Key words
training - safety - central stiffness - pulse wave velocity
Introduction
Low-load resistance exercise with blood flow restriction (BFR) is becoming
increasingly employed in rehabilitation [1] because
of the similar musculoskeletal benefits it confers compared to traditional heavy
load strength training, including muscle hypertrophy [2] and strength [3]. However, despite
BFR’s growth as an alternative exercise approach, there are still concerns
regarding its safety profile [4]
[5].
While peripheral hemodynamics (i. e. brachial BP measures) have been a focus
of research in elucidating the safety profile of BFR resistance exercise, less is
known regarding the central hemodynamic responses (i. e. aortic BP and
stiffness measures). A recent systematic review [6]
highlighted significant heterogeneities that exist in the limited body of literature
in this area, including differences in BFR prescriptive factors and repetition
schemes, as well as the absence of the “gold standard” pulse wave
velocity (PWV) assessment, for arterial stiffness in acute resistance exercise
investigations. PWV measures the difference in time delay of the systolic waveform
between a central (i. e. aorta) and peripheral (i. e. radial or
tibial artery) site and provides a measure of stiffness associated with the central
and peripheral arterial apparatus, respectively [7].
Significantly stiffer arteries may predispose exercisers to a higher risk of
cardiovascular events [7] during acute bouts of
exercise. Of potential significance are that daily transient increases of arterial
stiffness in aggregate can elevate the risk for CVD [8]. In addition, there is evidence acute reductions in arterial stiffness
can lower risk for cardiovascular disease and mortality [9], emphasizing the importance of continual investigations.
Evidence indicates that chronically elevated PWV independently predicts the presence
of cardiovascular risk factors (i. e. atherosclerosis) and hypertension
[7], as well as morbidity and mortality [10]. As such, it is prudent to understand the impact
that an acute resistance exercise session with BFR may play on PWV, particularly as
BFR is becoming more utilized in populations with hypertension [11], obesity [12] and
heart failure [13].
While research on PWV appears to indicate a deleterious effect on health when
chronically elevated, acute measures of central arterial stiffness (including PWV)
following resistance exercise appear to vary based on whether the exercise was
performed with the upper or lower body [14], the
contraction type (i. e. concentric versus eccentric) [15], the repetition scheme used [16], the exercise cadence employed [17], and the load used (i. e. 30%
1-rep max versus 70–80% 1-rep max) [18]. Arterial stiffness measures tend to increase acutely post-
resistance exercise and a rule of thumb is that chronic PWV elevations of
1 m/s increases all-cause mortality [19], although the relative importance of acute increases is less
established, particularly in the BFR literature. Therefore, understanding the impact
of resistance exercise with BFR on acute measures of arterial stiffness is important
but is currently understudied.
As BFR becomes more widely implemented in different practice settings [1], the availability of BFR equipment for consumer
purchase has increased. However, there is a dearth of research available on the
different types of BFR devices used, as well as features marketed to enhance its
safety, tolerability, and/or efficacy during application [20]. One of those features is autoregulation of
applied BFR pressures, whereby the applied pressure to the exercising limb from the
BFR cuff is kept relatively constant compared to a manually inflatable
(non-autoregulated) cuff that does not adjust and, therefore, may heighten
cardiovascular and perceptual exercise responses [21]. Autoregulation of applied pressures is likely of practical importance
due to the interaction of the cuff and the underlying musculature, as well as its
potential impact on modulating acute exercise-related responses. In the concentric
portion of the exercise, the cross-sectional area of the limb enlarges as the muscle
fibers within shorten. Under non-autoregulated conditions, the diameter of the cuff
does not change during the concentric phase, theoretically creating greater
pressures inside the limb. Autoregulated BFR devices accommodate for increases in
limb diameter during the concentric phase, and may afford similar limb pressures in
both phases of muscular contraction [21]. Studies
using cuffs capable of autoregulation of applied BFR pressures have been implemented
in healthy [22] and clinical populations [23]. Conversely, cuffs that are unable to autoregulate
have also been used in healthy [24] and clinical
populations [25]. Both forms of pressure regulation
appear to be safe [5], but it is currently unknown
whether autoregulation further enhances the safety profile and tolerance of BFR
exercise compared to non-autoregulated pressure application, as well as impact
measures of performance.
A recent study highlighted the potential for autoregulation of applied BFR pressures
to enhance the safety of BFR exercise compared to non-autoregulated cuff
applications. Jacobs et al. [26] showed a near 3x
risk reduction in minor adverse events (i. e. lightheadedness) in the
autoregulated compared to the non-autoregulated cuff condition when performing the
same exercise series. This study provided the first direct evidence that
autoregulation may enhance the acute safety profile of BFR exercise. However, the
hemodynamic (i. e. brachial blood pressures) and perceptual responses
(i. e. rate of perceived exertion/discomfort) to exercise between
conditions were largely the same, indicating the influence of other factors not
measured. A potential factor may be acute changes in central arterial stiffness. It
is important to also note a majority of BFR devices on the market today do not have
the ability to autoregulate, but the impact of blood flow restriction on central
stiffness has yet to be investigated. Thus, much is unknown regarding the impact of
autoregulation of applied pressures on arterial stiffness.
This study aimed to assess the acute impact of autoregulation of applied pressures on
arterial stiffness in a cohort of healthy, physically active adults during wall
squat exercise to volitional fatigue. In accordance with the results from previous
studies using both types of pressure regulation during BFR exercise that promote
similar long-term musculoskeletal benefits, we hypothesized that differences in
acute measures of arterial stiffness would be observed between the autoregulated and
non-autoregulated BFR cuff conditions, and autoregulated and low-load resistance
exercise without BFR.
Materials and Methods
Participants
Twenty-five physically active males and females were recruited at an institution
affiliated with one of the authors via a flyer and email outreach. Each female
participant served as their own control and their phases of menstrual cycle
followed normal patterns and were not controlled throughout the study period.
Previous investigations have concluded variations in menstrual phases and use of
hormonal contraceptives have little to no influence on indices of arterial
stiffness (our primary outcome measure) [27]
[28]
[29]. Physically
active was characterized as consistently exercising for greater than 6 months
of≥1,000 MET/min/week. Participants were initially
screened for study eligibility ([Table 1]) and,
if they met inclusion criteria and did not display any exclusion criteria, they
were scheduled for a familiarization session. None of the participants reported
tobacco use, however, potential exposure to secondhand smoke was not controlled.
Each participant signed an informed consent document in accordance with the
Declaration of Helsinki acknowledging potential risks and harms.
Table 1 Inclusion and exclusion criteria. BP, blood
pressure. MET, metabolic equivalent. BMI, body mass index. CHD,
chronic heart disease. CVD, cardiovascular disease.
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Inclusion Criteria
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Exclusion Criteria
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Study design
This intervention assessed differences in arterial stiffness following lower body
blood flow restriction (BFR) in exercise performed to volitional fatigue using
two different applied pressure settings (autoregulated [AR-BFR] versus
non-autoregulated [NAR-BFR]) in a sample of healthy, physically active
participants. In this crossover, randomized-controlled study, each participant
reported to the lab for four sessions. During the initial session
(familiarization), each participant was randomized to AR-BFR or NAR-BFR using a
randomization software (www.random.org/lists) and the exercise protocol
was performed without assessing outcomes. Following the familiarization session,
each participant was randomized into AR-BFR, NAR-BFR or No-BFR by the same
software and performed the identical exercise protocol as in the familiarization
session ([Fig. 1]), and the outcome variables
were assessed.
Fig. 1 Schematic of study protocol. AR-BFR, Autoregulated BFR
pressures; NAR-BFR, Non-autoregulated BFR pressures; No-BFR, Low-load
exercise without BFR.
Each session was separated by at least 7 days to reduce the potential impact on
exercise performance and recovery from soreness [30]. Participants were instructed to avoid caffeine and alcohol for
24 hours, and fully void before all sessions. They were advised to
continue their normal training activities while avoiding exercise-related
activities 24 hours before each session, including the familiarization
session. All testing occurred within±1 hour week-to-week to
minimize diurnal variations in responses. Measurements were conducted in the
Exercise Physiology Research Lab at an institution affiliated with one of the
authors after a 4-hour fast between 06:00 to 12:00 hours. Ambient
temperature was set at 21° Celsius (70° Fahrenheit) for all
testing sessions. The study was approved by the institutional ethics
committee.
Testing protocol
Initially, participants had their one repetition maximum (1-RM) determined and
then underwent a familiarization session to acclimate them to the sequencing of
data collection and the BFR stimulus. In all sessions, four sets to volitional
fatigue of dumbbell wall squats were performed using ~20% 1-RM
(to the nearest 5-pound increment) with (AR-BFR) or without (NAR-BFR)
autoregulation of applied pressures, in addition to a No-BFR condition.
Dumbbells were held with arms fully extended and the shoulder flexed at
0°. Due to the synthetic ice pad (Snipers Edge, Minneapolis, MN) mounted
on the wall, drag was minimized during the upward and downward phase of each
repetition. Rest between sets was 1 minute. Cadence was monitored via a
metronome (Seiko, Mahwah, NJ) for a 2-sec concentric and a 2-sec eccentric
phase, with range of motion set to 90° of knee flexion at the bottom and
full extension at the top. Volitional fatigue for each set was determined as the
inability to perform the technique to specifications (i. e. maintaining
back flat against the wall), inability to maintain appropriate cadence,
and/or desire to stop. A verbal warning was given upon the first
technique violation, and then the set was stopped with a second violation.
Ultrasonography of the carotid artery, applanation tonometry, and BP acquisition
were completed before and 10 minutes post-exercise for all training
sessions when the cuffs were not inflated. We selected 10 minutes
post-exercise due to feasibility and that a recent meta-analysis concluded data
collection periods between 0–14 minutes post-exercise yield
similar PWV values [31]. Rate of perceived
exertion (RPE), rate of perceived discomfort (RPD), and a subjective measure of
participant enjoyment of the session were assessed immediately post-exercise in
all training sessions while cuffs were inflated, as well as adverse events
monitored. In addition, total training volume and repetitions were recorded for
all trials ([Fig. 2]).
Fig. 2 Schematic of all treatment sessions. SBP, systolic blood
pressure; DBP, diastolic blood pressure; RPE, rating of perceived
exertion; RDP, rating of perceived discomfort; Perform again, 10-point
Likert scale assessing desire to perform exercise again; HR, heart rate;
RPP, rate pressure product.
BFR settings – autoregulation and limb occlusion pressure
For the AR-BFR or NAR-BFR familiarization and BFR exercise trials, two
11.5 cm variable contour pneumatic BFR cuffs (Delfi Personalized
Tourniquet Systems, Vancouver, Canada) were placed around the most proximal
portion of each thigh. Following a 10-min supine rest, limb occlusion pressure
(LOP) was determined in the supine position and subsequently set at 60%
LOP for the duration of exercise in accordance with practice recommendations
[32] and within the pressure recommendations
of a recent paper [33] that indicate a positive
impact in reducing repetitions to fatigue in comparison with load-matched
free-flow exercise. We acknowledge that there is a body of growing evidence
indicating that LOP changes as a function of position
(supine<sitting<standing in the legs) [34]
[35]. However, to the
authors’ knowledge, no study exists indicating that the differences in
%LOP significantly influence the acute responses to BFR exercise when
obtained in the position of exercise (i. e. standing) or in another
position (i. e. supine or sitting). Thus, we elected to assess LOP in
supine to increase reliability and align with most of the published literature
on BFR exercise. To minimize potential discrepancies, each participant used
34-inch length cuffs. The Delfi Personalized Tourniquet device has been
previously validated for its accuracy in determination of LOP compared to
doppler ultrasound [36] and has the capacity to
adjust the applied pressure (i. e. autoregulate) and cuff diameter to
the exercising limb to maintain a relatively consistent pressure despite changes
in muscle volume during exercise [21].
Furthermore, it has been shown to be the only commercially available BFR device
capable of maintaining set-interface pressure during BFR exercise compared to
four other BFR devices [37]. Conversely, during
NAR-BFR training, the cuff diameter does not adjust to the phase of muscle
contraction during exercise, possibly increasing cuff pressure on the limb above
60% LOP. NAR-BFR was performed by enabling a function that disabled
autoregulation of applied pressures while the cuff was still tethered to the
device with an air tube. BFR was applied continuously in both conditions,
inflating prior to the first set, and deflating after completion of the 4th set
following subjective assessments. Total time under tension was recorded for both
BFR sessions. Participants were blinded to the presence of autoregulation for
all trials but were not blinded when exercising in the No-BFR condition as no
cuffs were applied to the exercising limbs. A No-BFR training session utilizing
the same failure scheme and load but without the pneumatic cuffs applied to the
legs was also performed in a randomized order.
Familiarization session
A maximal dumbbell wall squat strength test (one-repetition maximum, 1-RM) was
completed prior to the familiarization session in the recreational center at an
institution affiliated with one of the authors in accordance with the guidelines
from the National Strength and Conditioning Association [38]. All 1-RMs were determined within five
attempts. Afterwards, participants walked to the laboratory and sat quietly with
legs uncrossed for 10 minutes prior to seated BP measurements following
standard guidelines [39]. Height, weight and body
composition assessment followed. Participants then rested supine on the
examination table for 10 minutes. Lastly, carotid ultrasound scans and
arterial tonometry were performed, which completed all pre-training assessments.
For all central arterial and peripheral assessments, the examiners were not
blinded to the group condition due to limitations in personnel. Participants
were then provided instructions on RPE, RPD, and shown a 1–10 Likert
scale assessing likelihood of performing the training again during the
familiarization session. The RPE scale ranged from 0 “no
exertion” to 10 “maximal effort” and participants were
cued with the same question, “How hard to you think you’re
working?” every time before answering. Similarly, the RPD scale ranged
from 0 “no discomfort” to 10 “maximal
discomfort” and participants were asked “How much discomfort do
you feel?” every time before responding. Participants were informed that
these would be assessed immediately post-exercise during sessions 2–4.
Each participant was then randomized into AR-BFR or NAR-BFR and performed the
exercise protocol as described above.
Outcome measures
Anthropometrics
Total body mass and height were measured during the familiarization session
on a medical scale and stadiometer (Detecto 439 Physician Beam Scale)
accurate to±0.1 kg and stadiometer between 0600 and 1200
after a void and 4-hour fast. Participants wore standard shorts and t-shirts
at the time of weighing. Air displacement plethysmography (BOD POD) (Cosmed
Metabolic Company, Rome) measured fat and fat-free body mass. Participants
wore tight clothing and sat quietly inside the BOD POD during three
sequential measurements.
Brachial Blood Pressure
Seated and supine brachial BP measurements were taken under quiet, ambient
(~21°) conditions. All BP measurements adhered strictly to
American Heart Association guidelines [39].
After a 5–10 minute rest period, both seated and supine
measurements were auscultated from the right brachial artery using an
automated sphygmomanometer (Welch, Allyn, New York). Systolic (SBP) and
diastolic (DBP) BPs were recorded every 2 minutes, and average SBP
and DBP readings were tabulated using 3 sequential measurements within
6 mmHg of each other.
Carotid Artery Ultrasonography
A doppler ultrasound machine probe (Terason t3300, Burlington, MA) was placed
approximately 2 cm distal from the carotid bulb after a 10-min
supine rest. Longitudinal B-mode images of the right common carotid artery
were recorded in 10-sec increments and measured manually offline. The
distance between the apical surface of the tunica media of the near and far
wall was used to determine systolic (maximal) and diastolic (minimal)
diameters. Three measurements of the systolic and diastolic diameters were
recorded and averaged.
Arterial Applanation Tonometry and Pulse Wave Velocity
Using a high-fidelity transducer (Complior Analytic Tonometer, Alam Medical,
Vincennes, France), right carotid arterial pressure waveforms and amplitudes
were recorded after a 10-min supine rest. The right brachial supine SBP and
DBP (explained above) and carotid waveforms were used to equate carotid SBP
(cSBP), DBP (cSBP) and mean arterial pressure (cMAP) through a proprietary
transfer function. All tonometry recordings were taken by the same
experienced researcher with excellent reproducibility (r>0.90;
p<0.05) for β-stiffness, PWV and arterial compliance
(AC).
Detailed procedures on PWV have been described elsewhere [40]. Briefly, after a 10-min supine rest for
conventional steady state conditions, three high-fidelity tonometers
(Complior Analytic Tonometer, Alam Medical, Vincennes, France)
simultaneously recorded and averaged 10 waveforms from the right side
carotid site lateral from the laryngeal prominence, radial site distal of
the scaphoid bone, and the femoral site at the most proximal portion of the
leg distal from the inguinal ligament. Distances between the arterial sites
were measured in the supine position with a caliper to the nearest
0.5 cm. Carotid-femoral PWV (CF-PWV) and carotid-radial PWV (CR-PWV)
were calculated by dividing the distance between arterial sites by the
foot-to-foot time delay between arterial waveforms (PWV=D
(m)/Δt (sec)).
β-Stiffness Index and Arterial Compliance
β-stiffness is an index of arterial stiffness, capturing the
nonlinear relationship between pressure and diameter independent of acute
changes to BP [7]. A detailed explanation of
the procedure can be found elsewhere [7].
Briefly, it was calculated as β=ln
(SBP/DBP)/[(systolic diameter-diastolic
diameter)/diastolic diameter] and expressed in arbitrary units.
Carotid systolic and diastolic diameters, cSBP and cDBP, were factored in
the β-stiffness calculations. Arterial compliance (AC) is an indice
of arterial stiffness and is sensitive to acute BP changes (29). AC was
calculated as (π(systolic diameter2 – diastolic
diameter2)÷4(SBP-DBP)) using carotid diameters and pressures [7].
Perceptual Experience Assessments
Immediately following completion of the final set and with the cuffs still
inflated, participants were requested to provide their rate of perceived
exertion (RPE) and rate of perceived discomfort (RPD), and were asked to
rate the likelihood that they would perform the same exercise again.
Participants were asked to anchor their response based on the entire
exercise session. Three 8-inch x 10-inch charts were held in front of the
participant by the same administrator in the following order: RPE, RPD and a
1–10 Likert scale. RPE and RPD scales were read to the participant
in accordance with a previous validation study [41]. Likelihood to perform this exercise again was assessed with
the question “on a scale of 1–10, how likely would you
perform the same exercise again? 10 being very likely and 0 being not likely
at all.” Participants were asked to report any adverse responses in
conjunction with the performance of each exercise bout. Any responses
gathered were classified according to a recently published review [5].
Statistical analysis
G*Power (Kiel University, Germany) software 3.1.9.7 calculated a sample
size of 20 participants using repeated measures analysis of variance (ANOVA) at
α=0.05 & β=0.80 to detect an effect size
of 0.35 [42]. The power calculation is based on
data from Stanford et al. [43] using central SBP
over time as the dependent variable and it was determined that twenty
participants were required for a moderate effect size. To account for the
expected attrition rate of 20%, 25 participants 18–40 years of
age of all races and ethnic backgrounds meeting the inclusion and exclusion
criteria ([Table 1]) were recruited for the
study. Statistical Package for the Social Sciences (IBM SPSS version 28, SPSS
Inc., Chicago IL) was used for both descriptive and inferential statistical
analyses. Shapiro-Wilk test was performed on variables to assess distribution
patterns. Paired samples t-tests assessed baseline (i. e. before
treatment) differences. A 3 (group) x 2 (time) two-way ANOVA was used to examine
the effects of treatment and the treatment-order interaction on variables of
interest. The Greenhouse-Geisser correction was used when there was failure to
meet assumptions of sphericity. Post hoc analysis (Tukey HSD) was performed on
variables with significant F-ratios. Findings with a p<0.05 were
considered significant, and all data are presented in means±standard
deviation (SD) unless otherwise stated. Effect sizes were reported as
Cohen’s d and were defined as: 0.2, small; 0.5, moderate;
and≥0.8, large [44].
Results
Participants
Participant characteristics are listed in [Table
2]. Thirteen males and 7 females completed the study. Attrition of
five participants occurred for various reasons, including time constraints,
missed appointments, acute illness, and loss of contact but not from BFR ([Fig. 1]). No injuries or adverse events related
to the treatments were reported. There were no significant baseline differences
recorded in pre-testing during the familiarization period in arterial stiffness,
cardiovascular ([Table 3]), or performance
([Table 4]) variables among any of the
randomized sessions.
Table 2 Baseline participant characteristics measured
during pretesting in the familiarization session. SD, standard
deviation. Yr, year. SBP, systolic blood pressure. DBP, diastolic
blood pressure. MAP, mean arterial pressure. 1 RM, one-repetition
maximum. LOP, limb occlusion pressure. MET, metabolic
equivalents.
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Variable
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Mean±SD
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Age, yr
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22.6±4.9
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Height, cm
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175.2±9.7
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Weight, kg
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79.7±15.9
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BMI, kg/m2
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25.6±4.9
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Body Fat, %
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15.9±8.9
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Fat Mass, kg
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12.7±8.5
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Fat Free Mass, kg
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66.8±14.3
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Seated SBP, mmHg
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124±10
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Seated DBP, mmHg
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74±7
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Seated MAP, mmHg
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90±7
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Dumbbell wall squat 1 RM, kg
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99.4±38.1
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Right leg LOP, mmHg
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205±17
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Left leg LOP, mmHg
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189±12
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MET ∙ min-1 ∙ wk-1
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2966±1400
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Table 3 Arterial stiffness, central hemodynamics and
muscle swelling responses pre- and post-intervention. Values
expressed as mean±SD; C, carotid; R, radial; F, femoral;
PWV, pulse wave velocity; SBP, systolic blood pressure; DBP,
diastolic blood pressure; PP, pulse pressure; MAP, mean arterial
pressure; β-SI, β stiffness index; AC, arterial
compliance; HR, heart rate; RPP, rate pressure product.
*P<0.05 Within Group; ‡ P<0.05
Between Group Effect with AR-BFR.
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AR-BFR
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NAR-BFR
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No-BFR
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Baseline Difference p values
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Variable
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PRE
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POST
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PRE
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POST
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PRE
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POST
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CF-PWV, m/s
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7.05±1.40
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7.12±1.43
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7.12±1.15
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7.69±1.65*‡
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6.87±1.04
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7.51±1.41*‡
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0.757
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CR-PWV, m/s
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9.33±3.07
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8.33±2.66
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9.35±2.50
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9.33±2.51
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8.91±1.74
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8.36±2.18*
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0.775
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Central SBP, mmHg
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117±15
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119±16*
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117±15
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119±14*
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115±12
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122±13*
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0.629
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Central DBP, mmHg
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67±8
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65±9
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67±8
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67±11
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67±8
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68±9
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0.840
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Central PP, mmHg
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50±12
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54±13*
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50±15
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53±13
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48±12
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55±13*
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0.641
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Central MAP, mmHg
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83±9
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83±10
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84±8
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84±10
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83±8
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86±9*
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0.713
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β-SI, U
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6.02±2.04
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5.42±1.44
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5.99±2.31
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5.85±1.60
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6.45±2.33
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6.08±2.93
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0.738
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AC, mm2/mmHg x 10–1
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1.41±0.40
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1.41±0.44
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1.44±0.68
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1.30±0.38
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1.31±0.42
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1.38±0.58
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0.593
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Supine SBP, mmHg
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121±10
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128±12*
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121±8
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129±15*
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122±9
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131±12*
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0.629
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Supine DBP, mmHg
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67±8
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65±9
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67±8
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67±11
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67±8
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68±9
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0.840
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Supine PP, mmHg
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55±7
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63±10*
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54±8
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63±10*
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55±9
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63±12*
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0.791
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Supine MAP, mmHg
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85±8
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86±9
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85±7
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87±11
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85±7
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89±9*
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0.642
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Supine HR, bpm
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63±9
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79±13*
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65±10
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83±12*‡
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62±11
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85±13*‡
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0.316
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Supine RPP, au
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7409±1426
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9474±1966*
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7625±1156
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9910±1766*‡
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7051±1026
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10415±1749*‡
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0.070
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Table 4 Reps, repetitions; TUT, time under tension; RPE,
rating of perceived exertion; RDP, rating of perceived discomfort;
Perform again, 10-point Likert scale assessing desire to perform
exercise again. ‡ P<0.05 Between difference with
No-BFR.
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Variable
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AR-BFR
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NAR-BFR
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No-BFR
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Total Reps
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53±20‡
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52±17‡
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83±27
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Volume
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2436±1263‡
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2341±1020‡
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3767±1771
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TUT, sec
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452±87
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444±74
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N/A
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RPE
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8.2±0.8
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8.5±1.0
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8.2±1.2
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RPD
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6.2±2.3‡
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6.6±2.2‡
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5.0±2.4
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Perform again
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6.85±2.39
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6.95±2.61
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7.50±2.50
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Hemodynamics
Several hemodynamic changes were identified ([Table
3]). As relative changes (pre- to post-) were similar to absolute
changes (pre- to post-), we elected to report only relative changes between
conditions in our Table. Central SBP increased (mean difference
(MD)=7±12 mmHg, 95% confidence interval (CI)
(2–13), p=0.004, effect size (ES)=0.65), central pulse
pressure (PP) (MD=7±12 mmHg, 95% CI
(1–13), p=0.012, ES=0.55), and central mean arterial
pressure (MAP) (MD=3±7 mmHg, 95% CI
(1–6), p=0.029, ES=0.45) in No-BFR. Compared to AR-BFR,
HR, SBP, and rate pressure product (RPP) were significantly higher immediately
following exercise in NAR-BFR (MD HR: 8±12 bpm, p=0.01,
95% CI (2–13), ES=0.63; MD RPP: 1297±1973 au,
p<0.01, 95% CI (373–2219), ES=0.66), and No-BFR
(MD HR: 8±14 bpm, 95% CI (2–14), p=0.01,
ES=0.58; MD SBP: 6±9 mmHg, 95% CI
(1–10), p=0.01, ES=0.58; MD RPP: 1773±2074 au,
95% CI (819–2726), p<0.01, ES=0.87) ([Fig. 3] and [4]).
All groups experienced a significant increase in supine heart rate (HR) and RPP
following treatment (all p<0.05). Additionally, NAR-BFR and No-BFR
experienced a significantly greater increase in supine HR (MD
NAR-BFR:4±9 bpm, 95% CI (1–8), p=0.046,
ES=0.41; MD No-BFR: 7±9 bpm, 95% CI (3–11),
p=0.002, ES=0.79) and supine RPP (MD NAR-BFR: 560±1215
au, 95% CI (26–1146), p=0.030, ES=0.46; MD
No-BFR: 1066±1391 au, 95% CI (396–1736),
p=0.002, ES=0.76) compared to AR-BFR.
Fig. 3 Heart rate immediately following exercise;
*P<0.05 Between difference with AR-BFR
Fig. 4 RPP immediately after exercise; *P<0.05
Between difference with AR-BFR
Arterial stiffness measures
Following the intervention, CF-PWV significantly increased in NAR-BFR
(MD=0.57±1.12 m/s, 95% CI (0.05–1.09),
p=0.017, ES=0.51) and No-BFR
(MD=0.63±1.42 m/s, 95% CI
(+0.04–1.3), p=0.032, ES=0.44) ([Table 3]). Compared to AR-BFR, NAR-BFR
experienced a greater increase in CF-PWV
(MD=0.70±1.60 m/s, 95% CI
(0.05–1.44), p=0.034, ES=0.43) ([Table 3]). CR-PWV significantly decreased after
the intervention in No-BFR (MD=-0.82±1.51 m/s,
95% CI (0.09–1.54), p=0.015, ES=0.54). No
statistical differences were detected in β-stiffness and AC ([Table 3]) (all p>0.05) with the
interventions.
Performance
Total reps and training volume were significantly lower in AR-BFR (reps:
–29.6±13.9, 95% CI (–23.11–36.08),
p<0.01, ES=2.13; volume: –1331±855, 95%
CI (–931–1731), p<0.01, ES=1.55) and NAR-BFR
(reps: –31.0±17.9, 95% CI
(–22.59–39.40), p<0.01, ES=1.73; volume:
–1426±999, 95% CI (–958–1893),
p<0.01, ES=1.42) compared to No-BFR ([Table 4]). Time under tension was not different between BFR
conditions (452±87 s vs. 444±74 s in AR-BFR and
NAR-BFR, respectively). RPD was significantly greater in AR-BFR
(1.2±1.4, 95% CI (0.5–1.9), p<0.01,
ES=0.86) and NAR-BFR (1.6±1.3, 95% CI (1.0–2.2),
p<0.01, ES=1.2) compared to No-BFR ([Table 4]). RPE and the 1–10 Likert scale assessing the
likelihood of performing the training again was not different between any
conditions (all p>0.05).
Discussion
This is the first study to examine the acute responses between AR-BFR, NAR-BFR, and
No-BFR exercise on arterial stiffness changes in a lower body resistance protocol to
volitional fatigue using a frequently studied BFR training device in healthy,
physically active adults. The main findings are (1) AR-BFR blunts exercise-induced
central arterial stiffness compared to NAR-BFR and No-BFR, and (2) no differences
were observed between perceptual outcomes or volume performed between AR-BFR and
NAR-BFR; however, both produced greater discomfort than No-BFR.
Arterial stiffness, central and peripheral hemodynamics
AR-BFR blunted the increase in CF-PWV compared to NAR-BFR 10 minutes
post-exercise with between-group differences of
~0.70 m/s with a small to moderate effect. We also
observed that No-BFR increased CF-PWV 0.63 m/s, although
between-group differences with AR-BFR did not reach significance. In addition,
supine RPP following exercise was elevated in both NAR-BFR and No-BFR trials
above AR-BFR, indicating heightened myocardial workload [45]. We also observed negligible or no
between-group differences in central hemodynamics (central
SBP/DBP/PP/MAP) and changes in β-stiffness or
AC. Explaining the potential reasons why these results may have occurred is
challenging, and likely not due to performance or perceptual-related factors, as
total volume, time under tension, and RPE/RPD were similar between BFR
conditions. Moreover, the increase in post-exercise CF-PWV occurred in No-BFR,
where participants performed ~34% more volume, lowering the
likelihood that volume modulates the CF-PWV response. There may be a
post-exercise temporal buffering effect on central stiffness with the
autoregulation feature as it is designed to accommodate limb diameter changes,
dissipating the forward and returning pulsatile forces similar to an elastic
aorta during systole. However, we did not observe similar reductions in CR-PWV
in AR-BFR. Perhaps this is an outcome of the peripheral arterial site used in
the measurement as the cuffs were place around the proximal thighs. However, we
measured peripheral changes using the radial artery, not the posterior tibial or
dorsalis pedis arteries, possibly missing the full impact on peripheral arterial
stiffness indices in the leg. However, we believe our measurement site
(i. e. femoral artery) is more suitable for capturing central stiffness
changes due to its proximal location compared to the radial artery assessment in
the upper extremity. Future studies are needed to clarify the possible influence
of thigh cuffs on arterial stiffness in the leg. As this was the first study of
its kind, no direct comparisons can be made with the existing body of BFR
literature.
Prior research has hypothesized that the acute increases in central stiffness
markers following high-intensity resistance exercise may be due to the unique
hemodynamics of strenuous exercise [46]. Pierce
et al. [46] proposed that large fluctuations in
BP and the use of the Valsalva maneuver mechanically compress the vasculature,
leading to a heightened pressor response and acute stiffening of the arteries.
However, over time (i. e. weeks to months), the central arterial
apparatus adapts, leading to negligible changes in central stiffness [18].
Nonetheless, while the relevance of the magnitude of acute changes in CF-PWV
speeds are uncertain, the current body of evidence indicates that changes
of+1 m/s increase age, sex and risk-factor adjusted
cardiovascular events, mortality, and all-cause mortality between
14–15% [47]. As our study
evidenced acute increases of 0.6–0.7 m/s in both NAR-BFR
and No-BFR, practitioners seeking to minimize adverse events during BFR exercise
may choose AR-BFR, as it prevented increases in central arterial stiffness.
While the values we report do not exceed what is currently known to be
associated with cardiovascular disease (+1 m/s), those
looking to reduce central stiffness may opt for AR-BFR, as it mitigated any
observable increase in CF-PWV. Although it is important to note that no adverse
events were recorded in any group throughout our entire study, necessitating
further research on the potential relevancy of our findings, particularly in
at-risk populations.
Performance, perceptual responses and safety
In this study, AR-BFR and NAR-BFR did not display differences in any of the
performance or perceptual measurements assessed. Both displayed similar total
repetitions, volume, time under tension, RPE, and RPD. In comparison, No-BFR
performed significantly more repetitions and total volume than both BFR
conditions with less RPD. These results align with the overall body of
literature on No-BFR versus BFR exercise on reducing exercise performance [32], as we observed a volume reduction of
approximately 34% in both BFR conditions. However, this partially
conflicts with a recent meta-analysis on perceptual demands [48] that indicated RPE/RPD was similar
between No-BFR as long as exercise was taken to volitional fatigue. Our study
reported high RPD in both BFR conditions compared to No-BFR with equal RPE.
Lastly, our results conflict with a recent study investigating autoregulation of
applied pressures using another commercially available BFR training device [26], indicating that acute responses to a BFR
training program with autoregulation are possibly device-specific [20].
In the only other study directly comparing the impact of AR-BFR on exercise
performance with cuffs of similar size, Jacobs et al. [26] had 56 participants perform a series of fixed and failure leg
extension BFR exercise protocols in a randomized order using 20% 1-RM.
Using the Smartcuffs device (cuff width 10.16 cm) capable of performing
both AR-BFR and NAR-BFR, it was observed that during failure protocols, AR-BFR
condition performed significantly more volume than NAR-BFR with similar RPE and
less RPD (albeit not likely clinically relevant). Interestingly, no clinically
relevant differences were observed in heart rate and brachial BP responses
between conditions, leaving unanswered questions regarding what could be
responsible for the observed differences. In contrast, our study with the Delfi
Personalized Tourniquet device did not show performance or perceptual
differences between the AR-BFR and NAR-BFR conditions. This observation may be
attributable to differences in device autoregulation responsiveness of being
able to maintain a consistent pressure between contraction phases, allowing for
some reperfusion between repetitions – although this is speculative and
requires comparative research in future studies.
In addition, it is important to note that no adverse events were reported in our
study despite all exercise sessions (including the familiarization session)
being conducted to failure, whereas Jacobs et al. [26] reported an adverse event in 7.14% of trials
(n=16 total) with a risk difference of 7% between NAR-BFR and
AR-BFR in favor of AR-BFR. It is challenging to understand why the occurrence of
adverse events was higher given that our study protocol had BFR exercise
performed to failure in all trials, whereas Jacobs et al. [26] had participants perform a fixed repetition
scheme more indicative of recommended practice [32] before performing a failure routine. More research is needed to
understand the participant, device and protocol-specific ways to minimize the
occurrence of adverse events during BFR exercise.
Limitations
This is the first study to investigate the arterial stiffness responses to an
acute exercise session to volitional fatigue with and without the presence of
autoregulation of applied BFR pressures, but it is not without limitations.
First, we sought to include both males and females to have a better
representative sampling of healthy, physically active young adults. However, our
study was likely not adequately powered to assess between-sex differences.
Recognizing the potential for different responses between sexes, we performed a
between-sex analysis. We noted divergent responses in CR-PWV in the NAR-BFR
condition in females, as well as a reduced overall volume of exercise performed
in all conditions, compared to males. However, nothing else reached significance
(Supp
[Table 1]). Therefore, we cannot say with
certainty that the responses between sexes are identical, warranting more
research that uses a sample size adequately powered to detect between-sex
differences. Second, while different menstrual phases do not appear to influence
indices of arterial stiffness, less is known how it impacts pain and perceived
effort to exercise. Thus, interpretation of these subjective scores should be
viewed with caution. Third, due to not having a leg press, we utilized a wall
squat. As this type of exercise is not confined to a predetermined range of
motion, there is likely a greater skill component than a traditional leg press
and different muscle activation patterns. In addition, participants performed
the wall squat leaning into a minimal friction wall, which may have altered the
load moved by the lower body. To control for this, we included a familiarization
session identical to the one performed in data collection. This likely allowed
for some motor learning to occur and potentially reduced the impact of the novel
wall squat exercise. Lastly, as participants were healthy, one cannot
extrapolate the findings to clinical populations without a degree of
caution.
Clinical implications
With more devices available for consumer purchase, it is prudent for research to
investigate whether certain features, such as autoregulation of applied
pressure, impact the acute response to BFR exercise. Our study provides two main
takeaways. The primary takeaway is that autoregulation of applied pressures has
the potential to limit the exercise-induced increases in CF-PWV in healthy,
physically active men and women. This may have relevance for increasing the
safety of BFR exercise as non-autoregulated pressures, as well as low-load
exercise to failure, increased CF-PWV to a similar degree. And second, our
results on performance and perceptual responses diverge from a recent study on
autoregulation [26], supporting that
autoregulation may have varying impact on the acute- and potentially long-term
responses to BFR exercise. As such, autoregulation of applied BFR pressures is
an important feature that warrants consideration in practice and future
research, particularly with respect to at-risk populations where attenuating the
central stiffness responses may be desired. The use of autoregulation may,
therefore, also serve a protective role in mitigating adverse responses to BFR
exercise.
Pre-Print
Part of the data collected from this study was published in pre-print on
SportRxiv [49].
Notice
This article was changed according to the following Erratum
on November 2nd 2023.
Erratum
The conflict of interest information of author N. Rolnick were
added to the article. This could not be displayed until now due
to a technical error.
