Menopause is associated with endothelial dysfunction,

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1 Menopause: The Journal of The North American Menopause Society Vol. 21, No. 10, pp. 000/000 DOI: /gme * 2014 by The North American Menopause Society Low-volume high-intensity interval training rapidly improves cardiopulmonary function in postmenopausal women Markos Klonizakis, PhD, 1 James Moss, PhD, 1 Stephen Gilbert, PhD, 1 David Broom, PhD, 1 Jeff Foster, MD, 2 and Garry A. Tew, PhD 3 Abstract Objective: This study compared the effects of a 2-week program of low-volume high-intensity interval training (HIT) with the effects of higher-volume moderate-intensity continuous training (CT) on cardiopulmonary and vascular functions in postmenopausal women. Methods: Twenty-two postmenopausal women were randomly assigned to undertake six HIT (n = 12) or CT (n = 10) sessions for 2 weeks. HIT sessions consisted of ten 1-minute intervals of cycling exercise at 100% of peak power output separated by 1 minute of active recovery. CT sessions involved 40 minutes of continuous cycling at 65% of peak power output. Variables assessed at baseline and 2 weeks included cardiopulmonary function (ventilatory threshold, peak oxygen uptake), macrovascular endothelial function (flow-mediated dilation of the brachial artery), and microvascular function (reactive hyperemia and local thermal hyperemia of forearm skin). Results: Eighteen participants completed the study (HIT, 11; CT, 7). Adherence to the exercise programs was excellent, with 107 of 108 sessions completed. Despite substantially lower total time commitment (È2.5 vs È5 h) and training volume (558 vs 1,237 kj) for HIT versus CT, increases from baseline in peak oxygen uptake achieved significance (P = 0.01) for the HIT group only ($ = 2.2 ml kg j1 min j1 ; P for interaction = 0.688). Improvements in exercise test duration were observed in both groups (HIT, 13%; CT, 5%; P for interaction = 0.194). There were no significant changes in macrovascular or microvascular function in either group. Conclusions: The findings suggest that low-volume HIT is feasible and can lead to rapid improvements in cardiopulmonary function in postmenopausal women. Key Words: Exercise Y Aerobic capacity Y Endothelial function Y Flow-mediated dilation Y Menopause. Menopause is associated with endothelial dysfunction, 1 which is defined as a decreased ability of the endothelium to induce vasodilation in response to specific stimuli due to an imbalance between vasoconstrictors and vasodilators. 2 Endothelial dysfunction is linked to classic risk factors for cardiovascular disease such as hypercholesterolemia, 3 hypertension, 4 and physical inactivity, 5 and is recognized as an early step in the development of atherosclerosis. 6 Therefore, endothelial function is an important therapeutic target and risk assessment marker in postmenopausal women. Brachial artery endothelial function is an accepted surrogate for coronary artery endothelial function 7 and can be noninvasively assessed using the well-established technique of flow-mediated dilation (FMD). 2,8 Received October 7, 2013; revised and accepted December 16, From the 1 Center for Sport and Exercise Science, Sheffield Hallam University, Sheffield, UK; 2 Sheffield Teaching Hospitals NHS Foundation Trust, Royal Hallamshire Hospital, Sheffield, UK; and 3 York Trials Unit, Department of Health Sciences, University of York, York, UK. Funding/support: None. Financial disclosure/conflicts of interest: None reported. Address correspondence to: Garry A. Tew, PhD, York Trials Unit, Department of Health Sciences, University of York, York YO10 5DD, UK. garry.tew@york.ac.uk Traditional endurance exercise training, which involves regular sessions of continuous moderate-intensity aerobic exercise for a period of weeks or months, is an effective strategy for reducing cardiovascular disease risk in postmenopausal women, 9,10 and improvements in endothelial function are suggested to account for a proportion of the associated risk reduction. 11 Although effective, continuous training (CT) is time-consuming, potentially limiting uptake and adherence. In recent years, high-intensity interval training (HIT) has gained considerable attention as an effective alternative to CT. Recent reviews of HIT studies highlight superior improvements in numerous physiological indices, including cardiopulmonary and endothelial functions, in comparison with CT in both healthy individuals and diseased populations. 12<14 However, most previous investigations used male participants and HIT and CT protocols that were matched for energy expenditure. No study has compared relatively timeefficient low-volume HIT to traditional higher-volume CT in postmenopausal women. Given that Black of time[ is a commonly cited barrier to doing more physical activity, 15 a low-volume HIT protocol might be a superior exercise strategy for uptake and adherence if the resultant physiological benefits are comparable. Two recent studies reported marked improvements in exercise capacity among recreationally active young men after a program of only six low-volume HIT sessions completed during a2-weekperiod. 16,17 In these studies, each exercise session Menopause, Vol. 21, No. 10,

2 KLONIZAKIS ET AL consisted of 8 to 12 1-minute intervals of cycling exercise at 100% of peak power output, separated by 75 seconds of recovery. Little et al 16 considered this protocol to be more Bpractical[ and Btolerable[ than those involving repeated Ball-out[ maximal cycling efforts (ie, repeated Wingate tests), which have been used in most other studies of low-volume HIT. The main purpose of the present study was to compare the effects of a similar 2-week protocol of low-volume HIT with the effects of higher-volume moderate-intensity CT on cardiopulmonary function and macrovascular endothelial function (brachial artery FMD) in postmenopausal women. These variables were selected because they are related to the future risk of mortality in asymptomatic women. 18,19 Given that endothelial dysfunction can occur in both the macrocirculation and the microcirculation and that these manifestations might be relatively independent of each other, 20 we also assessed changes in endothelial function in the microcirculation using laser Doppler flowmetry. Based on evidence from younger healthy populations, 16,17,21 we hypothesized that HIT and CT would evoke similar rapid improvements in cardiopulmonary and endothelial functions. METHODS Participants Twenty-two postmenopausal women were recruited via advertisements posted in and around Sheffield, UK. Inclusion criteria were as follows: postmenopause status (assessed by questionnaire), aged 55 to 85 years, and self-report of no regular purposeful exercise. Women who were hypertensive (systolic blood pressure 9140 mm Hg, diastolic blood pressure 990 mm Hg, or on medication for hypertension) or on hormone therapy were excluded, as were those with contraindications to exercise testing and training such as unstable angina and uncontrolled cardiac arrhythmias. Study design This was a two-arm, parallel-group, randomized trial. The experimental protocol consisted of familiarization procedures, baseline testing, a 2-week exercise training intervention, and posttraining measurements. After completion of baseline assessments, participants were randomly assigned either to a HIT or CT group. Allocation to HIT or CT was performed using a randomization sequence created by an independent researcher before study commencement (nquery Advisor 6.01; nquery Statistical Solutions, Cork, Ireland). The study researchers were made aware of this sequence on a case-by-case basis after baseline assessments had been completed. All participants commenced exercise training within 1 week of the baseline testing session, with posttraining measurements conducted 48 to 72 hours after the final training session. The study was approved by Sheffield Hallam University s Sport and Exercise Research Ethics Committee and conformed to the Declaration of Helsinki. All participants provided a written informed consent form before participation. All assessment sessions were performed in the morning (starting between 8:00 and 9:00 AM) after an overnight fast. Before testing sessions, participants were instructed to abstain from exercise for 24 hours, and from caffeine and alcohol consumption for 12 hours. The order of testing was as follows: macrovascular function, microvascular function, and cardiopulmonary function. There was a 30-minute break between the microvascular function test and the cardiopulmonary function test, during which participants consumed a light-carbohydrate snack. Macrovascular function In accordance with guidelines on FMD testing, 8 all assessments were performed with the participant lying supine in a dimly lit, temperature-controlled room (22-24-C). Before the commencement of measurements, participants rested quietly for a minimum of 15 minutes, with their right arm extended and supported approximately 80- from the torso. Resting blood pressure and heart rate were assessed on the left arm using an automated sphygmomanometer (Dinamap Dash 2500; GE Healthcare, Hatfield, UK). Ultrasound assessment of the brachial artery was performed on the distal third of the upper arm using a 7-MHz linear-array transducer attached to a highfrequency ultrasound system (Terason T3000; Teratech Corp, Burlington, MA). Once a suitable longitudinal B-mode view of the artery had been located, the probe was held stable, and image optimization was performed by manipulating depth and gain settings to allow a clear visualization of the near and far arterial walls and lumen. Doppler assessment of blood velocity was performed throughout all assessments. The angle of insonation to the arterial walls was set at 60- or less. Endothelium-dependent dilation (ie, endothelial function) was assessed using the brachial artery FMD technique. An occluding cuff (Hokanson E20 rapid cuff inflator; Hokanson Inc, Bellevue, WA), placed immediately distal to the olecranon process, was inflated to a pressure of 50 mm Hg above systolic blood pressure, with occlusion maintained for 5 minutes. Baseline recordings of vessel diameter were performed for 1 minute before cuff inflation. Recordings were restarted 30 seconds before cuff release and continued for a further 3 minutes thereafter. Continuous recording throughout the duration of the 3-minute postdeflation period was considered sufficient to ensure that peak vessel diameter had been recorded. 22 Glyceryl trinitrate (GTN) was used to induce endotheliumindependent arterial dilation. 8 Participants rested quietly for 15 minutes after completing the FMD assessment to allow arterial diameter to return to baseline values. The brachial artery was imaged as described for the FMD procedure, with the transducer held stable once an optimal view of the arterial lumen had been achieved. After a 1-minute baseline scan, a 400-KL dose of GTN was administered sublingually, with scanning maintained for 6 minutes thereafter. Recording was maintained for the duration of the scan to ensure capture of peak arterial dilation. Throughout vascular assessments, the ultrasound on-screen display was recorded at a frequency of 15 Hz using Camtasia Studio software (version 5.0.0; TechSmith Corp, Okemos, MI). Recorded files were analyzed using Brachial Analyzer for Research software (version ; Medical Imaging Applications, 2 Menopause, Vol. 21, No. 10, 2014 * 2014 The North American Menopause Society

3 INTERVAL TRAINING IN POSTMENOPAUSAL WOMEN Coralville, IA). Movements of the vessel wall within a selected region of interest were tracked by electronic calipers through each frame of the video file. Raw data for arterial diameter were subsequently smoothed using a rolling smoothing technique, where the median of a window of 50 data points was calculated before the window moved to the next 50 data points that shared a 20% overlap with the previous window. Baseline arterial diameter (D base ) was calculated as the mean of data acquired through the minute before cuff inflation or GTN administration. Peak diameter (D peak ) was considered the highest data point after cuff release or GTN administration. Arterial dilator response was calculated as the absolute diameter change (D peak j D base ; in mm), relative (%) diameter change ([(D peak j D base )/ D base ] 100), and allometrically scaled change to account for differences in baseline arterial diameter. 23 The latter was achieved by calculating the difference between the natural logarithm of baseline arterial diameter and the natural logarithm of peak arterial diameter (lnd peak j lnd base ), with the natural logarithm of baseline diameter included as a covariate. Data for allometrically scaled arterial diameter change were subsequently presented as a percentage of dilation ([antilog of the scaled diameter difference j 1] 100). Microvascular function Microvascular function was assessed in the skin of the ventral forearm using laser Doppler flowmetry coupled with two common blood flow provocation tests: postocclusive reactive hyperemia (PORH) and local thermal hyperemia (LTH). For these assessments, participants lay in supine position with the left arm extended to the side at heart level. Heart rate and blood pressure (right arm) were monitored at regular intervals throughout the protocol (Dinamap Dash 2500; GE Healthcare). As an index of skin blood flow, red blood cell flux was measured using three integrating laser Doppler probes (model 413; Perimed AB, Jarfalla, Sweden) connected to a laser Doppler flowmeter (Periflux 5000 System; Perimed AB) with associated software (PeriSoft for Windows 9.0 software; Perimed AB). Each probe was housed within a local heating ring (model 455; Perimed AB) that was positioned 5 cm or more apart on the ventral aspect of the left forearm. Calibration of the probes was verified using the manufacturer s motility standard immediately before any recording. Measurements of red blood cell flux were recorded at 0.03-second intervals. The temperature of the probes was standardized to 33-C during the PORH protocol and baseline period of the LTH test to avoid variations in skin temperature and, consequently, in the measurements of skin blood flow. 24 PORH refers to the transient increase in blood flow that occurs in response to a brief arterial occlusion. The PORH response is considered a global test of skin microvascular function because the vasodilatory response involves several factors: metabolic vasodilators, endothelial vasodilators, myogenic response, and sensory nerves. 24 During the PORH test, proximal arterial occlusion was performed by inflating the cuff placed on the distal part of the left upper arm to 200 mm Hg for 5minutes. 25 After cuff deflation, peak postocclusive flow was measured as the highest instantaneous value recorded at each probe. A 10-minute recovery period was allowed before initiating the LTH protocol. The increase in skin blood flow in response to a non-painful heat stimulus (ie, LTH) involves at least two independent phases: an initial transitory rise followed by a nadir, ultimately succeeded by a secondary rise and prolonged plateau. 26,27 The initial peak, which is typically observed within 2 to 3 minutes of the onset of heating, mostly depends on a local sensory nerve axon reflex 27,28 and, as such, has been considered an indicator of skin neurovascular function. 29 In contrast, the secondary plateau phase, which is usually recorded after 25 to 40 minutes of heating, is largely dependent on endothelium-derived nitric oxide 30 and is therefore considered an indicator of microvascular endothelial function. 29 The temperature of the local heating units surrounding the probes was increased from 33-C to 42-C, at a rate of 0.5-C for every 5 seconds, to investigate LTH. 25,28 Thereafter, this temperature was maintained until red blood cell flux had reached a stable plateau (typically 30 min). Participants did not feel any sensations of pain during any part of this protocol. Regions of the response were defined as follows: baselinevthe mean of the last 2 minutes of the initial 5-minute baseline period; initial peakvthe mean of the highest consecutive 30-second period within the distinct initial hyperemic response; and plateauvthe mean of the last 2 minutes of heating at 42-C. Measurements of red blood cell flux (recorded in arbitrary perfusion units) were divided by the corresponding mean arterial pressure (in mm Hg) and multiplied by 100 to give cutaneous vascular conductance (in arbitrary perfusion units mm Hg j1 ). Previous studies by Tew et al 31,32 have indicated this to be the most appropriate method of data expression for determining changes in microvascular function with exercise training, rather than normalizing data to baseline or maximal flow. For both microvascular tests, the mean value of data from all three laser Doppler probes was used as the final result for analysis. Cardiopulmonary function Assessment of cardiopulmonary function was performed by measuring ventilatory threshold and peak oxygen uptake (V O 2peak ). Participants underwent cardiopulmonary exercise testing following an incremental protocol using an electronically braked cycle ergometer (Excalibur Sport, Lode, The Netherlands). Each test was supervised by an exercise physiologist and a sports medicine physician. After 2 minutes of unloaded cycling, the intensity of exercise was increased by 15 W minute j1.the target cadence was 60 to 80 rev minute j1. Participants were encouraged to continue cycling to volitional exhaustion. Twelvelead electrocardiogram monitoring with ST segment analysis was performed continuously (Cardioperfect; Welch Allyn, Aston Abbotts, UK). The test was terminated if there was participant distress or development of ST depression of 2 mm or more in any lead. Rating of perceived exertion (RPE; Borg 6-20 scale 33 ) was recorded at the end of every minute. The volume of oxygen consumed during exercise was calculated from minute ventilation (measured using a pneumotachometer) Menopause, Vol. 21, No. 10,

4 KLONIZAKIS ET AL and simultaneous breath-by-breath analysis of expired gas fractions (Ultima CardiO2; MedGraphics, Gloucester, UK). Gas analyzers and flow probes were calibrated before each test. Oxygen consumption was expressed relative to body mass (ml kg j1 min j1 ). Ventilatory threshold was determined by an independent exercise physiologist who was blind to group allocation using the v slope and ventilatory equivalents methods. 34 V O 2peak was calculated as the highest consecutive 20-second period of V O 2 data in the last minute before volitional exhaustion, which generally occurred because of leg fatigue and/or breathlessness. Exercise interventions The training protocol consisted of six sessions completed within a 2-week period. All exercise sessions were performed on an electronically braked cycle ergometer (Bosch, Stuttgart, Germany), with sessions performed on Mondays, Wednesdays, and Fridays. For the HIT group, training consisted of 10 1-minute intervals at 100% of peak power output recorded during the baseline incremental cycling test, interspersed with 1 minute of light active recovery at 30 W. Training progression was implemented by increasing the power output of Bwork[ intervals as tolerated. For the CT group, training consisted of 40 minutes of continuous cycling at an intensity corresponding to 65% of peak power output. Training progression in the CT group was implemented by increasing the power output. In both groups, each session began and ended with 3 minutes of light cycling at 30 W. All training sessions were directly supervised by an exercise physiologist. Heart rate (FT2; Polar, Kempele, Finland) and RPE were recorded at the end of each work and rest interval in the HIT group, and at 5-minute intervals in the CT group. Participants were instructed to maintain normal dietary and physical activity practices throughout the study. Statistical analysis Statistical analyses were performed using the Statistical Package for the Social Sciences version 19.0 (SPSS UK Ltd, Bedfont Lakes, UK). Independent t tests were used to compare baseline characteristics between groups. A group-by-time analysis of variance with repeated measures on the time factor was the initial procedure for comparing the mean values of the cardiopulmonary function and vascular function data. Within-group changes were also examined using paired t tests. Pearson s correlation test was used to examine the relationship between baseline measures of macrovascular and microvascular functions. Statistical significance was set at P G 0.05, and data are presented as mean (SD). RESULTS Participant characteristics From the 22 participants recruited, 12 were allocated to HIT and 10 were allocated to CT. Four women (one woman in the HIT group and three women in the CT group) withdrew from the study, citing a change in health status unrelated to the study (n = 2), Bdomestic[ reasons (n = 1), and that the CT was too hard (n = 1). Data are presented for women who completed the study only. The mean (SD) age, body mass, and stature were 64 (7) years, 62.3 (6.0) kg, and 167 (5) cm for the HIT group, and 64 (4) years, 63.0 (13.5) kg, and 164 (5) cm for the CT group, respectively. There were no significant differences in baseline characteristics between groups. The mean (SD) body mass did not significantly change after training in either group (HIT: from 62.3 [6.0] to 62.0 [6.1] kg; CT: from 63.0 [13.5] to 63.0 [13.6] kg). Training adherence and responses Adherence to the exercise programs was excellent, with 107 of 108 sessions completed. The mean (SD) power output during the HIT (work intervals only) and CT sessions were 114 (28) and 82 (13) W, respectively. In the HIT group, the mean (SD) heart rate and RPE increased from 133 (14) beats minute j1 (79% [4%] of maximum) and 12 (1), respectively, at the end of the first work interval to 156 (15) beats minute j1 (93% [4%] of maximum) and 18 (1) at the end of the 10th work interval. In the CT group, the mean (SD) heart rate and RPE increased from 128 (10) beats minute j1 (79% [6%] of maximum) and 11 (1), respectively, after 5 minutes of exercise to 143 (9) beats minute j1 (88% [5%] of maximum) and 15 (2) after 40 minutes of exercise. Training time commitment during the six sessions was approximately 2.5 hours for HIT and approximately 5 hours for CT, and the total training volume was approximately 55% lower for HIT than for CT (mean [SD], 558 [105] vs 1,237 [416] kj; P G 0.001). Resting hemodynamics and cardiopulmonary function Resting hemodynamics and exercise testing data are presented in Table 1. There were no significant baseline differences between groups or group-by-time interactions for resting heart rate, resting systolic blood pressure, and resting diastolic blood pressure. There was, however, a significant main effect of time on resting heart rate (P = 0.037). Both groups decreased by a mean of 2 beats minute j1 ; however, the change from baseline was only significant in the CT group (P =0.031).In the HIT group, there was a trend for a decrease in systolic blood pressure after training ($ = j7 mm Hg; P = 0.073). In the CT group, there was a trend for a decrease in diastolic blood pressure ($ = j3 mmhg;p =0.086). There were no significant baseline differences between groups or group-by-time interactions for any of the exercise testing variables. There were, however, significant main effects of time on V O 2peak (P = 0.006) and test duration (P = 0.003). V O 2peak increased in the HIT and CT groups by 2.2 and 1.7 ml kg j1 minute j1, respectively, but the change from baseline was only significant in the HIT group (P =0.01andP =0.168,respectively). The increase in exercise test duration was significant after both HIT and CT (P = and P =0.046,respectively), but the improvement seemed larger for HIT (13% vs 5%). There were no significant changes in either group for ventilatory threshold or peak values of heart rate, RPE, and respiratory exchange ratio. Macrovascular and microvascular functions Vascular function data are presented in Table 2. There were no significant baseline differences between groups, group-by-time interactions, or main effects for any of the macrovascular or 4 Menopause, Vol. 21, No. 10, 2014 * 2014 The North American Menopause Society

5 INTERVAL TRAINING IN POSTMENOPAUSAL WOMEN TABLE 1. Resting hemodynamics and cardiopulmonary exercise test data before and after training HIT (n = 11) CT (n = 7) Baseline Follow-up Baseline Follow-up P for interaction Resting hemodynamics Heart rate, beatsimin j1 65 (9) 63 (7) 58 (10) 56 (11) a Systolic blood pressure, mm Hg 127 (17) 120 (14) 114 (13) 112 (12) Diastolic blood pressure, mm Hg 70 (4) 68 (6) 68 (7) 65 (6) Exercise test Ventilatory threshold, mlikg j1 Imin j (1.3) 15.4 (2.4) 16.4 (4.1) 17.7 (3.1) V O 2peak,mLIkg j1 Imin j (3.4) 22.6 (3.1) a 25.0 (7.4) 26.7 (5.4) Test duration, s 580 (98) 654 (64) a 621 (92) 655 (107) a Peak heart rate, beatsimin j1 163 (14) 163 (12) 158 (6) 156 (10) Peak RPE 18 (1) 18 (2) 19 (2) 18 (2) Peak respiratory exchange ratio 1.3 (0.2) 1.4 (0.1) 1.4 (0.1) 1.3 (0.1) Data are presented as mean (SD). HIT, high-intensity interval training; CT, continuous training; V O 2peak, peak oxygen uptake; RPE, rating of perceived exertion. a P G 0.05 versus pretraining. microvascular function variables. There were also no significant within-group changes. Absolute FMD was not associated with PORH (r = j0.033, P = 0.911), the initial peak (r =0.005, P = 0.987), or the plateau phase (r = j0.039, P =0.895)ofthe local thermal hyperemic response. This was also the case for relative FMD (PORH: r = j0.077, P = 0.793; initial peak: r =0.058,P = 0.843; secondary plateau: r =0.032,P =0.915). Similarly, absolute GTN-mediated dilation was not associated with PORH (r = j0.226, P = 0.403), the initial peak (r = j0.370, P = 0.236), or the secondary plateau (r = j0.237, P = 0.459), and there were no significant correlations for relative GTN-mediated dilation (PORH: r = j0.311, P = 0.325; initial peak: r = j0.392, P = 0.208; secondary plateau: r = j0.249, P =0.436). DISCUSSION This is the first study to explore the physiological effects of low-volume HIT in postmenopausal women. Marked increases in V O 2peak and exercise test duration were observed after only six sessions of low-volume HIT, and the changes seemed to be greater, albeit not statistically, compared with the comparison group that engaged in moderate-intensity CT with approximately double the volume. There were no changes in various indices of endothelial function, macrovascular function, and microvascular function in either group. Adherence to both exercise programs was excellent, and there were no adverse clinical events. In summary, our findings suggest that low-volume HIT is a feasible and time-efficient exercise strategy for promoting rapid improvements in cardiopulmonary function in postmenopausal women. The 11% increase in relative V O 2peak observed after lowvolume HIT is similar to the 8% increase reported in the study by Jacobs et al, 17 in which young, recreationally active men completed a similar 2-week program. This suggests that the older age of our participants did not limit their Btrainability,[ at least with regard to V O 2peak. The observed change, which equated to a mean absolute increase of 2.2 ml kg j1 minute j1, mightalsobeclinicallymeaningfulwithrespecttoareduced TABLE 2. Data on macrovascular and microvascular functions before and after training HIT (n = 11) CT (n = 7) Baseline Follow-up Baseline Follow-up P for interaction FMD indices Preocclusion diameter, mm 3.47 (0.45) 3.46 (0.36) 3.68 (0.31) 3.61 (0.40) Postocclusion peak diameter, mm 3.78 (0.45) 3.73 (0.33) 4.00 (0.22) 3.86 (0.41) Absolute FMD, mm 0.31 (0.21) 0.27 (0.14) 0.31 (0.27) 0.25 (0.15) Relative FMD, % 8.1 (7.2) 6.5 (3.7) 8.9 (7.9) 7.0 (4.3) Allometrically scaled FMD, % 8.5 (4.9) 7.4 (4.9) 9.7 (5.0) 7.4 (5.0) GTN indices Pre-GTN diameter, mm 3.67 (0.34) 3.52 (0.38) 3.52 (0.49) 3.74 (0.50) Post-GTN peak diameter, mm 4.08 (0.43) 3.96 (0.46) 4.02 (0.68) 4.17 (0.79) Absolute GTN-mediated dilation, mm 0.40 (0.17) 0.44 (0.22) 0.50 (0.26) 0.43 (0.32) Relative GTN-mediated dilation, % 11.0 (4.4) 12.6 (6.5) 13.9 (6.9) 10.9 (6.8) Allometrically scaled GTN-mediated dilation, % 10.0 (6.8) 14.8 (6.5) 14.9 (6.9) 10.6 (6.7) Microvascular function indices Reactive hyperemia CVC, APUImm Hg j1 137 (57) 147 (37) 139 (46) 153 (28) LTH initial peak CVC, APUImm Hg j1 225 (82) 228 (75) 220 (46) 234 (41) LTH secondary plateau CVC, APUImm Hg j1 283 (92) 286 (95) 286 (73) 314 (49) Data are presented as mean (SD). HIT, high-intensity interval training; CT, continuous training; FMD, flow-mediated dilation; GTN, glyceryl trinitrate; CVC, cutaneous vascular conductance; APU, arbitrary perfusion units; LTH, local thermal hyperemia. Menopause, Vol. 21, No. 10,

6 KLONIZAKIS ET AL risk of early mortality. Indeed, data from a prospective cohort study of 5,721 asymptomatic women suggested a 17% reduction in the risk of all-cause mortality (adjusted for Framingham risk score) for every 3.5 ml kg j1 minute j1 increase in V O 2peak. 19 We probably would have seen greater improvements in V O 2peak with a longer period of training. For example, Currie et al 35 recently reported a mean increase in V O 2peak of 4.7 ml kg j1 minute j1 after 12 weeks of a similar low-volume HIT in women with coronary artery disease. Although we did not explore the physiological mechanisms explaining the improvements in V O 2peak and exercise test duration, a recent study suggested that these changes are most probably attributed to increases in the oxidative capacity of the trained skeletal muscles due to an expansion of the mitochondrial network. 17 Contrary to our hypothesis, macrovascular endothelial function (assessed using brachial artery FMD) did not improve in either training group. This is in contrast to previous studies demonstrating enhanced FMD after 12 weeks of low-volume HIT in coronary artery disease patients 35 and after 2 weeks of CT in young, recreationally active men. 21 The absence of a training effect on FMD in the present study is difficult to explain but could possibly be attributable to the short training duration or the relatively high FMD scores at baseline (groups means of 8.1% and 8.9% for HIT and CT, respectively). Black et al 36 reported that brachial artery FMD was improved after 24 weeks (but not 12 weeks) of CT in postmenopausal women. In that study, baseline FMD was 4.5%Vmuch lower than what we observed. The reasons for this discrepancy are unclear because the testing procedures and participant characteristics (including V O 2peak ) were similar between the studies, with the exception of body mass index, which was lower in the present study (È23 vs 30 kg m j2 ). The lack of a significant correlation between the baseline data for FMD and the secondary plateau of the local thermal skin hyperemic response (indices of nitric oxideymediated endothelial function in the macrocirculation and microcirculation, respectively) supports the separate assessment of endothelial function in the microcirculation and suggests differential regulation of endothelial function in conduit arteries and skin microvessels. This finding is consistent with some 20,37 Vbut not all 38,39 Vstudies that have assessed the association between brachial artery and skin microvascular endothelium-dependent function, although it should be noted that all of these studies used laser Doppler flowmetry coupled with acetylcholine iontophoresis for the microvascular test. As for the macrovascular data, we observed no significant changes in the endothelial or vasodilatory function of the skin microcirculation in either training group. This may again be explained by the relatively short training duration, and a previous study of longer-term mild CT in postmenopausal women indicated that improvements in skin microvascular endothelial function (eg, LTH plateau and acetylcholine iontophoresis) did not become apparent until after 24 weeks. 10 Together, the endothelial function and vascular function data are consistent with the findings in the systematic review by Kessler et al, 14 which indicated that favorable changes in traditional markers of cardiovascular disease risk (eg, fasting blood glucose, high-density lipoprotein cholesterol, blood pressure) are not observed when the duration of HIT is less than 8 weeks. Limitations The present study did not include a control group that received no exercise intervention. Rather, we included a comparison group that underwent a higher-volume program of traditional CT. Previous interval training studies using a nonexercise control group reported no changes in physiological indices. 40,41 Another limitation was that we did not assess the relative acceptability of the different training strategies. Participants did not have the opportunity to try the exercise protocol to which they were not allocated; thus, we cannot say that HIT was preferred to CT or vice versa. However, our findings of excellent adherence rates and no adverse events are in line with previous reports that HIT is well tolerated and safe in a range of clinical and nonclinical populations. 14,42,43 Although the overall dropout rate was relatively high for a 2-week intervention when expressed as a percentage (18%), the absolute numbers were low (n = 4), most of the dropouts were from the CT group (n = 3), and most participants dropped out for reasons unrelated to the training programs. Based on our experiences with other older clinical populations, we would expect the dropout rate to be more like 10% to 15% if we had studied a larger group of postmenopausal women for a more typical 8- to 12-week intervention period. 44<46 Third, the findings should be viewed from the perspective that they are based on a relatively small number of motivated and consenting volunteers. Although this is the case for most of HIT studies in the literature, it nevertheless limits the generalizability of the findings and has implications for statistical power for detecting changes in outcome variables. Nevertheless, this study provides useful preliminary data regarding the relative impact of short-term low-volume HIT versus higher-volume CT on cardiopulmonary and vascular functions in postmenopausal women. CONCLUSIONS The results suggest that a practical model of low-volume HIT induces rapid improvements in cardiopulmonary function, but not vascular function, in postmenopausal women. The increase in cardiopulmonary function seems qualitatively (but not statistically) greater than that experienced with CT despite the HIT group performing approximately half the amount of work. Low-volume HIT might therefore represent a useful alternative training strategy to traditional higher-volume CT in this population, particularly when individuals have limited time to exercise. Further research is required to examine the relative acceptability and safety of HIT versus CT and the clinical implications of longer-term training. Acknowledgments: We would like to thank Tom Nadin, Liam Humphreys, and Alan Ruddock for helping us in supervising the exercise training sessions; Dr. Rebecca Robinson for helping us with 6 Menopause, Vol. 21, No. 10, 2014 * 2014 The North American Menopause Society

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