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An Alternative Approach to Isometric Exercise Training Prescription for Cardiovascular Health

Wiles, Jonathan, D.; Goldring, Natalie; O'Driscoll, Jamie, M.; Taylor, Katrina, A.; Coleman, Damian, A.

Translational Journal of the American College of Sports Medicine: January 15, 2018 - Volume 3 - Issue 2 - p 10–18
doi: 10.1249/TJX.0000000000000052
Original Investigation

ABSTRACT Isometric exercise training (IET) effectively reduces resting blood pressure (BP). Traditionally, IET protocols use hand grip or leg extension exercise. IET using larger muscle mass may influence the rate and magnitude of BP reductions. This study aimed to examine the efficacy of a novel isometric wall squat protocol. Twenty participants completed four laboratory visits over 2 wk. Two incremental isometric wall squat exercise (IWSE) tests were completed to determine the relationship of knee joint angle to heart rate (HR) and BP. The incremental IWSE test started at 135° of knee flexion decreasing by 10° every 2 min until 95° (final stage). Exercise was terminated upon completion of the 10-min test or at volitional fatigue. To ascertain the efficacy of prescribing IWSE training, the relationship between knee joint angle and mean HR was used to calculate the participant-specific knee joint angle required to elicit a target HR of 95% HRpeak for 2× IWSE training sessions. There were inverse relationships (P < 0.05) between knee joint angle and mean HR/BP parameters (r ≥ −0.99), with no difference in the peak HR during the last 30 s of the two incremental tests (P > 0.05). Although there were no differences (P > 0.05) in mean HR and BP values between repeated training sessions, there were differences (P < 0.05) within and between the four bouts during a session. In conclusion, data indicate that the IWSE test provides a reliable means of prescribing and monitoring isometric exercise intensity. The ability of IWSE to reduce resting BP when used for isometric exercise training prescription has been demonstrated in healthy participants. This finding could be significant in the nonpharmacological prevention, treatment, and management of hypertension.

Section of Sport and Exercise Sciences, School of Human and Life Sciences, Canterbury Christ Church University, Canterbury, UNITED KINGDOM

Address for correspondence: Jonathan D. Wiles, Ph.D., Canterbury Christ Church University, North Holmes Road, Canterbury CT1 1QU, United Kingdom (E-mail: jim.wiles@canterbury.ac.uk).

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INTRODUCTION

Arterial hypertension is a major global public health issue, adversely accelerating cardiovascular disease. As such, prevention, detection, and control measures are a key priority for policy makers. Small-scale research studies have demonstrated the powerful effect isometric exercise training (IET) has on reducing resting blood pressure (BP), with mean reductions of between 10 and 13 mm Hg systolic and 6 and 8 mm Hg for diastolic reported (1). Evidence suggests that an IET mode using larger compared with smaller muscle mass is able to produce a similar magnitude of resting BP reduction when conducted at significantly lower exercise intensity (2). Moreover, statistically significant reductions may occur at a faster rate, taking 3 wk after leg extension exercise (3) compared with 4–5 wk reported for hand grip exercise (4).

IET for the reduction of resting BP has tended to use either hand grip or leg extension exercise, and has mostly been prescribed using a constant force at a given percentage of maximal voluntary contraction (1). Previously, it has been shown that when constant EMG activity was used to determine exercise intensity, a “steady-state” heart rate (HR) response could be achieved during an incremental isometric exercise test (5). This physiological response suggests the potential for greater accuracy in IET prescription compared with a constant force protocol. However, scientifically reliable isometric exercise prescription using either of these methods uses relatively expensive and/or laboratory-based equipment and subsequently reduces the potential reach of application. In light of this, Goldring et al. (6) suggested that the constant position wall squat exercise (which uses a large muscle mass) might provide an inexpensive, accessible, and alternative means of performing IET. Furthermore, unlike many existing protocols this method can be performed easily in the home, increasing the access to the treatment of a wide patient demographic, providing patients and practitioners with greater choice, and helping to reduce healthcare inequalities. Ideally, the ultimate aim is to be able to prescribe isometric exercise to the growing number of relatively young individuals who are identified as prehypertensive, those newly diagnosed as stage 1 hypertensive without other risk factors (7), and people taking antihypertensive medication as an adjunctive therapy (8). However, there are limited evidence-based protocols aimed at allowing for accurate IET prescription outside the laboratory. In addition, if IET is to be accepted by health professionals as an alternative prescription for those with suboptimal BP control, then the principles of training must be adhered to. Therefore, a larger number of viable isometric exercise protocol options will allow for greater variation to be used during longer periods of IET prescription.

It was previously demonstrated that isometric wall squat exercise (IWSE) intensity could be adjusted by manipulating knee joint angle, and this was shown to result in reliable changes in HR and BP (6). As such, it could be proposed that constant position IWSE intensity might be set at a participant-specific knee joint angle to elicit a target HR. Indeed, the wall squat elicited similar cardiovascular responses to other isometric exercise modes that have successfully reduced resting BP. However, a practical method to determine isometric wall squat intensity to help provide a reliable and scientifically sound IET prescription remains to be established. Furthermore, despite the strong evidence supporting IET for BP reduction, few guidelines recommend its application. One possible reason for this is that there is limited research investigating the transient elevation in BP that occurs during isometric muscle contraction. The magnitude of the BP rise during isometric exercise has been shown to be pronounced, and as such, it has been recommended that isometric exercise should be avoided, especially in higher-risk populations (9). Therefore, this research also provides an ideal opportunity to evaluate the applicability of current American College of Sports Medicine (ACSM) BP safety thresholds, which are the only available guidelines for all populations during any form of exercise. Therefore, the aims of this study were to determine if a relationship exists between knee joint angle and HR and BP during an incremental isometric wall squat test and, if so, to evaluate the efficacy and reliability of prescribing target HR and BP ranges for isometric wall squat training. A further aim was to explore the BP response to isometric exercise involving a large muscle group, to ascertain if specific BP guidelines are required for isometric exercise interventions.

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METHODS

Participants

Twenty healthy men (age, 28.7 ± 7.4 yr; height, 1.80 ± 0.06 m; body mass, 78.8 ± 9.8 kg (mean ± SD)) with optimal to high normal BP (systolic BP (SBP), 124 ± 8 mm Hg; diastolic BP (DBP), 77 ± 4 mm Hg) defined according to European Society of Hypertension and European Society of Cardiology 2013 guidelines (10) volunteered to participate in the study. Before testing, each participant received a written explanation of the procedures including any potential risks, completed an exercise readiness questionnaire, and provided written informed consent. All participants were physically active, nonsmokers, free from any injury or disease, and not taking any medication during the investigation. Participants agreed to maintain regular dietary and physical activity habits throughout the testing period; abstain from food 2 h, caffeine 4 h, alcohol, and strenuous exercise 24 h before testing; and attend the laboratory at the same time of day for each test. The study was approved by the Canterbury Christ Church University Ethics Committee, and all procedures were conducted according to the 1964 Declaration of Helsinki.

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Equipment

BP

All BP parameters (SBP, DBP, and mean arterial pressure (MAP)) were recorded during rest and exercise. SBP and DBP were measured using a Finometer (Finapres Medical Systems BV, Amsterdam, the Netherlands), and the data were interfaced with a Windows PC using an analog I/O box (Finapres Medical Systems BV) and a 16-channel data acquisition system (PowerLab/16SP; ML795, ADInstruments Pty Ltd, Castle Hill, Australia). MAP was calculated instantaneously using the formulae: MAP = DBP + [1/3(SBP − DBP)]. All BP data were then continuously displayed on a computer using LabChart Pro software (version 7.1; ADInstruments Pty Ltd).

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HR

HR was recorded during rest and exercise via ECG using the PowerLab system. Participants were fitted with three single-use ECG electrodes (Ambu A/S, Ballerup, Denmark), which were placed in a standard three-lead bipolar ECG arrangement.

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Wall Squat Knee Joint Angle

Knee joint angle was measured during exercise using an MIE clinical goniometer (MIE Medical Research Ltd, Leeds, UK), which was attached to the participant's left leg using elastic Velcro straps ensuring that muscle compression did not occur (Fig. 1). The fulcrum was aligned with the lateral epicondyle of the femur, the moving arm was placed on the lateral midline of the femur using the greater trochanter for reference, and the stationary arm was placed on the lateral midline of the fibula using the lateral malleolus and fibular head for reference. A spirit level was attached to the stationary arm to ensure that the lower leg was kept vertical during exercise. The internal angle between the femur and fibula was measured.

Figure 1

Figure 1

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RPE

The Borg CR10 scale was used to measure RPE, specifically the localized discomfort felt in the participant's upper legs after isometric exercise (11).

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Procedures

Familiarization

Before collection of all data, participants were familiarized with the IWSE and testing protocols. All participants adhered to the testing requirements, and this was verbally confirmed by the participants before the start of each session. At the start of each testing session, participants were asked to ensure that they had an empty bladder.

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Overall Study Design

Each participant completed four separate laboratory visits over a 2-wk period. In week 1, participants completed two incremental tests (INC1 and INC2, respectively). In week 2, participants completed two IWSE training sessions (T1 and T2, respectively). All IWSE tests were performed using the same protocol, with HR and BP recorded continuously throughout all incremental tests and training sessions. For analysis, the mean for the last 5 s of each 30-s period were calculated offline, as well as the mean for the last 30 s of each incremental stage/training exercise bout. At the start of each test, participants rested in a seated position for 15 min. After an initial 10-min period, BP was measured continuously for 5 min with simultaneous HR recording.

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Test Protocol

The incremental isometric wall squat test required participants to perform continuous IWSE in stages of increasing intensity, which was determined by manipulating knee joint angle. On the basis of the work of Goldring et al. (6), the first stage began at 135° of knee flexion, and participants were instructed to hold this position for 2 min. Once each stage was complete, the knee joint angle was decreased by 10°. The angle was decreased every 2 min until the participant reached the end of 95° stage, or could no longer maintain the knee joint angle within 5° of the target value (volitional fatigue). Upon cessation, all participants reported an RPE of 10, which provided subjective evidence that the test had been completed to maximum.

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Training Protocol

On the basis of each participant's data from his initial incremental test (INC1), knee joint angle was plotted against the mean HR for the last 30 s of each incremental stage. The relationship between parameters was then used to calculate the specific knee joint angle required to elicit a target HR. The target HR selected for training was 95% HRpeak as used by Devereux et al. (12), with HRpeak defined as the mean HR of the final 30 s achieved during the incremental test. Target SBP, DBP, and MAP values were also calculated in the same manner for the prescribed target knee joint angle.

In addition, the individual target HR range (THRR) was established using the 95% reference interval (13). Target BP ranges for SBP, DBP, and MAP were also calculated using this method.

Each training session was composed of 4 × 2-min bouts of IWSE with 2-min rest between bouts. Participants started upright with their back against a wall, feet parallel and shoulder width apart, and hands by their side (Fig. 2). Participants then lowered their back down the wall while moving their feet forward into the required position, determined by the knee joint angle, as previously described by Wiles, Goldring, and Coleman (14). Participants were instructed to ensure that they kept their lower legs vertical and their trunk upright and were informed of the elapsed time. To avoid the Valsalva maneuver, participants were also instructed to breathe normally. All training sessions were completed at the same participant-specific knee joint angle prescribed from the first initial incremental test (INC1).

Figure 2

Figure 2

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Data Analysis

All data were checked for conformity with the parametric assumptions (15). Where the parametric assumptions were met, a factorial repeated-measures ANOVA was conducted for 1) incremental test data to explore possible differences in HR and BP between different time periods, 2) training session data to explore the differences in HR and BP between exercise bouts and time periods, and 3) individual exercise bouts to detect any differences in the percentage of time spent below, in, and above the target ranges. The LSD post hoc test was used to explore any significant differences detected. Where data were not normally distributed, log transformation was attempted to achieve a normal pattern. If normal distribution was still not achieved, the nonparametric Friedman test was carried out and the Wilcoxon signed ranks test was used for post hoc comparison. Data analysis was performed using IBM SPSS (BM SPSS Statistics for Windows, version 23.0; IBM Corporation, Armonk, NY).

The relationships between knee joint angle, HR, and BP for the incremental tests were explored using Pearson product–moment correlation coefficient using Microsoft Excel software (Microsoft Excel 10; Microsoft Corporation, Redmond, WA). If the relationship between any parameter and knee joint angle was nonlinear, the relationship was then explored through a one-phase exponential decay model using GraphPad Prism (GraphPad Prism version 5.01 for Windows; GraphPad Software, San Diego, CA). To compare the slopes and elevation (intercept) regression lines between incremental tests, an ANCOVA was completed using GraphPad Prism (GraphPad Prism version 5.01 for Windows; GraphPad Software).

For all data compared between 1) incremental tests (INC1 vs INC2), 2) training sessions (T1 vs T2), and 3) prescribed targets and actual values, a paired t-test/Wilcoxon signed ranks test was used for comparison. Within-participant variation, expressed as a coefficient of variation (CV), was calculated for all cardiovascular variables (HR, SBP, DBP, and MAP) at rest, for the incremental test and training session values, and also for the calculated training targets. The CV was derived by log-transformed two-way ANOVA as described by Atkinson and Nevill (16), together with the 95% confidence intervals (CI) for a normal distribution (17). Effect size (ES) statistics were calculated (with 95% CI) using the difference between comparisons divided by a pooled SD. For all tests, an alpha level of <0.05 was set as the threshold for statistical significance. All data are expressed as mean ± SD.

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RESULTS

Resting Measures

Mean resting values (computed from the four visits to the laboratory) were 69 ± 8 bpm for HR, 124 ± 8 mm Hg for SBP, 77 ± 4 mm Hg for DBP, and 93 ± 5 mm Hg for MAP.

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Exercising Measures

Incremental Test

Of the 40 incremental tests (20 participants × 2 tests [INC1 and INC2]), the mean final incremental stage reached was stage 5, and the mean duration of the test was 8 min 40 s ± 66 s, with no significant difference between INC1 and INC2 (t(19) = 0.654, P = 0.52, ES = 0.15, −0.33 to 0.63). RPE data confirmed that all participants had reached maximum.

The HR and BP data from all incremental stages were amalgamated, and the mean values for each 30-s time period were examined. There were no significant rises (F(3,57) = 1.727, P = 0.17) in mean HR during each 2-min work period when data were analyzed in this way. However, significant rises in SBP were noted over time (F(3,57) = 3.816, P = 0.015). The final 30-s period (90–120 s) was significantly higher than data for the first minute of exercise; however, there was no significant difference (P > 0.05) between the 60- and 120-s comparison: 2 ± 2 mm Hg for SBP (159 vs 161 mm Hg: P = 0.07, ES = 0.14, −0.013 to 0.29). There was no significant change over time for DBP (F(3,57) = 1.870, P = 0.145) or MAP (F(3,57) = 2.133, P = 0.106).

Mean HR shared a significant inverse curvilinear relationship to wall squat knee joint angle for both incremental tests (INC1 and INC2; r values ≥ −0.9940; P < 0.05). Figure 3A illustrates the mean inverse curvilinear relationship between knee joint angle and HR for both incremental tests (INC1 and INC2). Individual HR and knee joint angle relationships ranged from r = −0.8943 to r = −0.9998 (P < 0.05).

Figure

Figure

The mean BP parameter data revealed significant inverse linear relationships to wall squat knee joint angle for all incremental tests (r values ≥ −0.9902; P < 0.05; Fig. 3B–D). Individual relationships ranged from r = −0.8796 to r = −0.9993 (P < 0.05).

There was no significant difference in the HRpeak values attained during the last 30 s of the two incremental tests (INC1: 127 ± 16 bpm vs INC2: 126 ± 20 bpm; t(19) = 0.256, P =0.801, ES = −0.05, −0.21 to 0.11). The reliability of the HRpeak value was 6.3% (4.8%–9.2%) and the bias was −1.55 bpm; these values were input into the reference interval equation to calculate the participants' THRR.

From the HR and BP values attained, the target values for training were calculated as well as the ranges around these targets (Table 1). In addition to this, the bias values are displayed for the target BP as these data were entered into the reference interval equation to calculate the target BP ranges. The HR and BP training targets and ranges displayed for INC2 have been normalized to the knee joint angle predicted for INC1 for comparison; there were no significant differences between INC1 and INC2 for HR (t(19) = 1.840, P = 0.08) or SBP (t(19) = 0.694, P = 0.496).

TABLE 1

TABLE 1

The maximal BP responses attained during the incremental tests (INC1 and INC2) were 214 ± 14 mm Hg for SBP, 126 ± 8 mm Hg for DBP, and 154 ± 8 mm Hg for MAP.

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Training

All participants completed both wall squat exercise training sessions. All training sessions were performed at the knee joint angle prescribed from the first incremental test (mean, 104°; median, 102°; range, 97°–117°).

The mean HR and BP data for the whole and the last 30 s of each training session (T1 and T2) are shown in Table 2, along with the reliability data for each variable. There was no significant difference in the mean HR (t(19) = 0.866, P = 0.40), SBP (t(19) = 0.219, P = 0.83), DBP (t(19) = 0.215, P = 0.83), and MAP (t(19) = 0.032, P = 0.98) values between the two training sessions. Therefore, all further analyses were performed using the mean data of the two training sessions (T1 and T2).

TABLE 2

TABLE 2

When the mean HR and BP parameter values were compared between exercise bouts, there were significant differences between bouts 1 and 4: HR (F(3,57) = 55.526, P < 0.001), SBP (F(3,57) = 45.169, P < 0.001), DBP (χ2(156) = 218.03, P < 0.001), and MAP (F(3,57) = 44.830, P < 0.001). There were significant differences in the HR and BP parameter data when the means of all four bouts were compared over time during a training session (Fig. 4). There was no significant difference (P = 0.059) between 90 and 120 s for HR, but all BP values rose significantly during this period (P < 0.05).

Figure

Figure

When comparing the actual HR values achieved during training and the target HR, significant differences (P < 0.001) for both the whole training session (−16 ± 9 bpm, t(19) = 8.458, P < 0.001, ES = 1.1, 0.5–1.7) and for the last 30 s of training data (−8 ± 9 bpm, t(19) = 4.039, P < 0.001, ES = 0.9, 0.4–1.3) were found. A similar pattern was also present for the BP data during the whole training session (SBP: −19 ± 11 mm Hg, t(19) = 7.626, P < 0.001, ES = 1.2, 0.3–2; DBP: −10 ± 7 mm Hg, t(19) = 6.706, P < 0.001, ES = 1.9, 0.5–3.3; MAP: −12 ± 8 mm Hg, t(19) = 6.615, P < 0.001, ES = 1.6, 0.4–2.7, P < 0.01). However, there was no significant difference between the values attained during the last 30 s of training and the prescribed BP targets: SBP of −5 ± 12 mm Hg (t(19) = 1.886, P = 0.075, ES 0.29, −0.1 to 0.6), DBP of −3 ± 7 mm Hg (t(19) = 1.884, P = 0.075, ES = 0.5, −0.1 to 1.1), and MAP of −3 ± 9 mm Hg (t(19) = 1.581, P = 0.15, ES = 0.32, −0.2 to 0.8).

The target HR and BP ranges calculated from both the incremental tests were compared (Fig. 5) with the values attained during the training sessions (mean T1 and T2). Table 3 shows the percentage of time spent below, in, and above the target HR and BP ranges during the whole training session, and the percentage of time above the ACSM exercise termination guidelines (18) during the training was 0 ± 0% for SBP (>250 mm Hg) and 4% ± 7% for DBP (>115 mm Hg).

Figure 5

Figure 5

TABLE 3

TABLE 3

The maximal values attained during the IWSE training were 127 ± 16 bpm for HR, 211 ± 23 mm Hg for SBP, 120 ± 10 mm Hg for DBP, and 149 ± 14 mm Hg for MAP. Participants did not report any adverse symptoms such as shortness of breath, dizziness, chest pain, or light headedness during any of the incremental tests or training sessions (Figs. 4 and 5).

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DISCUSSION

This is the first study to provide detailed scientific justification to support the use of an IET prescription away from laboratory environments, for those wishing to reduce resting BP. One of the main findings of this study was that an incremental IWSE test produced strong significant inverse relationships between knee joint angle and both HR and BP as previously found during isolated bouts of wall squat exercise (6). Furthermore, HR and BP were shown to plateau during the last 30 s of each incremental stage. The relationships reported in this study are consistent with those previously reported from constant EMG incremental tests in which double-leg extension exercise was performed (5,12).

There were no significant differences observed between the two incremental tests, with regard to exercise stage, HR, or BP. In addition to this, the reliability of the mean HR and BP values attained during the training session was found to be reproducible. This finding is important to ensure a consistent, safe training prescription; this is due to the fact that isometric exercise intensity is calculated from these incremental test responses. The reliability values reported in the present study compare favorably with those established by Wiles et al. (5) for both HR (5.2% (4.0%–8.1%)) and SBP (5.3% (3.8%–9.1%)).

Because isometric exercise is known to produce an extreme pressor response (19), it was important to examine the maximal BP values attained during the incremental wall squat test and wall squat training session. It can be seen that SBP stayed within the ACSM exercise termination guidelines (<250 mm Hg), but DBP exceeded the upper limit (>115 mm Hg) (18). However, although DBP may have transiently exceeded the ACSM termination guidelines, it seems that this was only for a very short period of time. Indeed, the data indicate that the mean DBP value during the last 30 s of the incremental tests (mean INC1 and INC2) was 16 mm Hg lower than the maximal DBP value attained during the whole incremental test (110 ± 7 mm Hg vs 126 ± 8 mm Hg, respectively), which suggests that the maximal DBP represents a one-off value and does not represent the overall pressor response experienced during the incremental test. When the training BP values were compared with the ACSM guidelines for exercise termination, it can be seen that while 0% (0 s) of the time was spent above the SBP guideline (>250 mm Hg), 4% (~19 s) of the time was spent above the DBP guideline (>115 mm Hg) (18). However, it is important to note that in most cases, the time spent above the DBP guidelines did not occur during one constant time period and was instead spread across a training session. Indeed, typically the DBP guidelines were exceeded in bouts 2 (1%: ~1 s), 3 (4%: ~5 s), and 4 (10%: ~12 s). It is therefore suggested that this would be unlikely to cause any significant cardiovascular risk because the time that the body is subjected to such an extreme pressor response is minimal. As such, these results indicate that current ACSM BP guidelines (originally designed for aerobic exercise test termination) may not be appropriate for use with isometric exercise. Furthermore, current American Heart Association recommendations (9) clearly detail the need for more research investigating the BP surge (especially among individuals with raised BP) in order to reduce the current restrictions associated with isometric exercise prescription. Nonetheless, it is well established that the BP rise during isometric exercise is transient and followed with hypotension (20,21). Therefore, because the rise in DBP may be an important mechanism for the BP reductions seen after isometric exercise, it is conceivable that commissioned BP guidelines specific for isometric exercise are now required. However, further research is necessary to ascertain the safety of the transient BP responses in higher-risk populations to guide future clinic and home-based IET interventions.

On the basis of the relationship established between wall squat knee joint angle and HR, it was possible to interpolate the wall squat position (knee joint angle) at which a participant would have to train at to achieve the target HR. The mean target training wall squat knee joint angle was 104° (median, 102°; range, 97°–117°). It seems that the isometric training intensity prescribed for each participant was reliable, realistic, and manageable because all training sessions were fully completed. In addition to this, the mean HR and BP values attained between repeated training sessions were not significantly different and a good level of reliability was demonstrated.

When the training data were compared with the targets prescribed from the incremental test, the actual HR values attained during the training session were significantly lower than the prescribed target HR. The mean HR for the whole session was 16 bpm lower, whereas the last 30-s data was 8 bpm lower compared with the target. This may be due to interpolating a target HR from a continuous incremental test to the specific discontinuous training protocol used in this investigation. However, despite the discrepancy between the target and the actual values, the mean HR achieved during training in the present study was reliable and very similar to that achieved by Wiles et al. (22) in which BP was reduced. Indeed, recent research using this IWSE protocol in a healthy population (23) demonstrated statistically significant reductions in resting BP, which are comparable with the reductions seen by Wiles et al. (22).

Furthermore, it is suggested that for those wishing to use this method of prescribing IWSE to training, it is more useful to compare the actual training values attained with a target range rather than a single value because of the variable nature of HR (24). Indeed, a training session should aim to achieve a steady-state HR response, with the mean HR value lying within a THRR and close to the target HR (18). Monitoring HR in this manner can be used to detect and prevent large errors in training intensity (25), which is important when prescribing an effective but safe training program.

Prescribing isometric training through THRR provides a more stringent control of the training intensity in which both underload and, more importantly, too much overload can be monitored. As such, a further aim of this study was to devise a method to more precisely determine the THRR for training, rather than using the ±5% range previously applied within IET studies (12,22). To this end, the reference interval was used (13), which takes into account the variability of a measure such as HR. The size of the group's mean THRR in the present study (mean of INC1 and INC2 was 96–138 bpm) was much wider than the HR ranges used in the isometric training study of Wiles et al. (22), which produced a THRR of 96–106 bpm (target 95% HRpeak ± 5%). It was possible to use a narrower THRR in the study of Wiles et al. (22) because training was completed at a constant EMG value, which has been shown to produce an attenuated cardiovascular response (26,27). However, when the THRR calculated from the current study is compared with THRR used to determine the intensity of vigorous aerobic exercise (77%–95% HRmax), the size is similar and is therefore more representative of THRR commonly prescribed. For example, the vigorous-aerobic-exercise THRR was 147–181 bpm when calculated using the present study's group mean age (28).

When the actual HR values attained during the isometric wall squat training sessions were compared with the calculated THRR, it was found that participants spent 67% of the time (~5 min 22 s) within the calculated target range during the whole training session (8 min in total: 4 × 2-min wall squats). Thus, the time in the THRR per training session was found to be high, and therefore, the training intensity prescribed was sufficient for each participant to reach his target range. However, it is important to note that the mean HR of each exercise bout significantly increased throughout a training session, which suggests that the 2-min recovery period (commonly used) was not sufficient to achieve full cardiovascular recovery. This had an effect on the time spent in the THRR per exercise bout, such that the percentage of time spent in the target range significantly increased throughout the training session from bout 1 (48%) to bout 4 (81%). Therefore, it was the cumulative effect of these four exercise bouts (probably no different from patterns of cardiovascular response during most resistance training protocols where sets are used to achieve progressive overload and greater total training load) that resulted in the target HR being attained for a large duration (67%) of the whole training session. Because isometric exercise is known to produce significant cardiovascular stress (28), this gradational increase in intensity throughout the training session may inadvertently provide an additional level of safety, although this requires confirmation in higher-risk populations.

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Study Limitations

Our study is limited by a small sample size and comprised only healthy male, Caucasian participants. As such, the application of this isometric exercise intervention requires future research in female participants (6). In addition, future work is required in individuals with higher cardiovascular disease risk as well as different ethnic populations, in particular Afro-Caribbeans who are at greater risk for developing hypertension.

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CONCLUSIONS

It is proposed that the incremental test and calculated target HR and BP ranges provide a useful means by which IWSE intensity can be reliably prescribed and monitored. Indeed, a recent study performed in our laboratory recruiting healthy participants demonstrated statistically significant reductions in resting BP (23) after IWSE prescribed from the methodology detailed in this article. However, it remains to be established whether these beneficial responses can be replicated in clinical populations when using this protocol. Moreover, future research is required to explore the importance of the acute pressor response during IET as a potential determinant of BP adaptation and whether this is safe in higher-risk populations. This also raises the question as to whether specific BP guidelines are necessary for exercise professionals who may choose to prescribe isometric exercise.

The authors declare no funding and no conflict of interest.

The results of this study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation, and the results of the present study do not constitute endorsement by the American College of Sports Medicine.

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