Acute blood pressure responses to plank and wall sit isometric exercise in adults

Abstract

Objectives:

Isometric exercise is an accessible, time efficient intervention for reducing blood pressure (BP). However, guideline recommendations for its use remain sparse due to safety concerns. This study aimed to determine the proportion of participants whose BP exceeded predefined limits during isometric exercise. Secondary aims were to examine differences in BP responses between sexes and across exercises.

Methods:

Participants attended the laboratory for two sessions: screening and familiarisation, and exercise testing. In session two, participants performed the plank and wall sit in a randomised order while their BP was continuously monitored noninvasively using the Finapres NOVA. Each exercise was performed until volitional fatigue or until reaching the American College of Sports Medicine limits of SBP (>250 mmHg) or DBP (>115 mmHg).

Results:

Sixty-two apparently healthy participants completed the study (age 26.7 ± 8.2 years, 52% women, resting BP 121 ± 8/77 ± 6 mmHg). Almost all participants were stopped [plank: 97% (95% confidence interval, 95% CI 89–99); wall sit: 98% (95% CI 91–100)] due to DBP more than 115 mmHg after an average of approximately 50 s of exercise. All women exceeded the DBP limit [plank: 100% (95% CI 89–100); wall sit: 100% (95% CI 89–100)] compared to most males [plank: 93% (95% CI 78–99); wall sit: 97% (95% CI 83–99)]. No participants reached the SBP limit. No adverse events were reported.

Conclusion:

Common isometric exercises cause a marked increase in BP, particularly DBP. Despite this, no adverse events occurred. Future studies should examine the dose-response of different isometric exercises in higher risk populations to better inform its clinical suitability.

INTRODUCTION

Clinical guidelines recommend exercise, particularly aerobic exercise, for the management of hypertension [1–3]. A recent systematic review and network meta-analysis of 270 randomised controlled trials found that multiple exercise types [i.e. aerobic, dynamic resistance, combined, high-intensity interval training, isometric exercise training (IET)] significantly reduce blood pressure (BP) [4]. Of the exercise types reviewed however, IET was superior to aerobic, dynamic resistance, and high-intensity interval training for reducing SBP. Moreover, IET is perhaps the most accessible, adaptable for those with functional limitations, does not require specialised equipment, and is time efficient, requiring modest doses to reduce BP (typically 3 x 15 min sessions per week) compared to aerobic and dynamic resistance exercise. Thus, IET appears a viable exercise choice for the management of BP. However, recommendations for the use of IET in clinical practice for those with high-normal BP or hypertension are debated [5,6], with limited position stands or clinical guidelines currently advocating for IET as a BP management strategy [3,7,8].

There are two main reasons underpinning the reservation to recommend IET for people with high-normal BP or hypertension. First, the evidence base for IET in these cohorts is less established compared to aerobic or dynamic resistance training and the effect sizes may be influenced by small study bias [4,5]. Second, and probably more importantly, there have been historical safety concerns over the use of resistance training, including IET, for people with hypertension due to the exaggerated pressor response that occurs during exercise [9]. Indeed, even at relatively low doses and intensities (i.e. 2 min handgrip exercise at 30% maximal voluntary contraction), notable increases in SBP and DBP have been observed in people with high-normal BP or hypertension [10,11]. Moreover, other common isometric exercises like the wall sit can evoke even greater increases in BP during exercise, especially when performed at higher intensities [12,13].

Exercising BP, both low and high, is associated with the development of future cardiovascular disease [14–16]. However, these associations are based solely on studies of aerobic exercise. Moreover, the relationship between exercise hypertension and incident cardiovascular disease remains contentious, especially when confounding factors such as workload, sex differences, and underlying physiological changes are not considered [17,18]. Recommendations for BP during exercise and what constitutes a normal or exaggerated response is also debated [17,19]. Despite the sometimes pronounced increases in BP during isometric exercise, adverse events are rare [12,13,20]. Further, to our knowledge, no studies have established a threshold for a hypertensive response to isometric exercise, nor linked BP during isometric exercise to future cardiovascular disease. Nonetheless, concerns remain around the use of IET for the management of hypertension.

To date, relatively few studies have investigated how lower limb IET affects BP during exercise [21]. Moreover, many of the existing studies are limited by small samples sizes (< 30 participants) and poor generalisability due to primarily recruiting young males. Another key limitation is that most of the exercise protocols used in the previous studies (i.e. handgrip exercise at low intensity) do not reflect how isometric exercise is performed in recreational and clinical settings (i.e. involving larger muscle groups and performed at higher intensities or for longer durations). Additionally, no studies have investigated changes in BP during the plank, one of the most common isometric exercises, and one that is also suggested by recent clinical guidelines as an option for IET [8].

To better inform the evidence base for the prescription of IET in practice, it is important to understand how common isometric exercises, including those recommended by some clinical guidelines [8], affect BP in more diverse samples and whether it is associated with adverse events. Therefore, the primary aim of this study was to determine the proportion of participants whose BP exceeded predefined upper limits during the plank and wall sit exercises. The secondary aim was to determine whether the BP response during isometric exercise differed between sexes and across different exercises.

MATERIALS AND METHODS

This study was conducted in the Exercise Physiology laboratory and Lifestyle Clinic at the University of New South Wales, Sydney, Australia. The study was conducted in accordance with the Declaration of Helsinki. Ethical approval was obtained from the institutional Human Research Ethics Committee (HC190256) and all participants provided written informed consent. Participants were recruited via social media advertisements, institutional mailing lists, and flyers situated around campus. Participants were included if they were aged 18–65 years and were deemed medically suitable to perform isometric exercise after preexercise screening. Participants were excluded if they had an acute illness, were in the active phase of proliferative retinopathy, presented with unstable cardiovascular or metabolic disease, had resting SBP more than 200 mmHg and/or resting DBP more than 110 mmHg, or had any other condition that would make it medically unsafe for them to perform isometric exercise. The study was performed over two sessions, each approximately 35 min in duration and separated by at least 2 days. Participants were asked to avoid caffeine and strenuous exercise prior to each session. All sessions were conducted by undergraduate Exercise Physiology or Medicine students under the supervision of an Accredited Exercise Physiologist.

Session one: screening and familiarisation

At the beginning of the first session, participants’ suitability for exercise was determined using the Exercise and Sports Science Australia preexercise screening tool. Stage 1 was administered to identify any potential contraindications to exercise. Stages 2 and 3 (excluding fasting lipids and blood glucose) were administered to quantify the participants’ height, weight, body mass index (BMI), waist circumference, and resting BP. Resting BP was measured according to the recommendations of the International Society of Hypertension [1]. Briefly, participants sat quietly for 3–5 min, after which their BP was measured using a manual sphygmomanometer (Durashock DS54, Welch Allen, New York, USA). For those whose resting BP was less than 130/85 mmHg, no further measurement was taken. For those whose resting BP was above this threshold, two further measurements were taken, each separated by 1 min. Resting BP was then calculated as the average of the last two measurements. Participants also completed the Short-form International Physical Activity Questionnaire to measure their physical activity levels.

Following screening, eligible participants were familiarised with the isometric exercises they would be asked to perform in session two: the plank and the wall sit. These two exercises were chosen as they can be performed without any equipment, are some of the most common isometric exercises performed in recreational and clinical settings, and are also recommended by some clinical guidelines [8]. The experimenter demonstrated each exercise and provided verbal instructions on the correct technique. Participants were given the opportunity to practice, with their technique corrected when necessary. The experimenter then informed participants that in the second session they would be asked to perform each exercise for as long as possible, or to a certain BP limit, while their BP was continuously recorded.

Session two: exercise testing

At the start of the second session, participants were set up for noninvasive continuous BP monitoring using the Finapres NOVA (Finapres Medical Systems, Enschede, Netherlands). This device is considered the most reliable noninvasive method for BP measurement and is commonly used to assess haemodynamic parameters during exercise [22–26]. BP measures recorded from the Finapres during exercise are also moderately-highly correlated with those obtained using auscultation [26]. Details regarding the setup and calibration of the Finapres NOVA are described in detail elsewhere [23]. Briefly, the device was fitted to the participant's nondominant arm by attaching the inflatable finger cuff to the participant's middle finger between the distal and proximal interphalangeal joint. The front-end unit was then attached to the participant's wrist using a velcro strap. A height correction unit was fitted to automatically correct for hydrostatic pressure changes due to movement in the hand position relative to the heart. A brachial cuff was placed over the participant's arm and the device was the calibrated using the built-in programme (Finapres Medical Systems, Enschede, the Netherlands) according to the manufacturer's instructions. After calibration, built-in software (BeatScope, Finapres Medical Systems, Enschede, the Netherlands) was used to reconstruct mean brachial arterial pressure from the finger pulse pressure waveform and calculate BP and other haemodynamic parameters at rest and during exercise.

Following set up, participants sat quietly for 3–5 min while baseline readings were obtained, after which they completed the plank or wall sit in a randomised and counterbalanced order. The randomisation sequence for the exercises was computer generated using Microsoft Excel. During each exercise, participants were instructed to keep their hand and fingers relaxed so as not to interfere with the recordings from the Finapres NOVA. Participants were asked to perform the exercise until volitional failure or until their BP reached a predefined upper limit and stayed above this threshold for 3–5 s. We used the American College of Sports Medicine guidelines for exercise testing and prescription to determine the upper BP limits at which to terminate exercise: SBP more than 250 mmHg and/or DBP more than 115 mmHg [27]. We note that these guidelines were developed for maximal aerobic exercise testing, however the same thresholds were applied in this study as we are not aware of similar guidelines for resistance exercise. Volitional failure was used as the exercise endpoint as this better reflects how isometric exercised is used in real-world settings compared to the fixed duration protocols that are more common in research settings. During exercise, participants provided a rating of perceived exertion (RPE) every 30 s and at the end of exercise using the Borg 0–10 scale [28]. A rest period of approximately 5–7 min was then given to allow BP to return to baseline, after which time participants completed the second exercise. Any adverse events reported by participants were recorded.

Sample size calculation

The sample size calculation was performed using an online tool for estimating the proportion or apparent prevalence with specified precision [29]. This required imputation of the assumed prevalence, desired precision, and confidence interval (CI). Previous studies have shown normotensive and hypertensive individuals are unlikely to reach a SBP more than 250 mmHg during isometric exercise [12,13]. However, DBP more than 115 mmHg was reported in 20–25% of participants during isometric wall sit exercise [12]. Assuming the proportion of participants whose DBP exceeds 115 mmHg during isometric exercise is 20%, then 62 participants were required to detect this with 10% precision and a 95% CI.

Data processing and statistical analysis

Raw data were exported from the Finapres NOVA to a Microsoft Excel file. The average and maximum values for BP during the plank and wall sit were then calculated for analysis. We report numerical and categorical data for BMI, waist circumference, BP and physical activity. The categories were as follows: BMI (underweight <18.5; healthy weight 18.5–24.9; overweight 25–29.9; obese >30) [27]; waist circumference (men: increased risk >94 cm; greatly increased risk >102 cm; women: increased risk >80 cm increased risk; greatly increased risk >88 cm) [27]; and BP (normal: SBP <130 mmHg and DBP <85 mmHg; high-normal: SBP 130–139 mmHg and/or DBP 85–89 mmHg; Grade 1 hypertension: SBP 140–150 mmHg and/or DBP 90–99 mmHg; Grade 2 hypertension: SBP ≥160 mmHg and/or DBP ≥100 mmHg) [1]. Physical activity was classified as low, moderate, or high based on algorithms from a purpose-built electronic spreadsheet [30].

Statistical analysis was performed in IBM SPSS Statistics (version 26). Normality of the data was assessed using visual inspection of histograms and Q-Q plots as well as the Shapiro Wilk test. Numerical and categorical data were summarised using descriptive statistics, with exploratory subgroup analysis performed for people with high-normal BP or hypertension. For the primary aim, the proportion of participants whose BP exceeded the upper limit and corresponding 95% CI was estimated using the binomial exact calculation. For the secondary aims, paired t-tests were used to investigate the effect of the plank and wall sit on average and maximum BP during exercise as well as differences between the two exercises. Independent t-tests were used to examine sex differences in the effect of isometric exercise on BP. The threshold for statistical significance was set at P less than 0.05. Data are presented as mean ± standard deviation or mean difference (95% CI) unless otherwise stated.

RESULTS

Participant characteristics

Sixty-two participants (n = 32 women) completed the study. The average age of participants was 26.7 ± 8.2 years (range 19–53). Most participants had a normal BMI (71%), normal waist circumference (87.1%), normal BP (80.6%), and were physically active at a moderate to high level (95%). Six participants (9.7%) had high-normal BP and six (9.7%) had grade 1 hypertension. Participant characteristics are outlined in Table 1. There was a technical error during recording of the wall sit for one participant, so this data was not available for analysis. Otherwise, there was no missing data for any other participant.

TABLE 1.

Participant characteristics

	Total (n = 62)	Men (n = 30)	Women (n = 32)
Age (years)	26.7 ± 8.2	27.2 ± 8.5	26.3 ± 8.0
Height (cm)	169.8 ± 9	175.6 ± 7.0	164.3 ± 7.0
Weight (kg)	67.7 ± 10.9	72.6 ± 8.8	63.0 ± 10.7
BMI (kg/m2)	23.4 ± 3.5	23.4 ± 2.3	23.4 ± 4.4
Underweight	1 (1.6%)	0 (0%)	1 (3.1%)
Healthy	44 (70.9%)	21 (70%)	23 (71.9%)
Overweight	14 (22.6%)	9 (30%)	5 (15.6%)
Obese	3 (4.9%)	0 (0%)	3 (9.4%)
Waist circumference (cm)	78.0 ± 12.6	79.8 ± 7.4	76.4 ± 15.9
Healthy	54 (87.2%)	29 (96.7%)	25 (78.1%)
Increased risk	4 (6.4%)	1 (3.3%)	3 (9.4%)
Greatly increased risk	4 (6.4%)	0 (0%)	4 (12.5%)
Resting SBP (mmHg)	121 ± 8	124 ± 6	118 ± 8.0
Resting DBP (mmHg)	77 ± 6	76 ± 6	77 ± 6
Normotensive	50 (80.6%)	24 (80%)	26 (81.2%)
High-normal BP	6 (9.7%)	3 (10%)	3 (9.4%)
Grade 1 hypertension	6 (9.7%)	3 (10%)	3 (9.4%)
Physical activity (METmin/wk)	2689 ± 1520	3000 ± 1411	2395 ± 1583
Low	3 (4.8%)	1 (3.3%)	2 (6.3%)
Moderate	21 (33.9%)	9 (30%)	12 (37.5%)
High	38 (61.3%)	20 (66.7%)	18 (56.2%)

Data are presented as mean ± standard deviation or n (%).

Primary outcome – proportion of participants with blood pressure exceeding the upper limit during the plank and wall sit

During the plank, 60/62 participants (97%, 95% CI: 89–99) were stopped for DBP more than 115 mmHg. This occurred after 50 ± 34 s of exercise and corresponded to an RPE of 5.8 ± 1.8, denoting hard perceived effort. For males, 28/30 (93%, 95% CI: 78–99) were stopped for DBP more than 115 mmHg after 62 ± 37 s of exercise (range 10–132 s). The remaining two male participants stopped the plank due to volitional fatigue after 118 and 162 s, respectively. Average RPE for men at the end of the plank was RPE 6.0 ± 1.9. For women, all 32 (100%, 95% CI: 89–100) were stopped for DBP more than 115 mmHg after 39 ± 27 s of exercise (range 5–120 s). Average RPE for women at the end of the plank was 5.7 ± 1.8. No participants were stopped during the plank for SBP more than 250 mmHg, and no adverse symptoms or events were reported during or after exercise.

During the wall sit, 60/61 participants (98%, 95% CI: 91–100) were stopped for DBP more than 115 mmHg. This occurred after 53 ± 39 s of exercise and corresponded to an RPE of 6.0 ± 1.7, denoting hard perceived effort. For men, 29/30 (97%, 95% CI: 83–100) were stopped for DBP more than 115 mmHg after 65 ± 46 s of exercise (range 9–172 s). The remaining one male participant stopped the wall sit due to volitional fatigue after 218 s. Average RPE for men at the end of the wall sit was 6.0 ± 1.8. For women, all 31 (100%, 95% CI: 89–100) were stopped for DBP more than 115 mmHg after 42 ± 29 s of exercise (range 5–125 s). Average RPE for women at the end of the wall sit was 5.4 ± 1.6. No participants were stopped during the wall sit for SBP more than 250 mmHg and no adverse symptoms or events were reported during or after exercise.

Secondary outcome – sex differences in blood pressure during the plank and wall sit

Maximum SBP was significantly higher in men (180 ± 10) compared to women (173 ± 14) for the plank [mean difference (95% CI): 7 (1 to 14) mmHg, P = 0.021]. Maximum SBP was also significantly higher in men (193 ± 12) compared to women (185 ± 12) for the wall sit [mean difference (95% CI): 8 (2 to 14) mmHg, P = 0.013]. No other significant differences in BP were observed between men and women (Table 2).

TABLE 2.

Average and maximum blood pressure (mmHg) during the plank and wall sit

	Total	Men	Women	P (sex)	P (exercise)
Plank (n = 62)
Average SBP	155 ± 10.6	157 ± 10.2	153 ± 10.7	0.110	<0.001
Max SBP	177 ± 12.9	180 ± 10.1	173 ± 14.3	0.021	<0.001
Average DBP	106 ± 5.6	105 ± 6.4	107 ± 4.7	0.224	0.026
Max DBP	123 ± 3.5	123 ± 4.3	124 ± 2.7	0.642	0.370
Exceeded DBP limit (n, %)	60 (97%)	28 (93%)	32 (100%)
Wall sit (n = 61)
Average SBP	164 ± 13.2	166 ± 14.2	161 ± 11.8	0.110
Max SBP	189 ± 12.6	193 ± 12.1	185 ± 12.0	0.013
Average DBP	104 ± 7.3	102 ± 8.6	106 ± 5.5	0.093
Max DBP	123 ± 2.6	123 ± 3.9	123 ± 3.4	0.944
Exceeded DBP limit (n, %)	60 (98%)	29 (97%)	31 (100%)

Secondary outcome – differential effects of the plank and wall sit on blood pressure

The average and maximum SBP and DBP during the plank and wall sit are shown in Table 2. Average SBP was significantly higher during the wall sit (164 ± 13) compared to the plank (155 ± 11) [mean difference (95% CI): 9 (6 to 11) mmHg, P < 0.001]. Maximum SBP was significantly higher during the wall sit (189 ± 12) compared to the plank (177 ± 13) [mean difference (95% CI): 12 (9 to 15) mmHg, P < 0.001]. Average DBP was significantly higher during the plank (106 ± 6) compared to the wall sit (104 ± 7) [mean difference (95% CI): 2 (0.2 to 3.4) mmHg, P = 0.026].

Exploratory analysis – blood pressure responses to the plank and wall sit in participants with high-normal blood pressure or grade 1 hypertension

In the small subset of participants with high-normal BP (n = 6) or grade 1 hypertension (n = 6), average and maximum SBP during the plank was 153 ± 8 and 178 ± 14 mmHg, respectively, and average and maximum DBP was 105 ± 4 and 125 ± 5 mmHg, respectively. During the wall sit, average and maximum SBP was 166 ± 17 and 187 ± 15 mmHg, respectively, and average and maximum DBP was 106 ± 6 and 124 ± 3 mmHg, respectively. All participants with high-normal BP (100%, 95% CI 54–100) or grade 1 hypertension (100%, 95% CI 54–100) were stopped prior to volitional fatigue for DBP more than 115 mmHg after 34 ± 21 s (range 5–77 s) for the plank and 32 ± 20 s (range 5–68 s) for the wall sit. Average RPE during the plank and wall sit was 5 ± 1.9, denoting hard effort.

DISCUSSION

The results of this study found that in a sample of predominantly young and healthy normotensive adults, isometric plank and wall sit exercises significantly increase BP during exercise, with the ‘safe’ DBP limit reached for almost all participants during the plank [97% (95% CI 89–99)] and wall sit [98% (95% CI 91–100)] after approximately 50 s of exercise (and approximately 15–20 s earlier in participants with high-normal BP or grade 1 hypertension). Regarding the secondary outcomes, average and maximum intra-exercise SBP were greater for the wall sit compared to the plank. Sex differences were also observed, with maximum SBP higher in men compared to women, and women reaching the DBP limit approximately 20 s earlier than men for both exercises. No adverse events were reported.

To our knowledge, this is the first study to investigate the effect of the plank on intra-exercise BP. Our results showed that during the plank, the DBP limit (115 mmHg) was reached in 93% of men (95% CI: 78–99) after approximately 60 s of exercise and in 100% of women (95% CI: 89–100) after approximately 40 s of exercise. While exercise was stopped at this time, participants’ RPE was only approximately 6/10 on average, indicating they could have continued for longer if allowed (and anecdotally this was the case when participants were asked if they could have kept going). In recreational and clinical settings, people often perform the plank for at least a minute, sometimes longer when endurance allows. Thus, based on our results, these individuals may be regularly reaching DBP values above the recommended upper limit (albeit the upper limit suggested for aerobic exercise), but are presumably doing so without negative symptoms or adverse events. For exercisers who perform multiple sets, BP may be even higher as it tends to increase slightly with each subsequent set of isometric exercise [13,31], albeit not always [32].

The clinical implications of high BP during isometric exercise are not clear. To our knowledge, no studies have established a threshold for a hypertensive response to isometric exercise, nor linked BP during resistance exercise to future cardiovascular disease as shown for aerobic exercise [15,16]. Moreover, as high BP during isometric exercise is clearly apparent in low-risk participants, it may simply represent the normal physiological response, one that is complicated by variability in the physiological determinants of elevated exercising BP [18]. Together, these results suggest the need for some caution when interpreting BP during isometric exercise and the potential safety implications. Indeed, it is important not to discourage people with elevated BP or hypertension from exercising. Therefore, if there are concerns around the prescription of IET, more conservative protocols like low-intensity handgrip exercise may be more appropriate as these cause smaller increases in BP during exercise and are safe and effective in high-risk populations [4,33]. There remains a need to conduct large, high-quality longitudinal cohort studies to determine what link, if any, exists between significantly raised BP during isometric exercise and adverse outcomes in the immediate- and long-term as the current clinical significance of these BP increases remain unclear. The results of such studies would better inform the use of IET in clinical practice.

Our study adds to the growing body of research investigating the intra-exercise effect of the wall sit on BP. In the present study, participants performed a single wall sit at approximately 90 degrees knee joint flexion, typical of how this exercise is performed in recreational settings and clinical practice. In contrast, earlier studies have used an incremental approach where knee joint angle was decreased by 10 degrees every 2 min (135–125–115–105–95 degrees) [12,13] or 4 x 2 min contractions with intensity based on achieving a target heart rate [31,32]. Despite differences in exercise protocols, the results were largely comparable. That is, the wall sit caused an increase in SBP that did not reach the upper limit (250 mmHg) and typically did not exceed 180–200 mmHg. In contrast, increases in DBP were more pronounced, exceeding the upper limit (115 mmHg) for almost all participants in the present study and some participants in the earlier studies [12,13]. These results have implications for IET and BP management. Namely, performing a wall sit with a higher knee joint angle (lower intensity) may be preferred as this reduces resting BP [34] and causes a smaller increase in BP during exercise. For the same reason, lower intensity alternatives of the plank (e.g. performed on knees instead of toes) may also be preferred.

We chose to terminate exercise testing when participants’ BP reached the threshold for a hypertensive response to exercise as proposed by ACSM guidelines (i.e. SBP more than 250 mmHg or DBP more than 115 mmHg) [27]. We chose these thresholds in the absence of a similar threshold for resistance exercise but recognise they are one of several thresholds we could have chosen [35], all of which may have limited application in the context of IET. Indeed, the normal physiological DBP response to aerobic exercise (unchanged or slight decrease) is different to that of isometric exercise (increase), so adopting the same DBP thresholds to terminate these different types of exercise may not be appropriate. Instead, alternative criteria such as adverse symptoms (e.g. dizziness) may be more appropriate. Moreover, it is important to acknowledge that these thresholds are not without limitations, especially outside the context of aerobic exercise. These limitations have been discussed in detail elsewhere and are summarised as “conservative” [17] and “cautionary rather than evidence based” [19]. Indeed, the origin of the DBP criterion is unknown [19] and there is currently little evidence that exceeding the upper SBP or DBP threshold is linked to adverse events during aerobic exercise [36]. The same is true for resistance exercise where adverse events are rare despite sometimes exceedingly high BP values [37]. In their opinion piece, Currie et al. [19] call for a re-evaluation of existing exercise BP guidelines. We contend there is a need to extend this to resistance exercise, particularly IET, to allay historical safety concerns and better inform the use of this accessible, time-efficient, and effective exercise modality for reducing BP.

There are several lines of future research that would be useful to inform IET prescription in clinical settings. First, replicating the current study in a large sample of high-risk participants (e.g. elderly individuals or those with high-normal BP or hypertension) would provide more relevant data for clinicians and clinical guidelines. Second, investigation of the effect of different types, doses, and intensities of isometric exercise are needed. For example, wall sits with different knee angles (e.g., 90° vs. 120°) or planks with different support bases (e.g. on toes vs. knees) would allow comparison of the BP response at different exertion levels, providing useful practical information. Third, allowing participants to perform the exercise until volitional failure (as is more common in practice) would provide important data on the magnitude of BP changes during exhaustive isometric exercise. That is, do SBP and DBP continue to increase the longer the isometric contraction is held, or do they plateau? And if they continue to increase, are these changes associated with any adverse signs or symptoms? This would improve understanding of the safety of more real-world higher intensity, longer duration isometric exercise. Finally, there is a need for further exploration of the effect of sex given the differences between males and females in resting and exercising BP [18,38]. This was true in our study as well where females reached the DBP limit significantly faster than males in both exercises. The underlying physiological reasons for this were not examined but could be due to several factors such as differences in arterial stiffness and cardiac-peripheral performance, body size and composition (e.g. muscle mass), muscle strength, and sympathetic control [18,39].

There were a few notable strengths of our study. First, we used an isometric exercise protocol more typical of real-world settings which may improve generalisability of our findings. Second, our sample size was larger than most previous studies and included more females, allowing for investigation of sex differences which has seldom been done. Optimal resting BP is lower in females than males [38], so future studies could investigate if this is true for exercising BP as well, particularly as females in our study reached the DBP limit significantly faster. Our study also had limitations. While the sample size was larger than most previous studies, the 10% precision used for our sample size calculation may have been too liberal, contributing to the imprecise CIs for the primary outcome despite the high proportion of participants reaching the DBP limit during exercise. Another potential limitation was the method used to assess BP during exercise. While the Finapres NOVA is commonly used in research settings to measure changes in BP during exercise, and the values obtained are moderately-highly correlated to BP measured using auscultation [26], we are not aware of studies that have empirically validated its use for this purpose. This should be considered when interpreting the validity and reliability of our findings. Lastly, there is substantial variation in the threshold(s) used to define a hypertensive response to exercise [35]. While the criterion used in the current study is one of the most common, our results may have been different had another threshold been used [35].

In conclusion, the results of our study show that the plank and wall sit notably increase SBP and DBP during exercise, with the ‘safe’ upper DBP limit reached in almost all participants after approximately 50 s of exercise and approximately 15–20 s earlier in those with high-normal BP or grade 1 hypertension. The wall sit causes a more pronounced increase in SBP compared to the plank, and females reached the upper DBP limit faster than males. Despite the increases in BP, no adverse events were reported. Future studies should investigate the safety of different durations and intensities of common isometric exercises on BP, particularly in women and those with hypertension, to better inform their use in clinical practice.

ACKNOWLEDGEMENTS

None.

Conflicts of interest

No funding was received for the conduct of this study. A.E.S. is funded by an Investigator Grant from the National Health and Medical Research Council of Australia (APP2017504) and has received speaker honoraria from Servier, Abbott, Sanofi, AstraZeneca, Medtronic, Omron, and Aktiia and serves on scientific advisory boards for Medtronic, Alnylam, AstraZeneca, SiSU Health, and Sky Labs. The other authors have no conflict of interest, financial or otherwise, to be declared.