Hypertension, or elevated blood pressure, leads to increased risk of cardiovascular disease (CVD) and is the number one leading risk factor for mortality [1]. However, hypertension is also one of the most significant modifiable risk factors in the prevention of cardiovascular disease [2]. With a prevalence of hypertension in the US population around 29% [3] and its predicted continual increase [4], it is a critical health concern to reduce hypertension and improve cardiovascular disease risk. Although pharmacological interventions are often relied on to reduce blood pressure, lifestyle modification is the first line of therapy suggested by several governing bodies [5–7].

Lifestyle modifications often emphasize improvements in diet and exercise habits. The American College of Sports Medicine (ACSM), the American Heart Association, and others [8] have all provided professional exercise recommendations for adults with hypertension. However, most of these recommendations focus on aerobic exercise prescription [8]. The health benefits of aerobic exercise are well established [9–11], but less data exist in regards to the health benefits of resistance exercise, especially on cardiovascular health in individuals with elevated blood pressure [12]. Indeed, recent reviews and meta-analyses have established that aerobic and resistance exercise can both have a significant blood pressure lowering effect of approximately 3–4 mmHg in both systolic and diastolic blood pressure [13–16]. This small decrease has shown to be highly clinically relevant as it is estimated to reduce cardiac morbidity by 5%, stroke by 8–14%, and all-cause mortality by 4% in the average population.[13] Most earlier exercise studies on blood pressure and other CVD risk factors, however, have focused more on either aerobic or resistance training alone.

Recent studies have begun using a combination of aerobic and resistance exercise to determine if additive benefits exist in which an ~3 mmHg reduction in blood pressure has been observed [17]. Although a similar reduction in blood pressure compared to either aerobic or resistance training alone, both aerobic and resistance exercise provide independent, modality specific benefits toward cardiovascular disease risk factors. In general, aerobic exercise induces greater improvements in cardiorespiratory fitness and cardio-metabolic variables, whereas resistance exercise mainly effects muscular strength and has positive effects on body composition, such as muscle mass and bone density.

The combination of aerobic and resistance exercise could have an additive effect and further decrease the risk of CVD risk factors, however, there is a vast breadth of populations and co-morbidities included in studies on combination training, various exercise prescriptions and timing, and few with the sole focus on blood pressure reduction [17]. Additionally, many of the combined exercise training studies do not have an aerobic- or resistance-only group or are not a well-controlled randomized trials [15]. Therefore, it is unclear whether the additional benefits of combination exercise is simply due to extra exercise time rather than the independent additive benefits from each aerobic and resistance exercise. Further, data on well-controlled exercise interventions in middle-aged individuals with a heightened cardiovascular disease risk profile without overt disease are still scarce.

The purpose of this study was to compare the effects of time-matched aerobic training only, resistance training only, and combined aerobic and resistance training on blood pressure and CVD risk factors compared to a non-exercising control group. We conducted an 8-week randomized controlled trial in middle-aged adults with an elevated blood pressure or hypertension, overweight or obesity, and sedentary lifestyle. We hypothesized the combined aerobic and resistance training would elicit greater improvement in blood pressure and CVD risk factors compared with either training only group.

Fig 1 shows the flow of participants from recruitment to follow-up. Of the 69 individuals randomized, 66 (96%) completed the 8-week intervention. Mean exercise attendance was 96% in all groups except resistance training (92%). On average, aerobic exercise participants completed 100 ± 6% of the prescribed exercise amount in minutes, with a mean exercise intensity greater than what was prescribed (119 ± 13%) based on the heart rate measured during each exercise session throughout the intervention. Resistance training participants completed 100 ± 2% of prescribed sets, and exercised at the weight prescribed 99 ± 11% of the time. Based on pedometer counts, lifestyle physical activity outside the intervention did not change significantly over time (5689 ± 2005, 5429 ± 1883, 5404 ± 2125, 5664 ± 2285, 5466 ± 1896, 5535 ± 2116, 5191 ± 1893, and 5095 ± 2039 steps/day from week 1 to week 8, respectively; p = 0.69) and no difference was observed across groups (p>0.05). Also, no significant changes were noted in total calorie, fat, carbohydrate, protein, or sodium intake (p>0.05). No adverse events occurred in any of the intervention groups.

At baseline, participants were 58 ± 7 years old, 61% women, and had a body mass index of 32.4 ± 5.2 kg/m². Baseline resting systolic blood pressure was 131 ± 13 and diastolic blood pressure was 81 ±9 mmHg (Table 1).

Table 2 presents the data on the primary outcome variables. Following 8 weeks of exercise, the combination group reduced peripheral and central diastolic blood pressure -4 mmHg [95% CI: -6, -1] and -4 mmHg [95% CI: -7, -2], respectively. No significant improvements were observed in peripheral or central systolic blood pressure in any group. However, the aerobic and combination groups both showed a reduction in resting heart rate of -2 bpm [95% CI: -5, 1], which were significantly different from the control group (p<0.02).

Secondary outcomes of body composition, fitness, strength, and lipid profile are summarized in Table 3. Aerobic training yielded the greatest benefit in body composition with reductions in BMI (-0.3 kg/m² [95% CI: -0.7, 0.0]), weight (-1.0 kg [95% CI: -1.9, -0.1]) and fat mass (-0.9 kg [95% CI: -1.5, -0.2]). The resistance training group decreased waist circumference by -1.7 cm [95% CI: -3.3, -0.1] while the combination group increased both lean body mass (0.8 kg [95% CI: 0.1, 1.5]) and weight (0.9 kg [95% CI: 0.00, 1.8]). Improvements in both cardiorespiratory fitness and muscular strength were seen in the combined training group, with expected training benefits in aerobic and resistance training only. Cardiorespiratory fitness was increased in the aerobic training group and combination group by 7.7 ml/kg/min [95% CI: 3.9, 11.5] and 4.9 ml/kg/min [95% CI: 1.1, 8.7], respectively. However, no significant changes were observed in either the resistance or control group. Lower body muscular strength increased significantly in both the resistance (13 kg [95% CI: 4, 23]) and combination training groups (11 kg [95% CI: 2, 20]) in comparison with the control group. Upper body muscular strength increased in all exercise groups. Minimal changes occurred in fasting glucose and the lipid profile following training. The resistance training group reduced triglycerides (-26 mg/dL [95% CI: -47, -5]), as did the control group (-22 mg/dL [95% CI: -43, -1]).

Following the exercise intervention, the composite cardiovascular disease risk score indicated a slight reduction, although not significant, in the aerobic (-0.34 [95% CI: -1.46, 0.77]), resistance (-0.10 [95% CI: -1.25, 1.06]), and combination (-0.78 [95% CI: -1.89, 0.34]) training groups and increase in the control group (0.95 [95% CI: -0.20, 2.10]). In comparison, the reduction for the combination group was different from the control group (p = 0.04). We found similar results in other composite risk factor scores based on different CVD risk factors.

A specific goal of this study was to ensure that the total exercise time was consistent across exercise groups to be more applicable to the general population and help determine the effectiveness of each intervention style. The primary finding of this study was that only combined training provided significant changes in blood pressure, with reductions in diastolic pressure. Although participants in the aerobic and resistance training groups did receive benefit from exercise training in other aspects of cardiovascular health (i.e. body composition), the combined group experienced more cumulative benefits across all cardiovascular outcomes as indicated by the composite score. Despite aerobic exercise having the most well-known health benefits, this study supports the 2008 Physical Activity Guidelines recommendation of combination training and suggests individuals may receive greater and more complete CVD health benefits by performing both aerobic and resistance exercise.

Our training intervention was effective as we observed training-induced adaptations commonly reported after aerobic and resistance training: aerobic training led to significant increases in cardiorespiratory fitness and resistance training led to significant increases in muscular strength. Both aerobic and combination training had significant increases in cardiorespiratory fitness in this study, and the aerobic group had a larger increase in cardiorespiratory fitness than the combination group, although not statistically significant (effect size = 0.34). However, it is important to note that only the combined training group experienced significant benefits in both cardiorespiratory fitness and muscular strength in this study. While each exercise intervention elicited the expected changes in fitness and strength, the primary outcome of this study focused on blood pressure and CVD risk factors.

Our exercise intervention did not result in systolic blood pressure reductions. Meta-analyses and earlier studies have reported decreases in systolic blood pressure following aerobic or resistance exercise training alone or in combination [17,29–32]. This contradiction may reflect that most exercise interventions that elicit reductions in blood pressure have been at least 12 weeks long [30,33–35]. This could indicate that an 8-week intervention is not long enough to detect the expected blood pressure responses to exercise in this specific population. Additionally, participants qualified for the study based on orientation screening blood pressures, however, during the baseline visit 15 participants had a blood pressure <120/80. With normal baseline blood pressure, reductions with exercise are less likely to be detected [17] and could be another potential explanation to our results, although these participants were equally spread throughout each intervention group. Despite no changes in systolic blood pressure, significant reductions in diastolic blood pressure were seen with combination training.

Despite minimal changes in blood pressure, these data contribute to a growing body of literature. A recent meta-analysis was performed on randomized controlled trials looking at resistance training only in pre- and hypertensive individuals [12]. Only 5 studies were available, in which 4 had a population over the age of 60 years. Although many trials have been performed with aerobic training in this population, this stresses the importance of performing well-controlled resistance and combination exercise interventions in a middle-aged population with elevated risk factors without overt disease.

Exercise in conjunction with changes in body composition have previously shown benefits for blood pressure reduction and other CVD risk factors [36]. In regards to body composition, the aerobic training group had the most significant improvements with reductions in BMI, weight, fat mass, and body fat percentage. A recent meta-analysis of randomized controlled trials comparing aerobic, resistance, and combination training found aerobic training resulted in greater fat mass reductions than resistance training [37]. Although not significant, all training groups had a reduction in body fat percentage, which lends support to exercise training as an effective intervention for healthy weight management. Combination training increased weight slightly but elicited beneficial changes in body composition (increase in lean body mass). Overall, training-induced changes in fasting blood lipids and glucose were small and did not vary between training and control groups. It was not surprising that fasted lipids and glucose were unaltered by exercise training considering all groups were within normal ranges at baseline.

The primary outcome of our paper was blood pressure reduction, however, the study of a single CVD risk factor does not fully elucidate the cumulative benefit of exercise on multiple aspects of CVD risk studied, such as blood pressure, body composition, fitness, strength, and metabolism. Furthermore, individuals who do not show improvement in one CVD risk factor may show improvements in other CVD risk factors. One way to assess overall risk is to create a composite score using the sum of standardized scores [38,39]. Our composite score suggested that combination training elicits greater overall improvements in CVD risk factors than the control group. This provides support for the prescription of combination training for the greatest overall benefits from exercise training in middle-aged adults with elevated CVD risk.

Strengths of this study include the randomization that ensured participants were comparable at baseline in major confounding factors (e.g., age, sex), orientation sessions to minimize potential drop-out that led to high attendance and adherence, intention-to-treat analysis to include all participants randomized at baseline, and participants at high risk for developing CVD. Additionally, unlike previous studies [25,40], all exercise sessions had equal training time with objective verification of the amount and intensity of exercise being completed. Objective measurement of all exercises in this study eliminated the possibility of over- or under-reporting. Furthermore, the exercise prescriptions were well tolerated by sedentary, overweight/obese individuals, making them easily obtainable by a more general population. Lastly, participants wore a pedometer throughout the intervention period to minimize the uncertainty about changes in physical activity outside the intervention, commonly seen in other studies.

A randomized clinical trial is not without its limitations. One limitation comes from cardiorespiratory fitness being estimated using a submaximal treadmill test instead of a maximal effort. Although these values may be overestimated at baseline and with the change following training, the consistency in measurement technique still allows for a comparison to be made between groups and with training. Our control group was also not a true non-treatment group since they received the lifestyle educational session and information regarding diet prior to starting. Furthermore, although the reported steps and diet did not change significantly throughout the study, volunteers for an exercise intervention tend to be a motivated group. Thus, subtle lifestyle changes that could not be accounted for in this study may have contributed to our findings.

Among individuals at high risk of developing CVD with an elevated blood pressure or hypertension, a combination of aerobic and resistance exercise training resulted in improved diastolic blood pressure, increased lean body mass, and increased strength and cardiorespiratory fitness, even though the exercise intervention was only 8 weeks long. Moreover, these data suggest that combination training may be of better value than either aerobic or resistance training alone, as it appeared to have the most beneficial effect on the composite of CVD risk factors. However, further studies with a larger sample size and longer intervention is clearly warranted on the overall cardiovascular benefits of different types and combinations of exercise. Cumulatively our data suggest that concurrent aerobic and resistance training may be a more potent means to improve CVD risk factor burden among at-risk middle-aged adults than aerobic or resistance exercise alone.