Reductions in resting blood pressure after 4 weeks of isometric exercise training
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- Devereux, G.R., Wiles, J.D. & Swaine, I.L. Eur J Appl Physiol (2010) 109: 601. doi:10.1007/s00421-010-1394-x
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There is some evidence to suggest isometric training can reduce resting blood pressure in a shorter period than the typical 8 weeks, reported most commonly. The purpose of the present study was to explore whether 4 weeks of bilateral-leg isometric training can reduce resting blood pressure, and whether these changes are associated with altered cardiac output or total peripheral resistance. Thirteen participants volunteered for a 4-week crossover training study, involving three sessions per week (each session involving 4 × 2 min bilateral-leg isometric exercise). The training intensity used (95% peak HR) was equivalent to 24% MVC. In addition to blood pressure, resting heart rate, cardiac output, stroke volume, and total peripheral resistance were measured. Results demonstrated that bilateral-leg isometric exercise training for 4 weeks caused significant reductions in systolic, diastolic, and mean arterial pressure. Changes were −4.9 ± 5.8, −2.8 ± 3.2, and −2.7 ± 2.4 mmHg, respectively. No differences were observed in the other resting measures. In conclusion, this study has shown that it is possible to induce reductions in arterial blood pressure after 4 weeks of bilateral-leg isometric exercise.
KeywordsArterial blood pressureIsometric exerciseMuscle massVascular conductance
Isometric exercise training reduces resting blood pressure (Howden et al. 2002; McGowan et al. 2004; Ray and Carrasco 2000; Taylor et al. 2003; Wiles et al. 2009; Millar et al. 2007; Wiley et al. 1992). The duration of this training has been varied, with 8 weeks being the most common (McGowan et al. 2004; Wiles et al. 2009; Wiley et al. 1992). Two studies have shown significant reductions in resting systolic (SBP) or diastolic (DBP) blood pressure after interim measurements made at 3 weeks of a 5 week training program (Wiley et al. 1992; Howden et al. 2002). Neither of these studies reported concomitant reductions in mean arterial pressure (MAP), which is important, given that it represents the average arterial pressure during each cardiac cycle (Oblouck 1987). Measurement of MAP would be useful in understanding the rapid adaptations that occur after isometric training, if measured in concert with cardiac output (CO) and total peripheral resistance (TPR).
There is evidence for rapid adaptations in cardiovascular function after isometric training. In a 4-week study by Green et al. (1994) vascular adaptations to this kind of exercise training were demonstrated, including changes in peak vasodilator capacity. In another study, increased hyperemic blood flow and reduced vascular resistance were shown after 1 week of IHG training (Alomari and Welsch 2007). Wiles et al. (2009) measured SBP, DBP, and MAP in an 8-week bilateral-leg isometric training study, and showed that most of the reduction in resting SBP, DBP and MAP occurred in the first 4 weeks, although not statistically significant. They also measured CO and TPR at the same points but neither changed. There have not been any shorter isometric training studies that have attempted to show reductions in blood pressure in concert with measures of CO and TPR. The purpose of the present study was to investigate the effects of a 4-week bilateral-leg isometric exercise training program on resting blood pressure, CO, and TPR.
All tests were conducted using a Biodex System 3 Pro, isokinetic dynamometer (Biodex Medical Systems, Inc., Shirley, NY, USA), which has good mechanical intra-day and inter-day reliability for both position and torque (Drouin et al. 2004). A data link from the Biodex remote access to a 16-channel chart recorder (Powerlab, ADInstruments Ltd., Australia) was used to synchronize the time component of EMG and force recordings. The Biodex was fitted with a modified hip attachment that was inserted into the standard knee attachment, to allow for bilateral-leg extension exercise to be performed. Participants sat in the dynamometer in an upright position, with 90° of flexion at the hip and with the thighs supported. The movement arm (modified hip attachment) was secured 1 cm superior to the medial malleoli of the ankle. Participants were instructed to avoid using their upper body musculature to assist in generating force during isometric bilateral-leg exercise in order to standardize the level of stabilization and to avoid this affecting force (Magnusson et al. 1993; Mendler 1976).
A dual bio-amplifier was used to enable surface EMG measurement from both legs. The root mean square of the raw EMG signal was computed using the chart recording software (Chart 5 for Windows XP, ADInstruments Ltd). EMG was smoothed at 1 s using a high-pass digital filter. Surface EMG was recorded from the vastus lateralis, as this muscle has been shown to exhibit a linear relationship (r = 0.9961, P < 0.01) between EMG and force when performing isometric leg extension exercise (Alkner et al. 2000). Electrode placements were consistent among all participants using the vastus lateralis location (Surface Electromyography for the Non-Invasive Assessment of Muscles, http://www.seniam.org). Electrodes (R ECG pads, Ambu Inc., MD, USA) were placed at two-thirds on the line from the anterior spina iliaca superior to the lateral side of the patella, in the direction of the muscle fibers. EMG from each vastus lateralis were combined and averaged to give a single output.
HR was recorded via ECG using the 16-channel chart recorder. Participants were fitted with three blue sensor R ECG electrode pads (Ambu Inc., USA), as a standard three lead bipolar ECG arrangement, as recommended by ADInstruments was used. HR was sampled at a frequency of 1,000 Hz. Resting HR was recorded after a resting period of 15 min in a supine position in a silent, dimly lit room.
Arterial blood pressure
Resting BP measurements were made using an automated blood pressure monitor (Dinamap Pro 300 Critikon, GEMedical Systems, Slough, Berks, UK). Blood pressure was measured after 15 min rest in a seated position. The lowest of three measures, separated by 60 s, was used for analysis. This device has been assessed for accuracy and reliability of measurement (Beaubien et al. 2002) and found to overestimate diastolic blood pressure. However, the device appears to be similar in accuracy to other semi-automated devices (Lewis et al. 2002).
Cardiac output, stroke volume, and total peripheral resistance
The Innocor non-invasive CO monitor (Innovision A/S, Odense S, Denmark) was used to measure SV and CO at rest via inert gas re-breathing. HR was measured using a pulse oximeter (Innovision A/S). The relative levels of two inhaled inert gases of differing solubility in blood are measured, over three to four respirations. Innocor then calculates pulmonary blood flow. In the absence of significant intrapulmonary shunt, pulmonary blood flow is equal to cardiac output. The average value of two measures was used for analysis. Measures were separated by 5 min to ensure removal of previously inhaled insoluble gases. Participants sat in an upright position to prevent any interference with normal breathing (Damgaard and Norsk 2005). TPR was subsequently calculated using MAP from Dinamap blood pressure readings and CO from the Innocor device: TPR = MAP/CO (Turner et al. 1996; Walker et al. 1992). After an initial familiarization session, participants had all resting measures assessed at two points; pre- and post-training, immediately before the discontinuous incremental isometric exercise test.
Maximal voluntary contraction and EMGpeak
Maximal voluntary contraction and EMGpeak were determined prior to each discontinuous incremental test. Participants performed three maximal effort isometric bilateral-leg extensions against the fixed dynamometer arm. Each MVC was terminated after 2 s, and each was interspersed by a 120 s rest period. The isometric leg extension exercise was performed at a knee angle of 90° (180° corresponds to full knee extension) on the isokinetic dynamometer (Alkner et al. 2000). EMGpeak was determined from the MVC producing the highest torque and was established from the mean of the EMG activity recorded 0.25 s immediately prior to maximum torque (Wiles et al. 2007). The EMGpeak was then used to create %EMGpeak ‘targets’ for the subsequent incremental exercise test.
Discontinuous incremental isometric exercise test
Participants underwent a discontinuous incremental isometric exercise tests both pre- and post-training (after familiarization), immediately after the recording of resting measures. Participants began bilateral-leg isometric exercise at 10% EMGpeak for 2 min. Thereafter, the intensity increased in 5% increments, interspersed by 5-min rest periods, to volitional fatigue (or failure to maintain EMG signal within ±5% of the ‘target’ value). EMG was monitored and recorded. Average values (HR) for the final 60 s of each increment were used for analysis. Participants were instructed to breathe at a normal rhythm and depth at all times to avoid Valsalva manoeuvres.
The relationship between HR and EMG during the discontinuous incremental test was explored using Microsoft Prism software. This software also allows two regression lines to be compared before and after an intervening period using ANCOVA to compare the slopes and intercepts of the linear regression lines. The relationship between EMG and HR was linear for all participants (r values = 0.91–1.00; P < 0.05 in all cases) during all discontinuous incremental tests (pre- and post-exercise, and pre- and post-control). Comparison of pre- and post-control tests revealed no significant differences in the slope (P = 0.42) or elevation (P = 0.68) of the regression lines. The r values were 0.63 [P < 0.0001; SEE(95%) = 5.38%] and 0.70 [P < 0.0001; SEE(95%) = 3.44%] for pre and post tests, respectively.
Participants began training at a participant-specific EMG ‘target’ that equated to 95% HRpeak, interpolated from the regression line of HR versus EMG ascertained during the initial incremental test. Participants trained 3 days week−1 for 4 weeks. Training sessions were separated by at least 24 h. Training intensity (EMG mV) was updated during training if HR deviated from the target HR (95% HRpeak) by more than ±5%. Participants performed four bouts involving 2 min of isometric exercise separated by 3 min rest periods. EMG, HR, and torque were measured and recorded continuously throughout all 12 training sessions. Participants were instructed to breathe at a normal rhythm and depth at all times to avoid Valsalva manoeuvres. Training intensity set and maintained at constant EMG allows for a stable HR response to isometic exercise, whereas constant torque results in HR drift during an exercise bout.
All data were assessed for conformity with parametric assumptions (Field 2000). Pre–post differences were assessed using paired sample t test. Changes in blood pressure have been associated with initial values (Millar et al. 2007), so analysis of covariance (ANCOVA) was used to assess whether changes scores were influenced by initial baseline values. Differences in BP changes between the experimental and control condition were assessed using ANOVA within the ANCOVA test. An alpha level of <0.05 was set as the threshold for statistical significance, and the Bonferroni post-hoc procedure was used to explore any significant differences detected. Pearson product moment correlation was administered to assess for linear dependence between changes in MAP with CO and TPR, irrespective of significant differences pre- to post-intervention.
Group mean values for Systolic (SBP), diastolic (DBP) and mean arterial (MAP) pressure pre- and post-training and pre- and post-control
119.9 ± 11.6
115.0 ± 11.5*
69.0 ± 4.4
66.2 ± 5.0*
89.1 ± 4.7
86.5 ± 5.2*
119.5 ± 11.8
119.4 ± 12.3
66.4 ± 5.3
67.7 ± 3.8
87.9 ± 5.4
88.3 ± 6.1
The covariate of initial baseline values did not influence change scores for SBP (P = 0.35) or MAP (P = 0.87). However, baseline DBP values predicted DBP change scores (P = 0.02, F = 5.92).
This study showed that 4 weeks of bilateral-leg isometric exercise for 3 days week−1 caused reductions in resting SBP (−4.9 mmHg), DBP (−2.8 mmHg), and MAP (−2.7 mmHg). This is the shortest bilateral-leg isometric exercise training study to have shown reductions in all resting BP components, SBP, DBP, and MAP. Decreases in SBP (up to −10.0 mmHg) have been shown at interim points during longer training programs (after 3 weeks of a 5 week program) using bilateral-leg (Howden et al. 2002), and handgrip exercise (Wiley et al. 1992) at a higher relative intensity (50% MVC) and frequency (5 days week−1). Wiles et al. (2009) showed that bilateral-leg isometric exercise can cause reductions in SBP, DBP, and MAP after 8 weeks of training. This was similar to the changes reported in studies that have utilized isometric handgrip (IHG) or forearm exercise for 8 weeks (McGowan et al. 2004; Wiley et al. 1992) and 10 weeks (Taylor et al. 2003). Ray and Carrasco (2000) reported reductions in DBP and MAP, but not SBP, after 5 weeks of IHG. However, that study used a training frequency of 4 days week−1.
In the present study and that of Wiles et al. (2009), exercise intensity was prescribed as an EMG ‘target’ value, which related to a percentage of the highest HR achieved in the initial incremental exercise test (HRpeak). Howden et al. (2002) used percentage of maximal voluntary contraction (% MVC). That study used an exercise intensity of 20% MVC, and reported reductions in SBP after 3 weeks. By averaging torque produced in all training sessions for all participants against initial MVC trials in J.D Wiles’ study (personal communication), the exercise intensity was 22% MVC. Wiles et al. (2009) reported small but insignificant reductions after 4 weeks, but significant reductions in SBP, DBP, and MAP after 8 weeks. If torque values are averaged for all training sessions in the present study, the comparative exercise intensity would be 24% MVC.
The exercise intensities of bilateral-leg studies (20–24% MVC) appear to be lower than IHG studies (30–50% MVC). Differences in training frequency are also often evident (3 days week−1 bilateral-leg, and up to 5 days week−1 for IHG). The results of these studies and the present results, when taken together, appear to suggest that the rate of reduction of resting BP after isometric training is related to the mode of training. Muscle mass is an important factor in the magnitude of the pressor response to isometric exercise (Iellamo et al. 1999; McCloskey and Mitchell 1972; Mitchell et al. 1980). Seals et al. (1983) and Seals (1989) reported an increase in muscle sympathetic nerve activity (MSNA) with corresponding increases in active muscle mass. Increased MSNA is reported to contribute to the increases in BP and HR during exercise (Mitchell 1990; Mitchell and Schmidt 1983; Rowell and O’Leary 1990) and thus to the pressor response. For a given duration of isometric exercise, an individual performing bilateral-leg isometric exercise is likely to experience either a greater or accelerated pressor response during training than an individual who performs IHG, given the increased MSNA arising from the respective muscle masses of each mode of exercise.
The present data show that CO, SV, and TPR probably do not play a role in the observed BP adaptations to isometric exercise training. It is understood that in order for MAP to change, CO or TPR (or both) must change (Turner et al. 1996; Walker et al. 1992). One explanation for the finding that neither of these components changed is that the measurement of variables such as CO may be hampered by high variability (Damgaard and Norsk 2005; Pemberton et al. 2005). Measurement of CO by re-breathing is indirect and can exhibit higher variability when performed at rest compared to during exercise (Vanhees et al. 2000). Furthermore, our measure of TPR was derived from this indirect measure of CO using the formula TPR = MAP/CO. Therefore, the absence of corresponding changes in CO or TPR could be a reflection of their lack of sensitivity in detecting small changes after isometric exercise training.