvBRS (22). Specifically, low intensity (30% 1-repetition-maximum), low frequency (maximum 3 days/week) isometric handgrip exercise training for 8 weeks improved cvBRS in treated hypertensives (19). Lower cvBRS has been observed in males who strength trained 5 days/weeks for >2 years compared to untrained controls (22). In contrary, cvBRS was unchanged follow 8 weeks of 3 days/week of heavy resistance training in young adults (20), and 10 weeks of 3 days/week of periodized total body resistance training also in young adults (21). Given the contradicting results in the field, the primary purpose of this study was to (1) determine the relationship between muscle strengthening activity frequency and cvBRS, and (2) assess the impact of meeting the muscle strengthening activities guidelines (i.e., 2 days/week) on cvBRS. Given the conflicting reports (19, 20, 22), we did not have a directional hypothesis; however, with the support of public health activity guidelines, we believe that higher frequency habitual muscle strengthening will be associated with higher cvBRS.

Detailed participant characteristics are presented in Table 1. Average overall cvBRS was 14.9 ± 9.1 ms/mmHg (3.1–48.4 ms/mmHg) and average BEI was 0.48 ± 0.15 (0.05–0.94).

Muscle strengthening activity frequency was positively correlated with overall cvBRS in the covariate-adjusted model (overall model: adjusted R² = 0.419, β = 2.17, both, p < 0.001; raw weight 0.27; Figure 1, Table 2), as well as with up- and down-sequences (Supplementary Figures S1, S2). A sensitivity analysis removing those who engaged in 0 days/week did not change any results. Relative weights analysis confirmed that muscle strengthening activity frequency was the primary predictor for overall-, up-, and down- cvBRS (range: 50%–68%), with age being the only other significant predictor of overall- and down- cvBRS (range: 26%–38%) (Table 2). Meeting muscle strengthening frequency guidelines was also associated with a greater overall cvBRS (overall model: adjusted R² = 0.306, β = 7.23, both, p < 0.001; raw weight 0.17; Figure 2; Supplementary Table S2), and up- and down-sequences (Supplementary Figures S3, S4). Relative weights analysis confirmed that meeting muscle strengthening guidelines was the primary predictor for overall-, up- cvBRS, and a secondary predictor for down- cvBRS (range: 41%–55%), with age being the only other significant predictor for overall- and down- cvBRS (range: 33%–46%) (Supplementary Table S2).

Muscle strengthening activity frequency was not associated with SBP (overall model: adjusted R² = 0.120, p < 0.001, β = 0.32, p = 0.497) or DBP (overall model: adjusted R² = 0.129, p < 0.001, β = −0.19, p = 0.681), but was positively associated with RRI (overall model: adjusted R² = 0.255, β = 0.03, both, p < 0.001; raw weight 0.10; Supplementary Table S1). Relative weights confirmed that age (55%) and muscle strengthening activity frequency (35%) were both predictors of RRI. Neither muscle strengthening frequency nor meeting guidelines were associated with BEI, up-BEI, or down-BEI (all, p > 0.14; Supplementary Table S3).

This study aimed to provide clarification regarding the impact of muscle strengthening activity frequency on cvBRS. In a heterogeneous group of adults, participants who engaged in more frequent weekly muscle strengthening activities had higher cvBRS after adjusting for MVPA, age, sex, and BMI, with muscle strengthening frequency exhibiting the greatest relative importance. This study highlights the benefits of regular muscle strengthening activities and meeting the associated guidelines on vagally-mediated blood pressure regulation. This study also provides a novel perspective on how more frequent bouts of muscle strengthening activity may be efficacious for beat-by-beat blood pressure control and helps clarify conflicting reports on the impact of muscle strengthening exercise on cvBRS (19, 20, 22).

The benefits of exercise on overall- and cardiovascular-specific health markers have been well established (32–36). The current resistance/muscle strengthening exercise literature on cardiovagal baroreflex function is limited and provides conflicting results (19, 20, 22). Our observations that more frequent muscle strengthening exercise sessions was associated with better cvBRS is consistent with previous findings implementing a lower body resistance training program (4–5 sets of 6–10 repetitions, combination of sets and repetitions depended on the week of the program participants were in, 2 days/week, 19 weeks) (37). Acutely, handgrip exercise augments cvBRS in healthy adults, whereby it was increased 10-minutes after a single bout of handgrip exercise completed for 4 sets of 2 min at 30% of maximal contraction force (29). In contrast to the current study and these submaximal handgrip and resistance training studies, opposing results have been observed in smaller samples of healthy young adults following heavy strength training (20) and cross-sectional studies of young males who frequently engaged in resistance training (22, 38). The current study has been conducted in a large sample including males and females of a variety of ages. The current study also focused on muscle strengthening activities specifically to align with (inter)national guidelines that may include resistance training. However, it is quite possible that these activities are completed at a much lower intensity or volume (e.g., gardening) than those of Nakamura et al. (22), in which participants were long duration (>2 years) and high frequency (>5 days/week) resistance training, and Figueroa et al. (38) whose participants were more experienced with resistance training (4–7 years) at moderate to high intensity (≥2 days/week). In addition, different methodologies of assessing cvBRS spontaneous in the present study vs. during dynamic maneuvers [e.g., handgrip exercise (38), Valsalva's Maneuver (22), etc.] that engage several physiological processes beyond the vagal arm of the arterial baroreflex (i.e., cardiopulmonary afferents, chemoreceptor afferents, pulmonary afferents, etc.) may contribute to the divergent observations. Despite the heterogeneity regarding the impact of muscle strengthening exercises on cvBRS, our study adds to the literature by supporting the overarching notion that more frequent muscle strengthening exercise is associated with better vagally-mediated blood pressure regulation, which may partially explain the positive cardiovascular effects of strengthening exercise.

This study provides evidence that participants who met muscle strengthening guidelines had greater cvBRS. Higher cvBRS indicates that there is a greater ability of the vagal arm of the baroreflex to modify beat-by-beat SBP, which is characteristic of a lower risk of cardiovascular events and hypertension (39), and maintaining normal blood pressure (40). The physiological mechanisms underpinning our observations are unclear but may be a result of muscle strengthening activity-induced vascular or neural adaptations. An umbrella review (41) reported that low-to-moderate intensity (42, 43), but not high intensity (44), resistance training led to reduced central artery arterial stiffness. A greater compliance of barosensory-containing arteries (e.g., carotid arteries) to distend during pressure would result in an improved mechanical component of the cardiovagal baroreflex (43). As well, physical activity has been linked to greater concentrations of muscarinic receptors at the sinoatrial node (45), and structural remodelling of the neurons in the nucleus tractus solitarius of the medulla oblongata, a brain region that receives neural signals from baroreceptors involved in the cardiovagal baroreflex arc (46). Whether these neural changes are stimulated by frequent muscle strengthening activities is unclear but worthy of future study.

This present study is strengthened by its investigation into a large sample of male and female participants across a wide age range. This study is also strengthened in its assessment of habitual muscle strengthening activities in consideration of (inter)national guidelines, which coincides with public health messaging. We did not focus on direct measures of muscle strength as the objective of this study was to determine how muscle strengthening frequencies and meeting physical activity guidelines, not muscle strength per se, was associated with cvBRS. However, this study does not come without certain limitations. This study is cross-sectional in nature, limiting the ability to establish causal relationships between muscle strengthening and cvBRS. Future studies should consider following a longitudinal approach to confirm causality by integrating muscle strengthening training and examining its impact on cvBRS. Another limitation is the use of self-report questionnaires to gather muscle strengthening frequency data from participants in which the validity of resistance training questionnaire is not well-established (e.g., number of muscle strengthening sessions reported vs. an objective criterion) and they do not specify the mode of muscle strengthening that participants engaged in. Our study was limited to the data collected based on the specific wording of the question and were unable to collect data on specific exercise type, intensity, or history of exercise. Additionally, self-report measures may introduce over- or underestimation of frequencies. Future studies should consider using objective measures of measuring muscle strengthening frequency and collecting specific data on exercise type and intensity. Interventional studies could also consider implementing groups who complete different types of exercise to determine how this influences the results we have reported. An inability to account for diet and sex hormones in the analysis can also be considered a limitation, as a healthier diet and higher levels of estrogen may augment cvBRS, although neither BMI nor sex were associated with cvBRS in our study. Nevertheless, future studies should control for diet and hormones during testing sessions if possible. Additionally, it is important to note that we were unable to replicate previous findings of a positive relationship between objectively measured MVPA and cvBRS, which was reported using a subsample of the population from this study, providing further evidence to support the discrepancy between objective vs. subjective assessment of MVPA. This study is not mechanistic in approach and does not include measures that might explain changes in cvBRS such as arterial distensibility (47), central artery stiffness (41), neural changes to the concentration of muscarinic receptors on the sinoatrial node (45), the anatomy of the neurons in the nucleus tractus solitarius (46), or vagus nerve activity using microneurography (48). Future work should focus on measuring the neural adaptations and arterial stiffness to clarify the effects of exercise on these structures involved in autonomic function to improve the understanding of the mechanisms underlying this relationship.

This study provides evidence that higher frequencies of muscle strengthening activities are associated with improved cvBRS and meeting the current physical activity guidelines recommending muscle strengthening activities at least twice per week was independently associated with better vagally-mediated blood pressure regulation. Overall, this study highlights the cardiovascular benefits of individuals engaging in muscle strengthening activities and provides evidence for some of the benefits of achieving public health recommendations for muscle strengthening activities on beat-by-beat blood pressure regulation.