Background Regular aerobic exercise prevents and reverses arterial stiffening, but the association between resistance training and arterial stiffness is unclear.
Aim This study was performed to conduct a systematic review and meta-analysis of randomised controlled clinical trials (RCTs) assessing the associations between resistance training and changes in arterial stiffness.
Methods MEDLINE and SPORTDiscus databases were searched from January 1980 through to April 2011. RCTs evaluating the ability of resistance training to increase arterial stiffness in comparison with a control group were included in the meta-analysis. Two independent reviewers extracted data and assessed the quality of the included studies. Data from 185 reports of eight RCTs (193 participants) were included. Pooled mean differences in arterial stiffness indices (carotid arterial β stiffness and pulse wave velocity (PWV)) between intervention and control groups were calculated using a random-effects model.
Results The overall association of resistance training versus control with relative changes in carotid β index or PWV (eight studies; 193 participants) was 10.7% (95% CI 3.4% to 18.0%; I2, 89%; heterogeneity, p<0.001). Five studies indicated that resistance training in young subjects (n=115) was significantly associated with an increase in stiffness index of 14.3% (95% CI 8.5% to 20.1%; I2, 71%; heterogeneity, p<0.001) compared with controls. However, three studies showed that resistance training in middle-aged subjects (n=78) was not associated with changes in arterial stiffness. In addition, although high-intensity resistance training (n=87) was significantly associated with an increase in stiffness of 11.6%, moderate-intensity resistance training (n=106) showed no such association.
Conclusion High-intensity resistance training is associated with increased arterial stiffness in young subjects with low baseline levels of arterial stiffness.
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Increases in arterial stiffness impair arterial buffering function and contribute to elevation of systolic blood pressure, left ventricular hypertrophy, coronary ischaemic disease and reduction of arterial baroreflex sensitivity.1,–,3 Indeed, greater arterial stiffness is associated with a higher rate of mortality in patients with end-stage renal failure and essential hypertension,4 as well as hypertension in normotensive men.5 ,6 Therefore, prevention and treatment of pathological changes in arterial stiffness are of paramount importance.
Previous studies have demonstrated that regular aerobic exercise is efficacious in preventing and reversing arterial stiffening in healthy adults.7,–,9 In recent years, resistance exercise, another common exercise modality, has gained widespread acceptance in exercise prescription and cardiopulmonary rehabilitation programmes, and has become an integral component of comprehensive health programmes endorsed by major health organisations.10,–,13 These recommendations are based primarily on the documented impact of resistance training on the attenuation of osteoporosis and sarcopaenia and related risks, including falling and functional disability.10 ,14 However, little information is available regarding the potential influence of resistance training on non-musculoskeletal components, in particular, cardiovascular function. In marked contrast to the favourable effects of regular aerobic exercise on arterial stiffness,7,–,9 we and other groups have found that several months of resistance training ‘increases’ central arterial stiffness in healthy men.15,–,21 On the other hand, no such changes were observed in several other studies.22,–,25
The small sample sizes in these studies may have been responsible, at least in part, for the observed discrepancies. Meta-analysis is especially appropriate when the number of studies is small and/or the number of subjects that can be enroled in any one study is small. To our knowledge, there have been no previous meta-analyses to examine the effects of resistance training on arterial stiffness. Therefore, this study was performed to conduct a systematic review and meta-analysis of the effects of resistance training on arterial stiffness.
Search strategy and study selection
We searched the MEDLINE (accessed via PubMed) and SPORTDiscus electronic databases covering the period from January 1980 through April 2011. In addition, we searched the references of published studies manually. The initial search consisted of the terms (resistance or strength or weight) and training and (artery or arterial) and (compliance or stiffness) associated with a high-sensitivity strategy for the search of randomised control trials (RCTs). Only eligible full articles in English were considered for review.
We included RCTs that compared any category of resistance exercise training with a control group that evaluated arterial stiffness as an outcome, and reported means or differences between means and respective dispersion values of arterial stiffness at baseline and after the intervention. Studies using outcomes other than pulse wave velocity (PWV) and carotid β stiffness index were excluded.3 ,26 ,27
Titles and abstracts of retrieved articles were independently evaluated by two investigators (M. M. and N. T.). Abstracts that did not provide sufficient information regarding the inclusion and exclusion criteria were retrieved for full-text evaluation. Two reviewers independently evaluated full-text articles and determined the eligibility for this study. Moreover, they independently conducted data extraction from eligible studies. The major categories of variables encoded were (1) physical characteristics of subjects, (2) stiffness assessment characteristics, (3) training programme characteristics and (4) treatment effects (mean and SD values of changes in arterial stiffness for baseline and postintervention in training and control groups). The majority of studies included in this meta-analysis reported only mean±SD for baseline and postintervention. The corresponding authors of all five of these studies were contacted to obtain data regarding the treatment effects. Four of these five authors responded, and the treatment effects for the remaining one study were calculated in accordance with the previous study.28
Statistical analysis was performed with Review Manager Software (RevMan 5.0; Cochrane Collaboration, Oxford, UK). Absolute changes in carotid β index and PWV are reported as differences between arithmetic means before and after interventions. Moreover, relative (%) changes were also calculated. Pooled-effect estimates were obtained by comparing the least-squares mean percentage changes from baseline to the end of the study for each group, and were expressed as the weighted mean differences between groups. Calculations were performed using a random-effects model. In all analyses, p<0.05 was considered to indicate statistical significance. Subgroup analysis was performed according to age (young: <40 years old; middle-aged: ≥40 years old) and training intensity (high: >70% 1 repetition maximum (1RM); moderate: 40–70% 1RM). The cut-off point of training intensity was based on American Heart Associationreview in which high and moderate intensity were defined as ~80% and 30%–60% of 1RM, respectively.29
Statistical heterogeneity of the treatment effect among studies was assessed using the Cochran Q test. A threshold p value of 0.1 was considered statistically significant. The inconsistency I2 test was performed and values >50% were considered indicative of high heterogeneity. Selection bias was examined visually using the funnel plot method.
Description of studies
From 185 potentially relevant citations retrieved from electronic databases and searches of reference lists, eight RCTs fulfiled the inclusion criteria. A flow diagram of search and selection is shown in figure 1. The included studies had a total of 193 participants. The characteristics of these studies are summarised in table 1.
Association of resistance training with arterial stiffness
The overall association of any resistance training versus control with relative changes in carotid β index or PWV (eight studies; 193 participants) was 10.7% (95% CI 3.4% to 18.0%; I2, 89%; heterogeneity, p<0.001) (figure 2). A funnel plot of sample size against the effect size was examined. The plot did not show any asymmetry, indicating that significant publication bias was unlikely. From observations of data, for example, publication year, randomisation process and losses to follow-up, selection bias did not markedly affect the results of the present study. In addition, the related factors of age, sex, training intensity or volume, and BMI varied, and these factors were examined visually. The results detected biases by age and training intensity. Thus, subgroup analysis was performed according to age (young or middle-aged) and training intensity (high or moderate).
Five studies demonstrated that resistance training in young subjects (n=115) was significantly associated with an increase in stiffness of 14.3% (95% CI 8.5% to 20.1%; I2, 71%; heterogeneity, p<0.001) in comparison with control (figure 2). Four of the five studies used high-intensity resistance training, while the other study used training of moderate intensity. On the other hand, three studies showed that resistance training in middle-aged subjects (n=78) was not associated with a change in arterial stiffness (mean difference: −0.6%, 95% CI −10.8% to 9.6%). All of the three studies used moderate-intensity training.
Although high-intensity resistance training (n=87) was significantly associated with an increase in stiffness of 11.6% (95% CI 7.3% to 15.9%; I2, 54%; heterogeneity, p<0.001), moderate-intensity resistance training (n=106) showed no such association, because there was a large degree of variability between the studies (mean difference: 9.1%, 95% CI −5.9% to 24.2%).
Our systematic review and meta-analysis demonstrated that resistance training is associated with an increase of ~11.0% in arterial stiffness. Interestingly, resistance training in middle-aged subjects was not associated with an increase in arterial stiffness, but that in young subjects showed a significant association with an increase in stiffness compared with controls. To our knowledge, this is the first systematic review and meta-analysis to assess the association between resistance training and arterial stiffening.
The current guidelines endorsed by major health organisations recommend resistance training.11,–,13 The present finding that resistance training is associated with arterial stiffening may discourage this practice. However, we should emphasise that the magnitude of the relative increase in arterial stiffness was only ~11%, the absolute increase in carotid β was 1.12 AU (95% CI 0.66 to 1.58 AU), and the increase in PWV was 72 cm/s (95% CI −3 to 148 cm/s). Such levels of arterial stiffening may not have clinically adverse effects, especially in young adults with low baseline levels of arterial stiffness.26 ,27 ,30 In addition, although high-intensity resistance training (n=87) was significantly associated with an increase in stiffness of 11.6%, moderate-intensity resistance training (n=106) showed no such association. Furthermore, the blood pressure response during dynamic resistance exercise using large muscle groups may be attenuated in middle-aged men relative to young men.31 Taken together, based on the role of resistance training on the maintenance of functional ability and the prevention of osteoporosis and sarcopaenia, ‘properly prescribed’ moderate-intensity resistance training based on evidence and guidelines should still be encouraged, particularly in middle-aged and older adults.
The critical question remains whether any type of high-intensity resistance training can be performed regularly without inducing arterial stiffening in young adults. Recent studies suggested that simultaneously performed aerobic exercise training, for example, walking and jogging for 30 min at moderate-intensity,17 lower-limb resistance training20 or eccentric resistance training19 did not induce stiffening of central arteries. In contrast to resistance training, regular aerobic exercise is known to be efficacious for preventing and reversing arterial stiffening in healthy adults.7 ,32,–,34 In fact, the current guidelines endorsed by the major health organisations recommend muscle-strengthening activity and moderate-intensity to vigorous-intensity aerobic activity.10,–,12 Therefore, although further studies are necessary, simultaneous aerobic exercise (moderate-intensity walking or jogging for ~30 min) could prevent the stiffening of central arteries caused by high-intensity resistance training in healthy young adults.
The physiological mechanisms underlying the arterial stiffening associated with high-intensity resistance training are not yet clear. Intense resistance training is known to be a strong stimulus to increase sympathetic nervous system activity,35 ,36 which may act to increase arterial stiffness by providing chronic restraint on the arterial wall via greater sympathetic adrenergic vasoconstrictor tone.37 During each bout of high-intensity resistance exercise, arterial blood pressure is also known to increase to as high as ~320/250 mm Hg.38 These acute intermittent elevations in arterial blood pressure during resistance exercise may alter the arterial structure, or arterial load-bearing properties or both. Further studies are required to determine the physiological mechanisms underlying the influence of resistance training on central arterial stiffness.
There were some limitations to this meta-analysis. First, although only RCTs were included in the analysis, the limitation may be acceptable as the quality of intervention studies may be affected by many confounding biases. Our systematic review identified three non-RCT intervention studies examining the effects of resistance training on arterial stiffness.16 ,22 ,24 Second, publication bias is always a concern in meta-analyses. We performed electronic searches, including a manual search, and examined sample size against effect size by funnel plot analysis. The funnel plot suggested that there was little influence of publication bias on the effect size. Third, this analysis was confined to articles in the English language literature, and data extraction was unblinded, both of which are potential sources of bias. Fourth, this meta-analysis did not include studies regarding the effects of light-intensity (<30% 1RM) resistance training or those in older subjects (>65 years old), and further investigations are therefore necessary. Finally, the author of the present study is also the principal investigator of two articles selected in this meta-analysis.17 ,18 However, this potential conflict did not affect the procedures or results of this meta-analysis because evaluation for eligibility and data extraction were performed by two independent reviewers.
High-intensity resistance training is associated with an increase in arterial stiffness in young participants with low baseline levels of arterial stiffness. Further studies are warranted to address whether the relatively small increase in arterial stiffness reported with high-intensity resistance training is relevant to cardiovascular health.
What is already known on this topic
The current guidelines endorsed by major health organisations recommend resistance training.
Regular aerobic exercise prevents and reverses arterial stiffening, but it is unclear and in controversy whether resistance training increases arterial stiffness.
What this study adds
This meta-analysis indicates that resistance training is associated with an increase of ∼11% in arterial stiffness.
Although high-intensity resistance training is associated with increased arterial stiffness in young with low baseline levels of arterial stiffness, moderate-intensity resistance training in middle-aged was not.
The prescribed moderate-intensity resistance training should be encouraged, particularly in middle-aged and older adults.
The author thanks Dr Noriko Tanaka, Dr Kyoko Taku and the corresponding authors of eligible studies in this meta-analysis for their help in data acquisition and analyses.
Contributors The author contributed to (1) the conception and design, or analysis and interpretation of data, (2) drafting the article or revising it critically for important intellectual content and (3) final approval of the version to be published.
Funding This study was supported by a Grant-in-Aid for Scientific Research (#232400089, M Miyachi) and a Grant-in-Aid for Scientific Research from the Ministry of Health, Labour and Welfare of Japan (M Miyachi).
Competing interests None.
Provenance and peer review Not commissioned; externally peer reviewed.
▸ References to this paper are available online at http://bjsm.bmjgroup.com