2. 2015 influence of relative blood flow restriction pressure on muscle activation and muscle...

8/18/2019 2. 2015 Influence of Relative Blood Flow Restriction Pressure on Muscle Activation and Muscle Adaptation

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INFLUENCE OF RELATIVE BLOOD FLOW RESTRICTION PRESSURE ON

MUSCLE ACTIVATION AND MUSCLE ADAPTATION

BRITTANY R. COUNTS, BS,1 SCOTT J. DANKEL, BS,1 BRIAN E. BARNETT, BS,1 DAEYEOL KIM, MS,2

J. GRANT MOUSER, BS,2 KIRSTEN M. ALLEN, BS,2 ROBERT S. THIEBAUD, PhD,3 TAKASHI ABE, PhD,1

MICHAEL G. BEMBEN, PhD,2 and JEREMY P. LOENNEKE, PhD1

1Kevser Ermin Applied Physiology Laboratory, Department of Health, Exercise Science, and Recreation Management, University of Mississippi, P.O. Box 1848, University, Mississippi 38677, USA

2Department of Health and Exercise Science, Neuromuscular Research Laboratory, University of Oklahoma, Norman, Oklahoma, USA 3Department of Kinesiology, Texas Wesleyan University, Fort Worth, Texas, USA

Accepted 30 June 2015

ABSTRACT: Introduction: The aim of this study was to investi-gate the acute and chronic skeletal muscle response to differinglevels of blood flow restriction (BFR) pressure. Methods: Four-teen participants completed elbow flexion exercise with pres-sures from 40% to 90% of arterial occlusion. Pre/post torquemeasurements and electromyographic (EMG) amplitude ofeach set were quantified for each condition. This was followedby a separate 8-week training study of the effect of high (90%arterial occlusion) and low (40% arterial occlusion) pressure on

muscle size and function. Results:

For the acute study,decreases in torque were similar between pressures [–15.5(5.9) Nm, P 50.344]. For amplitude of the first 3 and last 3reps there was a time effect. After training, increases in musclesize (10%), peak isotonic strength (18%), peak isokinetic torque(1808 /s523%, 608 /s511%), and muscular endurance (62%)changed similarly between pressures. Conclusion: We suggestthat higher relative pressures may not be necessary when exer-cising under BFR.

Muscle Nerve 53: 438–445, 2016

Low-load resistance exercise [20%–30% concen-tric 1-repetition maximum (1RM)] in combination

with blood flow restriction (BFR) increases muscle

size and strength in a variety of populations.1–3

When applied appropriately, this stimulus hasbeen found to provide a safe and effective stimulusin the absence of measurable muscle damage.4,5

The mechanisms behind these beneficial effectsare not completely known, but metabolic accumu-lation–induced fatigue may be playing an influen-tial role in the muscle adaptations observed afterthis type of exercise. To illustrate, metabolic accu-mulation in combination with a reduced oxygenenvironment may increase recruitment of higherthreshold (type II) muscle fibers.6,7 This suggests

that higher pressures, resulting in a greater reduc-tion in oxygen and subsequent increase in meta-bolic accumulation,8 may augment muscle fiber

recruitment with low-load resistance exercise incombination with BFR. Muscle fiber recruitment may be important, as it has been previously sug-gested that increased recruitment is related tosome degree with changes in muscle protein syn-thesis.9 To illustrate, lower body low-load exerciseto volitional fatigue results in high levels of muscle

activation,10–12

and has also been found to pro-duce muscle protein synthetic13 and muscle hyper-trophic responses similar to higher load resistancetraining.14,15

We recently observed that higher relative pres-sures (pressures based on individual limb circumfer-ence) may not augment muscle activation in thelower body.12 However, due to the lack of statisticalpower to compare across groups, only qualitativeanalyses could be completed across pressures(40%–60% estimated arterial occlusion). Further-more, no published study to date has compared thehypertrophic responses of BFR training under dif-

ferent occlusion pressures. Thus, the purpose of this study was 2-fold. First, we sought to determine,using a within-subject design, whether or not higherrelative pressures provide an increase in muscle acti-

vation over lower pressures. We hypothesized that muscle activation would not be augmented to a large degree with higher pressures. Second, basedon the acute muscle activation data, we sought todetermine whether differences in muscle adaptation

would be observed after 8 weeks of resistance train-ing with either high or low pressures applied.

Although similar muscle activation was reportedacross pressures, we hypothesized that exercising

with higher relative pressures may attenuate someof the gains in muscle mass due to the reduction intotal exercise volume observed with higher pres-sures from the acute study.

METHODS

Participants. For experiment 1, 14 physically activeparticipants (10 men, 4 women) were recruited.“Physically active” was defined as being active 3 ormore days per week with an upper body resistancetraining component 2 or more days per week for at least the previous 3 months. Physically active

Abbreviations: 1RM, 1-repetition maximum; ANOVA, analysis of var-iance; bSBP, brachial systolic blood pressure; EMG, electromyography;FR, blood flow restriction; MVC, maximal voluntary contraction

Additional Supporting Information may be found in the online version of this article.

Key words: arterial occlusion; hypertrophy; KAATSU; perceptualresponse; resistance training; vascular occlusion trainingCorrespondence to: J.P. Loenneke; e-mail: [email protected]

VC 2015 Wiley Periodicals, Inc.Published online 2 July 2015 in Wiley Online Library (wileyonlinelibrary.com).DOI 10.1002/mus.24756

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participants were used to better reflect the actual acuteresponses to different exercise and limit the possibility of a training effect due to repeated testing. For experi-ment 2, a total of 8 non–resistance-trained men(n 5 5) and women (n 5 3) volunteered to participatein this study. One man enrolled but dropped out

before the first visit, therefore analysis was conductedon the remaining 7 participants. Participants wereexcluded if they had at least 1 risk factor for throm-boembolism. The experiments were approved by theuniversity’s institutional review board, and eachparticipant gave written informed consent beforeparticipation.

Experiment 1 Study Design. During the initial visit participants had standing arterial occlusion pres-sure determined and were then tested on eacharm for the unilateral dumbbell elbow flexion 1-repetition maximum (1RM). Participants werethen familiarized with the BFR stimulus and maxi-mal voluntary contraction (MVC) testing. Next,participants were scheduled for the first of 3 test-ing visits with a minimum of 5 and a maximum of 10 days between visits. Participants completed allof the exercise conditions in random order (1 con-dition per arm) across 3 separate visits (2 condi-tions per visit). The exercise bouts within each day

were separated by 10 min of rest. For each condi-tion, the participants were instructed to complete1 set of 30 repetitions followed by 3 sets of 15 rep-etitions at 30% of their concentric 1RM at 40%,50%, 60%, 70%, 80%, or 90% of their standing

arterial occlusion pressure. All conditions were sep-arated by 30-s rest periods between sets. A metro-nome was used to ensure that the participantsheld the cadence of 1 s for the concentric muscleaction and 1 s for the eccentric muscle action dur-ing the unilateral elbow flexion exercise. If theparticipant could not maintain the cadence duringa particular set, the set was stopped, and the partic-ipant rested for 30 s until the next set. Muscle acti-

vation was measured at pre-exercise (no BFR) andduring each set of exercise (with BFR). The elbow flexor MVC was performed on an isokinetic dyna-mometer pre- and post-exercise to determine

fatigue. All testing sessions were completed beforethe participant exercised for that day, and each

visit was completed at least 24 h after the last upper body workout.

Electromyography and Isometric Fatigue. Electro-myographic (EMG) signals were recorded from thebiceps brachii of the arm during exercise. Electro-des were placed on a line between the medial acro-mion and the antecubital fossa at a distance of one-third from the antecubital fossa. The skin wasshaved, abraded, and cleaned with alcohol wipes.Bipolar electrodes were placed over the muscle

belly with an inter-electrode distance of 20 mm.The ground electrode was placed on the seventhcervical vertebrae at the neck. The surface electro-des were connected to an amplifier and digitized(Biopac Systems, Inc., Goleta, California). The sig-nal was filtered (low-pass filter 500 HZ, high-pass

filter 10 HZ

), amplified (1,0003

), and sampled at a rate of 1 kHZ. Before the exercise bout, the par-ticipant performed 2 isometric MVCs with thebiceps brachii at a joint angle of 908 with a 30-srest between MVCs on an isokinetic dynamometer.The EMG was recorded continuously from thebiceps brachii during each exercise bout. LabView 7.1 (National Instrument Corp., Austin, Texas)computer software was used to analyze the data.EMG amplitude (root mean square, RMS) was ana-lyzed from the average of the first 3 repetitionsand the average of the last 3 repetitions for eachset and expressed relative to the highest pre-

exercise MVC (%MVC).Experiment 2 Study Design. Based on findingsfrom the acute study, we sought to determine

whether the acute changes would translate tochronic muscle adaptation. Thus, participants com-pleted 8 weeks of low-load unilateral elbow flexiontraining with 1 arm exercising at low pressure(40% arterial occlusion) and the other arm exer-cising at higher pressure (90% arterial occlusion).The participants visited the laboratory for a totalof 26 visits. The first 2 pre-training visits consistedof paperwork and baseline measurements, followed

by 22 separate training sessions and 2 post-training visits (48–72 h after last training session) that measured changes caused by the exercise interven-tion (Fig. 1). Participants trained 2 times per weekfor the first 2 weeks followed by 3 training sessionsper week for weeks 3–8. A similar number of train-ing sessions has previously been shown to producemeasurable changes in muscle size andstrength.16,17 The goal reps for each exercise pro-tocol included 1 set of 30 repetitions followed by 3sets of 15, with 30-s rest periods between sets. Exer-cise was completed to a metronome with 1 s forthe concentric and 1 s for the eccentric portion of

the exercise. Participants were stopped beforecompleting the goal number of repetitions, whenthey were unable to lift the load with proper formor keep to the beat of the metronome. Trainingload was adjusted every 2 weeks to maintain 30%of 1RM. A non-BFR control condition was not included, as previous studies have consistently shown that repetition-matched protocols without BFR do not lead to meaningful changes in musclesize and strength.1

Determination of 1RM. For experiments 1 and 2,the maximum load that could be lifted for the

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unilateral dumbbell curl through a full range of motion with proper form was assessed andrecorded as the concentric 1RM. Briefly, partici-pants completed 5 reps of light weight (3.41 kg)

as a warm-up, and then weight was progressively increased until the load could not be lifted success-fully through a full range of motion.18 Each arm

was tested in a random order, and all participantsreached their 1RM within 5 attempts. To ensurestrict form, participants completed their concentric1RM with their back and heels against a wall and

with feet shoulder width apart.

Determination of Arterial Occlusion Pressure. Forexperiments 1 and 2, a narrow (5-cm-wide blad-der) nylon cuff was applied to the most proximalpart of the arm. Pressure was regulated using a

cuff inflator system (E 20 Rapid Cuff Inflator;Hokanson, Bellevue, Washington). The pulse at the wrist (arterial blood blow) was detected using a hand-held bidirectional Doppler probe placed onthe radial artery. The cuffs were inflated to 50 mmHg and quickly raised to the participant’s previ-ously measured systolic blood pressure. Pressure

was then slowly increased until the arterial flow was no longer detected during inflation. Arterialocclusion pressure was recorded to the nearest 1 mm Hg as the lowest cuff pressure at which a pulse was not present.

Muscle Thickness. For experiment 2, muscle size was estimated by B-mode ultrasound (SSD-500 witha 5-MHZ probe; Aloka). Ultrasound measurementsof the biceps brachii were taken halfway betweenthe acromion process and lateral epicondyle and10 cm proximal to the lateral epicondyle. Musclesize of the anterior forearm was measured at 30%proximal between the styloid process and the headof the ulna. Three images were taken at each site,printed, and analyzed by an investigator who wasblinded to the arm’s condition. The average of the3 measurements was used for final analysis. Muscle

thicknesses of the deltoid and triceps were alsomeasured to demonstrate stability of the measure-ment across time, as those muscle groups were not expected to change with strict elbow flexion exer-

cise. The minimal difference (i.e., reliability)needed to be considered real for the anterior por-tion of the upper and lower arm was calculated tobe 0.2 cm.

Muscle Endurance. Participants completed asmany repetitions of unilateral elbow flexion exer-cise as they could to a metronome with 1 s for theconcentric and 1 s for the eccentric portion of thelift. The load used was 30% of the predetermined1RM for that test day. All participants kept theirback and heels against a wall with their feet shoulder width apart to ensure strict form through-

out testing.

Isokinetic Elbow Flexion Strength. Isokinetic torque was measured using an isokinetic dynamometer(Quickset System 4; Biodex) Measurements weretaken on both arms in random order. First, partici-pants completed 2 sets of 3 at 1808/s separated by 90 s of rest. This was then repeated at 608/s. All

values were gravity corrected. The minimal differ-ences needed for changes to be considered real

were calculated as 5 Nm for 1808/s and 3 Nm for608/s.

Ratings of Discomfort. Ratings of discomfort werequantified using the Borg discomfort scale

(CR101) before each exercise bout and after eachset for all training sessions, Methods have beendescribed in detail previously.19

Statistical Analyses. All data were analyzed usingSPSS 22.0 software (SPSS, Inc., Chicago, Illinois)

with variability represented as standard deviation(SD). For experiment 1, there were no baselinedifferences in MVC, thus a 1-way analysis of var-iance (ANOVA) was completed for the MVCchange scores (mean decrease from baseline) and

FIGURE 1. Outline of experiment 2. Mth, muscle thickness; 1RM, 1-repetition maximum; 30% to failure is test of muscle endurance.



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overall exercise volume to determine whether dif-ferences existed between conditions. For EMG, a 6(condition) 3 4 (time) repeated-measures ANOVA

was used. A significant result from the repeated-measures ANOVA was followed by a 1-way ANOVA to determine where the difference occurred across

time within each visit and within each time-point across visits. Statistical significance was set at analpha level of 0.05.

For experiment 2, a 2 (condition) 3 3 (time)repeated-measures ANOVA was completed for mus-cle thickness, maximal isotonic strength, and exer-cise volume. A significant result from the repeated-measures ANOVA was followed by a 1-way ANOVA to determine where the difference occurred acrosstime within each pressure, and a paired-sample t -test was used to determine where the differencesoccurred between pressures within each time-point. A 2 (condition) 3 2 (time) repeated-

measures ANOVA was completed for isokineticstrength. Follow-up tests included paired sample t -tests across time within each pressure and acrosspressures within each time-point. For ratings of dis-comfort, Wilcoxon-related samples non-parametrictests determined differences between pressures

within each set of exercise. Statistical significance was set at an alpha level of 0.05.

RESULTS

Experiment 1. Participants. Participants (n 5 14),on average, were 246 3 years old, 17467 cm inheight, 79.76 11.3 kg in weight, and had a 1RM for

the right arm of 186

6 kg and a 1RM for the left arm of 1966 kg, and had a standing arterial occlu-sion pressure of 1406 14 mm Hg for the right armand 1436 17 mm Hg for the left arm.

Maximal Voluntary Contraction. There were nosignificant differences across arterial occlusionpressures in the MVC change scores from baseline(P 5 0.344). The grand mean decline in torquefrom baseline was 215.56 5.9 Nm.

Exercise Volume. There were significant differ-ences in exercise volume across pressures, with less

volume being completed at the highest pressures(P < 0.001; Fig. 2B).

EMG. There was no significant interaction with amplitude of the first 3 repetitions (P 5 0.456;Table 1). In addition, there was no significant main effect for condition (P 5 0.850), but there

was for time (P < 0.001), with amplitude increasingfrom the first set. For the last repetitions, there

was no significant interaction with EMG amplitude(P 5 0.450; Table 1). In addition, there was no sig-nificant main effect for condition (P 5 0.881), but there was for time (P 5 0.021).

Experiment 2. Participants. Participants (n 5 7),on average, were 2363 years old, 169.66 9.5 cm

in height, 56.76 11.3 kg in weight, and had a standing arterial occlusion pressure of 1296 19mm Hg for the high-pressure arm and 1336 19mm Hg for the low-pressure arm. Thus, the meanpressure used during exercise was 116617 mmHg and 536 7 mm Hg for the high- and low-pressure arms, respectively. Of the 22 training ses-sions, 2 participants missed 1 training session each,translating into an overall completion rate of 99%.

Muscle Thickness. There was no significant interaction with muscle thickness at the mid–upper

arm (P 5 0.258; Fig. 3A) or 10 cm above the elbow joint (P 50.674; Fig. 3B). In addition, there was nosignificant main effect for condition (P 0.151),but there was for time (P < 0.001). With the fore-arm, there was no interaction (P 50.338) or maineffect of condition, but there was a main effect of time (P 5 0.04). Follow-up tests for forearm musclesize identified a significant increase from pre topost [1.86 0.1 cm vs. 1.96 0.2 cm], but this differ-ence did not exceed the error of our measurement.In addition, no significant differences wereobserved across time for the triceps or deltoid (data not shown).

Muscle Strength. There was no significant inter-action with muscle strength (P 5 0.909). In addition,there was no significant main effect for condition(P 5 0.409), but there was for time (P < 0.001). Maxi-mal isotonic strength (1RM) increased from pre tomid [11.26 5 kg vs. 12.26 5.5 kg] to post [13.26 5.8 kg], with significant differences betweeneach time-point (P 0.006).

Isokinetic Torque. There was no significant inter-action with isokinetic strength at 1808/s (P 5 0.480;Fig. 3C) or 608/s (P 5 0.386; Fig. 3D). In addition,

FIGURE 2. Mean total exercise volume completed across pres-

sures in the acute study (experiment 1). Conditions with differ-

ent letters represent significant differences between conditions

(P 0.05). Variability is represented as standard deviations.



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there was no significant main effect for condition(P 0.633), but there was for time (P 0.014).

Muscle Endurance. There was no significant interaction with muscle endurance (P 5 0.901). Inaddition, there was no significant main effect forcondition (P 5 0.265), but there was for time(P < 0.001). The number of repetitions completedto failure increased from pre [37 (7) repetitions]to post [60 (13) repetitions].

Rating of Discomfort. Ratings of discomfort between pressures were statistically compared inthe first, eleventh, and last training sessions. Rat-

ings of discomfort were significantly different

between pressures for most sets of exercise (Table2). When plotted across time, the peak discomfort

was almost always higher in the high-pressure arm(see Fig. S1 in the Supplementary Material, avail-able online).

Exercise Volume. There was no significant inter-action with the average repetitions completed inthe first set in weeks 1, 4, or 8 (P 5 0.08; Table 3).In addition, there was no significant main effect for condition (P 5 0.08) or time (P 5 0.10). Forthe average repetitions completed in sets 2–4,there was no significant interaction (P 5 0.416;

Table 3); however, there was a significant main

Table 1. Muscle activation from experiment 1.

EMG amplitude first 3 reps (%MVC) Time

Arterial occlusion Set 1 Set 2 Set 3 Set 4 1 vs. 2, 3, 4

40% 33 (9) 46 (19) 48 (18) 44 (14)

50% 38 (13) 51 (17) 56 (21) 53 (23)

60% 43 (31) 58 (32) 56 (30) 56 (28)

70% 36 (20) 49 (26) 52 (26) 49 (23)80% 37 (13) 53 (23) 45 (15) 55 (31)

90% 36 (20) 53 (37) 53 (39) 51 (33)

EMG amplitude last 3 reps (%MVC)

Arterial occlusion Set 1 Set 2 Set 3 Set 4 2 vs. 3, 4; 3 vs. 4

40% 53 (16) 61 (22) 56 (23) 49 (16)

50% 62 (27) 74 (34) 64 (38) 63 (38)

60% 71 (45) 71 (37) 65 (39) 60 (35)

70% 62 (43) 65 (37) 59 (30) 55 (30)

80% 61 (26) 68 (41) 66 (48) 61 (40)

90% 57 (35) 64 (53) 58 (49) 56 (43)

Variability represented as standard deviations. Main effects of time are noted in the “Time” column at far right. The different numbers represent significant

differences between sets (P 0.05).

FIGURE 3. Mean changes across applied pressures in muscle thickness at the 10-cm site (A), muscle thickness of the mid-upper arm

(B), and isokinetic peak torque at 1808 /s (C) and 608 /s (D). Dagger (†) indicates a main effect of time. Time-points with different letters

represent significant differences between time-points in (A) and (B). Variability is represented as standard deviations. To maintain suffi-

cient statistical power, only pre-exercise, day 11, and post-exercise were compared.



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effect for condition (P 5 0.004) and time(P < 0.001). For the average exercise volume com-pleted in weeks 1, 4, or 8, a 2 3 3 repeated-measures ANOVA did not reveal a significant inter-action (P 5 0.766) or main effect of condition

(P 5 0.127), but there was a main effect for time(P < 0.001; Table 3).

DISCUSSION

These findings suggest that relatively highpressures may not be needed to maximize theacute or chronic response to BFR exercise. Forexample, although a wide range of relative pres-sures were used in the acute experiment, theincrease in fatigue and muscle activation acrosspressures was similar. Thus, we speculated that lower pressures may produce similar changes in

muscle size and strength as higher pressures. Toprovide further insight, we completed a small-scale training study to determine if differences inmuscle adaptation could be observed after exer-cise in combination with 2 different pressures(40% vs. 90% arterial occlusion). Our chronicdata are in agreement with the acute experiment and suggests that both relative pressuresincreased muscle size and strength to a similarextent after low-load training in combination withBFR. Contrary to our hypothesis, the difference

in exercise volume between pressures did not appear to affect muscle adaptation.

Experiment 1. Previous studies in the upper body have identified increases in EMG amplitude duringlow-load resistance exercise in combination withBFR.7,17,20–23 The increase in EMG amplitude may

be due to a metabolic “overload” (i.e., depletion of phosphocreatine stores and decrease in musclepH) induced fatigue within the muscle.6 The meta-bolic accumulation in concert with a reduced oxy-gen environment from the restriction of bloodflow may increase recruitment of higher thresholdfibers through stimulation of group III and IV afferent fibers.7 The muscle activation of the last 3repetitions marginally decreased in some of thesets. This is likely due to the participant “cheatingthe weight up” with muscles other than the bicepsbrachii. This occurred despite our efforts to make

the exercise execution as strict as possible. To ourknowledge, only 1 other study 21 has addressedthose changes across different pressures [80%,100%, and 120% of brachial systolic blood pres-sure (bSBP)] in the upper body. In that study, theauthors observed that muscle activation increasedprogressively in all groups. However, the amplitude

was significantly greater with 120% bSBP than a work-matched non-BFR condition from the end of 30 repetitive contractions to the end of the secondset of 15 contractions. In addition, previous data in the lower body suggested that EMG amplitudeis increased from 40% to 50% estimated arterial

occlusion, but no further increase was observed when the pressure was increased to 60% estimatedarterial occlusion.12 Our finding of a lack of aug-mentation with increasing pressure is in contrast to the 2 previous studies. Possible reasons for thisdiscrepancy may be related to the setting of restric-tion pressure. In our study we set the pressure rela-tive to the actual cuff used during exercise, but theaforementioned investigation by Yasuda et al .21 didnot. Second, the previous study in our laboratory

was completed with narrow cuffs in the lower

Table 2. Ratings of discomfort from experiment 2.

Ratings of discomfort (0–101)

Day 1 Set 1 Set 2* Set 3* Set 4*

High 2 (0.5–2.5) 3 (3–4) 3.5 (3–5) 4 (3–7)

Low 0.5 (0.3–2) 1 (0.3–2.5) 2 (0.3–3) 2.5 ( 0.5–5)

Day 11 Set 1* Set 2* Set 3* Set 4*

High 2 (1.5–3) 2.5 (2–4) 3 (1.5–5) 3 (2–5)Low 0.7 (0.5–1) 1 (0.5–2) 1 (1–2) 1.5 (1–2)

Day 22 Set 1 Set 2* Set 3* Set 4*

High 1.5 (1–2) 2 (1.5–3) 3 (2.5–3) 3 (3–3)

Low 1 (0.3–1.5) 1 (0.5–2) 1 (0.7–2) 1.5 (1–2.5)

Data presented as 50th percentile (25th–75th percentiles).

*Significant differences between pressures for that set (P 0.05).

Table 3. Exercise volume from experiment 2.

Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 Week 7 Week 8

First set

High 27 (3) 28 (2) 28 (1) 30 (0) 30 (0) 30 (0) 30 (0) 30 (0)

Low 29 (1) 29 (1) 30 (0) 30 (0) 30 (0) 30 (0) 30 (0) 30 (0)

Sets 2–4 Week 1* Week 2 Week 3 Week 4† Week 5 Week 6 Week 7 Week 8‡

High* 5 (2) 5 (2) 6 (2) 9 (4) 8 (3) 9 (4) 11 (4) 11 (4)

Low† 10 (4) 10 (4) 11 (3) 13 (3) 12 (2) 13 (2) 14 (1) 14 (1)

Volume (kg) Week 1* Week 2 Week 3 Week 4† Week 5 Week 6 Week 7 Week 8‡

High 151.7 (88.5) 155.5 (76) 171.9 (86) 203.7 (99.7) 212.8 (107.8) 221.2 (107.9) 250.3 (119.5) 254.7 (115.5)

Low 185. 5 (81.9) 186.1 (74) 207.7 (97.1) 229.3 (117.9) 252.7 (124.8) 257.4 (122.4) 282.5 (131.8) 283.7 (132.8)

Weeks with different symbols represents significant differences between weeks. Conditions with different symbols represent significant differences between

conditions. To maintain sufficient statistical power, only weeks 1, 4, and 8 were compared. Variability represented as standard deviation.



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body,12 thus arterial occlusion could only be esti-mated.24 It is possible that the estimated value inthe lower body may have been less than 40% arte-rial occlusion. However, in the present study, we

were able to determine arterial occlusion in every individual, thus we likely have a truer representa-

tion of 40% arterial occlusion in the upper body.It may also be that there are intrinsic differencesbetween the upper and lower body musculature.

Experiment 2. Although research has shown that low-load exercise with BFR increases muscle massand strength,1,3 it was unknown whether the appliedpressure affected the overall adaptive response. Wefound no difference in muscle size, strength, orendurance between pressures, despite differences inexercise volume. It has been previously hypothesizedthat one needs to surpass a certain volume thresholdto maximize the hypertrophic response9; however,our results suggest that threshold may be lower thanthe commonly prescribed 75-repetition protocol.This finding coincides with a previous study suggest-ing that more volume does not always augment mus-cle size and strength.25 Given that both groups hadsimilar volumes of work in the first set, this may sug-gest that, in this population, the first set of approxi-mately 30 repetitions may be the most important

with the following sets being of less importance,assuming the muscle reaches maximal fatigue. How-ever, we also cannot rule out the possibility that high relative pressure has a physiologic effect onmuscle, making the overall exercise volume of less

importance.It has been hypothesized that a hypothetical

range may exist for observing beneficial adapta-tions with low-load exercise in combination withBFR, and higher pressures increase the possibility of an adverse event.26 Our results show that muscleadaptions were similar, but there was an overallhigher rating of discomfort during exercise withthe higher applied pressure. Although the differ-ences in discomfort were small, these differences

were maintained throughout the training study.Further, peak ratings of discomfort for each ses-sion were almost always greater with higher applied

pressures compared with lower applied pressures(see Fig. S1 online). It is important to note that our rating quantified discomfort during exerciseand not the rest periods. Most participantsreported anecdotally much greater discomfort dur-ing the rest period with high relative pressures,

which suggests that our measurement time-point was inadequate to show the true differencesbetween pressures. Taken together, 40% arterialocclusion may be all that is needed to maximizethe anabolic response to low-load BFR training

when compared with 90% arterial occlusion, with-

out the greater discomfort that was observed with90% arterial occlusion.

Limitations. In view of the results presented, thisstudy has some limitations. First, the training study had a relatively small sample size. However, meanchanges in muscle size, strength, and endurance

were similar between arms, which suggests that thesimilar change between pressures was unlikely dueto a statistical power issue. Further, the acute data presented here, along with a previous study,12 cor-roborate the finding that higher relative pressuresmay not augment muscle adaptation. Our estimateof muscle growth was muscle thickness and not the“gold standard” estimate from magnetic resonanceimaging, although previous studies indicated a strong relationship between ultrasound estimatesand more sophisticated measures.27–29 Regardless,the significant increases in biceps brachii thickness

exceeded the error of our blinded tester (minimaldifference), which gives confidence to the results.In addition, post-exercise muscle thickness meas-urements were taken 48–72 h after exercise despiteprevious data suggesting that swelling from upperbody exercise lasts less than 24 h.30 A final poten-tial limitation could be the cross-education of strength from one limb to the other; however, it has been noted previously that the cross-educationeffect is minimal or nonexistent when both limbsare training with different protocols.14

In conclusion, these findings indicate that mus-cle activation is not affected to a large degree by

relative differences in applied pressure. Further-more, we found that low-load exercise in combina-tion with either 40% or 90% arterial occlusionproduced similar increases in muscle size, strength,and endurance. In addition, the higher pressurecondition produced indicated higher ratings of dis-comfort throughout the training program. Basedon these preliminary data, we suggest that higherrelative pressures may not be necessary with low-load resistance training in combination with BFR.

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