Increased wave reflection (augmented pressure (AP) and augmentation index (AIx)) and reduced muscle strength may increase cardiovascular risk in postmenopausal women. We evaluated the effects of whole-body vibration exercise training (WBVET) on aortic haemodynamics and leg muscle strength. Twenty-eight postmenopausal women (age, 56±3 years; brachial systolic blood pressure (SBP) 138±12 mm Hg; body mass index, 33.9±3.7 kg m−2) were randomized to 6 weeks of WBVET (n=15) or no-exercise control groups. Aortic SBP, diastolic blood pressure (DBP), pulse pressure (PP), AP, AIx, tension time index (TTI, myocardial oxygen demand) and leg press muscle strength were measured before and after 6 weeks. WBVET significantly (P<0.05) decreased aortic SBP (∼10 mm Hg), DBP (∼5 mm Hg), PP (∼5 mm Hg), AP (∼5 mm Hg), AIx (∼10%) and TTI (∼311 mm Hg s per minute), while increased muscle strength (∼9%) compared with no changes after control. Changes in AP and leg muscle strength were correlated (r=−0.58, P=0.02). Our data demonstrated that WBVET reduced pressure wave reflection magnitude and aortic blood pressure in postmenopausal women with prehypertension or hypertension. Our study suggests that WBVET may decrease cardiovascular risk in postmenopausal women by improving wave reflection and muscle strength.
As a result of ageing and obesity,1 impaired vasomotor tone increases pressure wave reflection from peripheral arteries to the aorta causing augmented pressure (AP).2 AP is the main determinant of increased aortic systolic blood pressure (SBP), pulse pressure (PP) and augmentation index (AIx), and is greater in older women than in men.3, 4 This high pressure wave reflection predisposes women to myocardial ischemia due to increased oxygen demand.2
In addition to arterial ageing, there is a greater loss of leg muscle strength than muscle mass in older adults.5 Interestingly, leg muscle strength loss, but not muscle mass, has been associated with mortality in older individuals.6 It appears that high muscle strength has a protective effect on all-cause mortality in men with hypertension.7 A recent cross-sectional study reported an inverse relationship between explosive leg muscle strength and AIx in older adults.8 Because the age-related increase in wave reflection and subsequent risk for heart failure is greater in women than in men,4, 9 strategies that counteract or improve the negative consequences of wave reflection and muscle weakness in older women are needed.
Although the effectiveness of moderate to high-intensity resistance exercise training for improving muscle strength in middle-aged and postmenopausal women is well recognized,10, 11, 12 resistance exercise training may cause either a negative13 or no impact on AIx.11, 12, 14, 15 Alternatively, whole-body vibration exercise (WBV) training has shown to be a potential rehabilitation modality for muscular and arterial function. WBV training has proven to improve muscle strength in young16, 17 and postmenopausal women.18, 19 Recently, we found a decrease in aortic SBP and AIx following 6 weeks of WBV training in young normotensive overweight and obese women,16 but AP was not analysed. Acute WBV has been shown to decrease AP20 and induce arterial vasodilation in the exercised legs.21 Similarly, acute and short-term administration of vasodilatory drugs can attenuate AP by reducing wave reflection magnitude from peripheral arteries to the aorta.9, 22, 23 Therefore, WBV training would reduce AP and AIx through a reduction in wave reflection magnitude.
The purpose of this study was to examine whether WBV training improves aortic blood pressure (BP), wave reflection and muscle strength in obese postmenopausal women with prehypertension or hypertension. We hypothesized that WBV training would improve aortic haemodynamics and muscle strength and the improvements will be inversely related.
Materials and methods
Thirty women volunteered for this randomized parallel design study. Participants were postmenopausal (>1 year without menstruation),24 overweight or obese (body mass index (BMI)>25 kg m−2), prehypertensives or stage-1 hypertensives (SBP>120 mm Hg), and sedentary (<60 min of regular exercise training).25 Participants were non-smokers, free of apparent cardiovascular diseases as assessed by medical history and not taking vasoactive medication or hormone therapy. Participants were excluded if they had diabetes, joint prosthetic devices, recent thrombosis or wounds in the legs. All participants gave written informed consent. The study protocol was approved by The Florida State University Human Subject Committee and registered in ClinicalTrials.gov (NCT01741779).
Women were randomly assigned to either 6 weeks of WBV training (n=15) or non-exercising control group (n=15) (Figure 1). Participants reported to the laboratory following an overnight fast and refrained from caffeine and alcohol for 24 h or moderate-to-intense physical activity for 48–72 h before each visit. Haemodynamics were assessed after 15 min in the supine position in a quiet temperature-controlled room (23±1 °C). Thereafter, body composition and leg muscle strength were assessed. All measurements were repeated after the 6-week intervention period. After WBV training, cardiovascular testing was performed at least 24 h after the last WBV session. Women were advised not to change their regular lifestyles during the study. The control group was instructed to refrain from any mode of structured exercise training.
Pulse wave analysis
The average of two measurements of brachial SBP and diastolic blood pressure (DBP) was used to calibrate radial waveforms obtained in duplicate from a 10-s epoch using a high-fidelity tonometer (SPT-301B; Millar Instruments, Houston, TX, USA). Aortic pressure waveforms were derived using a generalized validated transfer function (SphygmoCor, AtCor Medical, Sydney, Australia). PP was calculated as SBP-DBP. AP is the difference between the second (P2) and first systolic peak (P1). The AIx was calculated as the AP expressed as a percentage of aortic PP ((P2−P1)/PP × 100). AIx normalized for a heart rate of 75 beats per minute (AIx@75) was also calculated. TTI, defined as the systolic pressure integral as a function of time per minute, was considered as a measure of myocardial oxygen demand.26 The average of two measurements of aortic haemodynamic with high quality (operator index 80%) was used in the analysis. In our laboratory, the intraclass correlation coefficient for resting aortic SBP, AP and AIx taken on 2 separate days is 0.95.
Leg muscle strength
Leg muscle strength was measured by the eight-repetition maximum (8RM) test27 using variable resistance equipment for the leg press exercise (MedX Corp., Ocala, FL, USA). The highest weight moved eight times through the full range of motion using a good form was considered as the 8RM. Changes in muscle strength are presented as percentage of the baseline 8RM (%strength).
Height was measured to the nearest 0.5 cm using a stadiometer and body weight was measured to the nearest 0.1 kg using a seca scale (Sunbeam Products Inc., Boca Raton, FL, USA). BMI was calculated as kg m−2. Body fat percentage (%) and bilateral arm and leg lean mass were determined from whole-body dual-energy x-ray absorptiometry scans (GE Lunar DPX-IQ, Madison, WI, USA).
Whole-body vibration exercise training
Participants underwent three supervised training sessions per week separated by at least 48 h for 6 weeks. The WBV training included leg exercises standing on a WBV platform (Power Plate pro5 AIRdaptive, Northbrook, IL, USA). Exercises consisted of both dynamic and static for semi-squats and lunges with a 120° knee angle (considering 180° as full knee extension), squats with a 90° knee angle and calf-raises. The dynamic exercises were performed with slow movements at a rate of 2 s for the concentric phase and 3 s for the eccentric phase. The vibration intensity was progressed by increasing the frequency (25–35 Hz) and amplitude (1 mm). The duration (30–45 s) and number of the sets (1–2) was progressively increased, while the interest rest period was maintained at 60 s. The selected training protocol is similar to that previously used in postmenopausal and obese women.16, 18
All variables were normally distributed (Shapiro–Wilk test). Based on previous data,16 it was calculated that 14 participants per group would provide 80% power (two-sided α=0.05) to detect a 6% difference in AIx. Unpaired t-test was used to identify possible group differences at baseline. The effect of the intervention over time was evaluated by a 2 × 2 analysis of variance with repeated measurements (group (control vs WBV) × time (before and after 6 weeks)). When a significant group-by-time interaction and/or time effect was identified, post hoc comparisons were made with paired t-tests to detect within-group differences across time. Pearson’s correlations were used to analyse the relationship between changes in wave reflection and %strength. A value of P<0.05 was accepted as statistically significant. Variables are presented as mean±s.d.
Two participants in the control group dropped out for personal reasons. Data are presented for 15 and 13 participants in the WBV and control groups, respectively (Figure 1). Time since menopause did not differ between the WBV (5.7±2.2 years (range: 3–10 years)) and control (5.9±2.1 years (range: 3–9 years)) group. Attendance to the exercise sessions, addressed from training logs, was >98%.
Table 1 shows participant characteristics, body composition, muscle strength and brachial BP before and after the interventions. There were no significant differences in these parameters at baseline. A significant group-by-time interaction (P<0.05) was detected for leg muscle strength and brachial BP. The 8RM increased after WBV training compared with no change after control. Brachial SBP and DBP significantly (P<0.01) decreased after WBV training compared with no changes after control. Body weight, BMI, body fat% and lean mass of arms and legs did not change in both groups.
Table 2 shows haemodynamic variables before and after 6 weeks of WBV training and control. There were no group differences in any variable at baseline. There were group-by-time interactions (P<0.05) for aortic SBP, DBP, PP, AP, AIx, AIx@75, P2 and TTI. Aortic SBP, DBP, PP, AP (Figure 2a), AIx (Figure 2b), AIx@75, P2 and TTI significantly (P<0.01) decreased after WBV training, but no significant changes occurred after control.
Correlations between wave reflection and muscle strength
The changes in %strength were correlated with changes in AP (r=−0.58, P=0.024) (Figure 3), but not with changes in AIx (r=−0.54, P=0.058).
The present study examined the impact of WBV training on pressure wave reflection in obese postmenopausal women with high BP. The main findings were that 6 weeks of WBV training decreased wave reflection magnitude and aortic BP. In addition, the relative increase in leg muscle strength was correlated with the decrease in AP.
Pressure wave reflection can be examined as the difference between the second and first systolic peak relative to aortic PP (AIx) or in absolute amount of AP. The increased AIx observed in our participants before WBV training may be explained by age, female sex and obesity.1, 3, 4 The reduction in AIx (−10%) found following WBV training in the present study is in agreement with our previous report (−8%) in young normotensive women.16 However, the magnitude of the decrease in AIx appears to be greater in older women with prehypertension and hypertension. In middle-aged adults, an increase in the reflected wave magnitude augments aortic SBP and PP, which in turn, increases the AIx.3, 28 This age-related increase in wave reflection, which is higher in women than in men,3, 4, 29 may contribute to the development of left ventricular diastolic dysfunction4 and coronary artery disease.30 A recent meta-analysis revealed that increases in AIx and aortic BP (SBP and PP) by 10% and 10 mm Hg would increase the risk for cardiovascular events by 31.8, 8.8 and 13.7%, respectively.31 In the present study, concurrently with the reduction in AIx, we noted decreases in aortic SBP (−11 mm Hg) and PP (−5 mm Hg) after WBV training. Thus, WBV training may reduce cardiovascular risk in postmenopausal women.
In the present study, AP (−5.1 mm Hg) and magnitude of the reflected wave (P2) were attenuated with WBV training, while the first systolic peak (P1) was not affected. The increase in AP in middle-aged adults is attributed to an increase in wave reflection magnitude.3, 29 AP increases by ∼6.7 mm Hg in middle-aged women over a 10-year period.9 The age-related increase in aortic haemodynamics in women may be induced by reduced nitric oxide-dependent vasodilation as acute9 and 4 weeks22 administration of nitrovasodilators decrease AP by 7.5–9 mm Hg. With attenuated magnitude of the reflected wave due to peripheral artery vasodilation, aortic SBP decreases, and thereby AP and left ventricular afterload.22, 23 Our present study has shown that WBV training have favourable effects on aortic haemodynamics in postmenopausal women with prehypertension and stage-1 hypertension, which would be attributed to the vasodilatory effect of WBV on leg muscular arteries.21
A negative relationship between radial AIx and explosive strength has been reported in older adults.8 Because AP progressively increases with ageing, while AIx may decrease with older age,3, 9 AP may be a more accurate marker of wave reflection than AIx in middle-aged and older adults.32 We observed an increase in leg muscle strength following WBV training with no significant increase in lean mass. Strength gain is a well-documented finding following WBV training.16, 18, 19 We found a negative relationship between changes in relative leg muscle strength and AP. This relationship may have important implications for older women with hypertension as increased muscle strength provides protection against all-cause mortality in men with hypertension.7 The protection conferred by leg muscle strength on the risk for coronary artery disease33 may be related to reduction in AP.30, 32 In middle-aged adults with high cardiovascular risk, aortic AP, but not AIx, was independently associated with prevalent coronary artery disease via reduced myocardial perfusion.32 In the current study, the reduction in aortic AP may have influenced the decrease in TTI, a marker of myocardial oxygen demand.26 Of note, reductions in AIx and AP have not been associated with strength gains following resistance exercise training in older adults.12, 14, 34
Our study has some limitations. Our participants were obese postmenopausal women with prehypertension or stage-1 hypertension and the observed findings may be influenced by adiposity, age, sex and BP category. Furthermore, blood gonadotropins and sex hormone levels were not measured and the postmenopausal status of our participants was self-reported. Another limitation of the present study may be the lack of an exercise no-WBV group. However, exercise without WBV does not induce an acute decrease in AIx, but an increase.20
In summary, we have shown that WBV training reduces wave reflection magnitude, aortic BP while increases leg muscle strength in obese postmenopausal women with prehypertension or hypertension. Our findings demonstrated an association between improvements in wave reflection magnitude and leg muscle strength, suggesting that WBV training may decrease cardiovascular and physical disability risk in postmenopausal women.
We thank Power Plate International for providing the vibration platforms. We also are grateful to the participants.
About this article
Benefits of whole-body vibration training on arterial function and muscle strength in young overweight/obese women
Hypertension Research (2017)