The acute and early phase effects of blood flow restriction training on ratings of perceived exertion,performance fatigability,and muscular strength in women

Abstract

BACKGROUND:

Blood flow restriction (BFR) resistance training (RT) has garnered recent interest, but female-specific data remains scarce.

OBJECTIVE:

The purpose was to examine the effects of 2-wks of low-load concentric, isokinetic, reciprocal forearm flexion and extension training, with and without BFR on perceptual responses, performance fatigability, and muscular strength.

METHODS:

Twenty women were assigned to a BFRT or a non-BFRT group. Each group trained at 30% of concentric peak moment. Each session consisted of 75 concentric, isokinetic, reciprocal forearm flexion extension muscle actions. RPEs were recorded following each set. Pretest and posttest maximal voluntary isometric contraction (MVIC) force was measured, and percent decline was defined as performance fatigability.

RESULTS:

The RPE values ( $p<$ 0.05) increased across sets. Strength (collapsed across muscle action) increased ( $p<$ 0.05) from 0-wk (23.7 $\pm$ 3.2 Nm) to 2-wk (26.8 $\pm$ 2.7 Nm). Independent of group and muscle action, performance fatigability ( $p<$ 0.05) increased from 0-wk (10.9 $\pm$ 5.0%) to 2-wk (14.1 $\pm$ 4.4%).

CONCLUSIONS:

2-wks of low-load concentric, reciprocal forearm flexion and extension training resulted in similar training-induced changes in perceptual responses, performance fatigability, and muscular strength between BFRT and non-BFRT. These findings may reduce concerns of increased perceptual responses following BFRRT compared to non-BFRRT.

Keywords

Venous occlusion perceptual responses KAATSU reciprocal forearm isokinetic muscle actions

1. Introduction

It is well known that there is historical and current sex bias in biomedical research, which results in significantly lower reporting of data related to women [1]. Recent studies [2, 3, 4, 5] investigating the effects of blood flow restriction resistance training (BFRRT) and BFR aerobic training on perceptual as well as behavioral responses, however, have utilized primarily male samples. Thus, additional research in women is needed to understand the efficacy of low-load ( $\leqslant$ 40% of one repetition maximum [1RM]) BFRRT, which has garnered interest as a potential method to achieve comparable muscular adaptations as high-load (70–85% 1RM) non-BFRRT [6, 7, 8, 9]. Typical BFRRT protocols utilize pneumatic cuffs to limit the arterial blood flow to the active skeletal muscle, while completely occluding venous blood flow [7, 10, 11, 12]. This reduced arterial blood flow has been hypothesized to induce an hypoxic environment within the muscle fiber, which may enhance training-induced muscular adaptations (i.e. strength, hypertrophy) [13]. Venous occlusion can potentially lead to increased metabolite accumulation and cell swelling, which may activate anabolic signaling pathways related to hypertrophy and increased strength [13]. The precise mechanisms underlying BFR-related muscular adaptations, however, remain debated. Furthermore, the potential for BFRRT to elicit muscular adaptions inherent to high-load resistance training, but with low-loads, has led to the implementation of low-load BFRRT in rehabilitation [9, 14] and older individuals [6, 15]. For example, Takarada et al. [14] demonstrated that blood flow occlusion alone (absent of exercise) can be used as an early-phase method of rehabilitation due to effectively attenuating the rate of postoperative disuse atrophy of the leg extensors. There are, however, concerns related to the perceptual (i.e. perception pain, discomfort, exertion, etc.) and cardiovascular responses following BFRRT, which may limit its application [16, 17].

Previous investigations [15, 16, 17, 18, 19] have examined ratings of perceived exertion (RPE) following BFRRT. For example, Loenneke et al. [17] examined 15 men who on average reported significantly higher RPE values following 2 sets of low-load (30% 1RM) bilateral leg extensions to failure with BFR than without occlusion (6 vs. 5 RPE units, respectively, on the Borg CR-10 Scale). Mattocks et al. [16] recently examined 40 participants across 8 weeks of progressive BFRRT and non-BFRRT and reported that there was a significantly greater reduction in RPE for the BFR group compared to the non-BFR group. Due to the study design (i.e. increasing the number of sets across training sessions), however, the authors [16] only analyzed the RPE value from the first set of each session, which was presented as a limitation. Thus, how RPE changes across set for occlusion and non-occlusion conditions in women remains unknown. Hollander et al. [20], however, have examined RPE on a repetition-by-repetition basis to estimate a metabolic break point (RPE $=$ 6; 0–10 scale), and the authors reported there was only a difference in the number of repetitions to reach RPE $=$ 6 between BFRRT and non-BFRRT during the first of three sets. Thus, there is evidence to support that the factors underlying RPE vary across set. It is currently unknown, however, if and to what extent RPE values (affected by strain, intensity, discomfort, and fatigue [21]) change on a set-by-set basis in response to BFRRT compared to non-BFRRT in women. These findings would provide health as well as human performance professionals insight for prescribing either BFRRT or non-BFRRT for the promotion of exercise adherence.

Human performance and the ability to continue exercise are limited by the perception of fatigue and the overall exercise-induced decrease in force or power of a muscle, termed performance fatigability [22]. Performance fatigability has recently been reviewed in the context of its manifestation and its relevancy pertaining to sex-related differences [22]. It has been suggested that inducing performance fatigability through performing repetitions to failure was necessary to elicit maximum increases muscular strength [23, 24], but this notion remains debatable. In theory, the ability to attenuate performance fatigability would promote the maintenance of force throughout a work bout and training session, which would likely lead to greater improvement in strength via increased exercise volume over time [25]. In conjunction with this concept, Hureau et al. [26] have hypothesized that exercise training may “raise” an individual’s tolerance to sensations associated with the cessation of exercise, which would result in greater magnitude of performance fatigability. Currently, however, little is known regarding the relationship between BFRRT and performance fatigability as well as how this relationship contributes to muscular adaptations in women. For men, however, Evans et al. [27] reported that 4-wks of plantar flexion (3 $\times$ /wk; 4 $\times$ 50) BFRRT elicited an 18% increase in maximal voluntary isometric contraction (MVIC), which was independent of a change in performance fatigability. Furthermore, within a non-BFRRT model, there have been mixed findings related to training-induced changes in performance fatigability such as an increase [28, 29] or no change [30] across time. For example, Izquierdo et al. [28] reported that following 7 weeks of non-BFRRT, performance fatigability and muscular strength increased. Unlike Izquierdo et al. [28], however, Pedersen et al. [30] reported that following 6 weeks of leg flexion non-BFRRT, there was an increase in strength without a change in performance fatigability. Mendonca et al. [31] reported that men and women exhibited a greater magnitude of performance fatigability following a single bout of low-load (20% 1RM; 60% of arterial occlusion) BFR resistance exercise compared to a single bout of high load (75% 1RM) resistance exercise. Although, this difference was diminished after covarying for the non-equated volumes inherent to each respective conditions [31]. Therefore, it largely remains unknown to what extent the magnitude of performance fatigability in women is affected by BFRRT compared to non-BFRRT and if training-induced changes in performance fatigability are related to changes in muscular strength.

Short-term low-load BFRRT has been used to investigate changes in muscular strength [6, 32]. For example, Abe et al. [32] demonstrated that 2 weeks of BFRRT at 20% 1RM resulted in a 16.8% increase for barbell squat strength. Abe et al. [32], however, used a variable cuff pressure across training sessions, so it remains unknown if short-term BFRRT with a constant cuff pressure elicits similar increases in muscular strength as well as if there are concurrent changes in perceptual and performance fatigability responses in women. Therefore, the purpose of the present study was to examine the effects of 2 weeks of low-load concentric, isokinetic, reciprocal forearm flexion and extension training, with and without BFR, on perceptual responses, performance fatigability, and muscular strength in women. Based on previous studies [7, 16, 18, 29], it was hypothesized that: 1) BFRRT would result in greater acute and chronic RPE values than non-BFRRT; 2) Performance fatigability would increase as a result of both BFRRT and non-BFRRT; and 3) BFRRT and non-BFRRT would elicit similar increases in forearm flexion and extension strength.

2. Methods

2.1 Participants

Twenty recreationally active women [33] with no known cardiovascular, metabolic, or muscular diseases volunteered to participate in this investigation. The subjects were a part of a large multi-dependent variable study, but there is no overlap between the current study and previously published data [34]. Furthermore, in the present study, the women were randomly assigned to either the BFRRT group ( $\bar{X}$ $\pm$ SD: age $=$ 22 $\pm$ 2 yrs; height $=$ 165.1 $\pm$ 7.4 cm; and body mass; 60.1 $\pm$ 5.3 kg; $n=$ 10) or the non-BFRRT group (age $=$ 21 $\pm$ 1 yrs; height $=$ 163.8 $\pm$ 5.3 cm, and body mass $=$ 60.6 $\pm$ 4.2 kg; $n=$ 10). The subjects visited the laboratory on 8 occasions (1 familiarization visit and 7 visits consisting of 7 training sessions and 2 testing days [during the 1 ${}^{\text{st}}$ and 7 ${}^{\text{th}}$ training visits]) over a 2-wk period. In addition, the participants started the investigation at the beginning of the menses phase of their ovulatory cycle, and all testing and training procedures were scheduled at approximately the same time of day. All procedures were conducted ethically in accordance with the principles of the World Medical Association Declaration of Helsinki. Also, the University Institutional Review Board for Human Subjects approved (ID#: 20180918559EP) the study and during the first visit all subjects completed a health history questionnaire and signed a written informed consent prior to testing.

Figure 1.

Timeline of investigation. Note: Changes in maximum strength reflect the mean increases from pretraining MVICs 1 to pretraining MVICs 2. Changes in performance fatigability reflect the mean differences between the percent decline from pretraining MVIC 1 to post-training MVICs 1 and the percent decline from pretraining MVICs 2 to post-training MVICs 2.

2.2 Experimental design

This was a randomized, repeated-measures, between-group, parallel design investigation (Fig. 1). Women were separated into two different groups: a BFRRT and a non-BFRRT group. The BFRRT group wore a cuff on their upper arm at a pressure of 40% of the lowest pressure necessary to completely occlude the arterial inward flow, which completely occluded venous return from the training limb. Each group completed the same training 2-wk program, which consisted of reciprocal concentric forearm flexion-extensions at a load of 30% maximum for each respective muscle action (i.e. 30% flexion maximum and 30% extension maximum). During each day of training, the participants completed 4 sets (1 ${}^{\text{st}}$ set $=$ 30 reps, 2 ${}^{\text{nd}}$ – 4 ${}^{\text{th}}$ set $=$ 15 reps each; total number of repetitions/sessions $=$ 75) of these reciprocal muscle actions at the previously described load. There was a total of 7 training sessions across a 2-wk timespan. Before the first training session and immediately after the 7 ${}^{\text{th}}$ training session, the participants completed testing procedures, which included the assessment of maximal strength and performance fatigability. Specifically, during the 1 ${}^{\text{st}}$ and 7 ${}^{\text{th}}$ training visit strength was assessed before any training (i.e. before the first set [1 $\times$ 30]). In addition, performance fatigability was quantified as the exercise-induced percent decline in maximum strength from pre- to post-training session (i.e. before and after the 75 training repetitions on a respective training day). Of note, the training-induced changes to maximal strength were only influenced by the first 6 training sessions. Participants only completed the seventh training session to get a measure of performance fatigability.

2.3 Familiarization visit

During the familiarization visit, the subjects’ height (Cardinal Detecto 439 Mechanical Beam, Webb City, MO USA) and body mass (Cardinal Detecto 439 Mechanical Beam, Webb City, MO USA) were recorded. The subjects were oriented to their seated position on the isokinetic dynamometer based on manufacturers recommendations (Biodex System 3; Biodex Medical Systems, Inc. Shirley, NY, USA). Once positioned on the dynamometer, the subjects performed isometric and concentric, isokinetic, forearm flexion and extension muscle actions at submaximal and maximal intensities. The training arm of each participant was randomized so that there were an equal number of participants training their dominant and non-dominant arm (Dominant arm was defined as the arm used to throw a ball). The subjects were oriented to the training procedures by performing reciprocal concentric-only isokinetic forearm flexion and extension muscle actions at 30% of their forearm flexion concentric peak moment and forearm extension concentric peak moment. During all visits, the subjects were able to visually track moment production on a digital monitor. All concentric, isokinetic muscle actions were performed at 120 ${}^{\circ}$ /s and completed through a 120 ${}^{\circ}$ range of motion (0–120 ${}^{\circ}$ of forearm flexion with 0 ${}^{\circ}$ corresponding to full extension).

2.4 Training visits

Each training session consisted of 75 concentric, isokinetic, reciprocal forearm flexion-extension muscle actions performed over four sets (1 $\times$ 30 $+$ 3 $\times$ 15), which were each separated by 30-s of rest. Prior to the first set on the first and seventh training sessions, the subjects performed a standardized warm-up consisting of 10 submaximal ( $\sim$ 50% maximal effort), concentric, isokinetic, reciprocal forearm flexion-extension muscle actions. This was followed by 5-min of rest and subsequent maximal effort tests. Specifically, the subjects performed randomly ordered maximal, concentric, isokinetic, forearm flexion and forearm extension muscle actions as well as MVIC trials. The subjects performed each muscle action twice and the highest value of each was used to calculate training loads and was also used for subsequent analyses. Also, following the final set (4 ${}^{\text{th}}$ set of the prescribed training load) on the first and seventh training sessions, the subjects immediately performed a posttest MVIC. For the BFRRT group, prior to completing the posttest MVIC, the BFR cuff was rapidly deflated and removed. Furthermore, the percent decline from that day’s pre-training MVIC to post-training MVIC (only during the first and seventh training sessions) for forearm flexion and extension was defined as performance fatigability, and all MVIC trials were completed at an elbow joint angle of 45 ${}^{\circ}$ and for a duration of 3-s.

2.5 Blood flow restriction

During all training session sets (i.e. 1 $\times$ 30, 3 $\times$ 15), the subjects in the BFRRT group wore a 30-mm wide cuff (KAATSU Master, Sato Sports Plaza, Tokyo, Japan) on the most proximal portion of the upper arm (below deltoid) of their training arm. The cuff was inflated to the target pressure for each subject and remained inflated for the entire duration ( $\sim$ 5 min) of the 75-repetition training protocol including the 30-s rest between sets. The target pressure was defined as 40% [10, 11, 35] of the lowest amount of pressure needed to completely occlude the brachial artery as evidenced by Pulse Wave Doppler (GE Logiq, USA), which was identified prior to any testing procedures and conducted before training visit 1.

2.6 Ratings of perceived exertion

Following each training set, the subjects were asked to provide an RPE value that corresponded to the OMNI-RES [36, 37]. Thus, the subjects provided a total of 4 RPE values per training visit. Prior to the first training visit, the subjects were familiarized and permitted to ask questions regarding the OMNI-RES. Also, prior to the first set of each training visit, the subjects were reminded to be attentive to sensations such as strain, intensity, discomfort, and fatigue felt during the set when providing a post-set RPE value [21].

2.7 Statistical analyses

Independent $t$ -tests were used to examine mean differences in daily training loads between groups. A 2 (Group: BFRRT and non-BFRRT) $\times$ 2 (Time: Pretraining and Post-training) $\times$ 2 (Trial: Pretest MVIC and Posttest MVIC) $\times$ 2 (Muscle Action: Flexion and Extension) mixed factorial ANOVA was used to examine mean differences in performance fatigability. Changes in strength were assessed with a 2 (Group: BFRRT and non-BFRRT) $\times$ 2 (Time: Pretraining and Post-training) $\times$ 2 (Muscle action: Forearm flexion and Forearm extension) mixed factorial ANOVA. In addition, a 2 (Group: BFRRT and non-BFRRT) $\times$ 4 (Set: 1–4) $\times$ 7 (Training Visit: 1–7) mixed factorial ANOVA was used to examine mean differences in RPE values. Significant interactions were followed up with repeated measure ANOVA or independent $t$ -tests when appropriate. Effect sizes were reported as partial eta squared ( $\eta_{p}^{2}$ ) and Cohen’s $d$ , respectively. A $p$ -value $\leqslant$ 0.05 was considered statistically significant for all analyses, and the analyses were conducted using Statistical Package for the Social Sciences software (version 26.0. SPSS Inc. Chicago, IL, USA).

Figure 2.

A. Individual subject (unique symbols) and mean (collapsed across Muscle Action and Group) ( $\pm$ SD) values for 0-wk (23.7 $\pm$ 3.2 Nm) pretest MVIC and 2-wk (26.8 $\pm$ 2.7 Nm) pretest MVIC. ${}^{\ast}$ Denotes a significant ( $p<$ 0.05) mean increase from the 0-wk to 2-wk timepoint. B. Mean (collapsed across Group and Muscle Action) ( $\pm$ SD) values for pretest MVIC and posttest MVIC at 0-wk (49.5 $\pm$ 5.3 Nm vs. 43.9 $\pm$ 3.2 nm) and 2-wk (55.7 $\pm$ 4.2 Nm vs. 47.8 $\pm$ 3.6 Nm). ${}^{{\dagger}}$ Denotes a significant ( $p<$ 0.05) decrease from the pretest to posttest MVIC.

Table 1

The pairwise comparisons (collapsed across Group) of the RPE values reported following each set resulting from the seven (i.e. one for each Training Visit) follow-up 1-way repeated measures ANOVAs

Training visit	$p$ -value ( $\eta_{p}^{2}$ )	Set 1	Set 2	Set 3	Set 4
1	0.01 (0.490)	5.4 $\pm$ 1.2	5.7 $\pm$ 1.6	6.3 $\pm$ 1.6 ${}^{\text{a,b}}$	6.6 $\pm$ 1.5 ${}^{\text{a,b,c}}$
2	$<$ 0.001 (0.622)	5.4 $\pm$ 1.2	5.4 $\pm$ 1.2 ${}^{\text{a}}$	6.0 $\pm$ 1.2 ${}^{\text{a,b}}$	6.5 $\pm$ 0.9 ${}^{\text{a,b}}$
3	0.005 (0.462)	5.5 $\pm$ 1.1	6.1 $\pm$ 1.1	6.5 $\pm$ 1.0 ${}^{\text{a}}$	6.8 $\pm$ 1.3 ${}^{\text{a,b}}$
4	0.006 (0.501)	5.5 $\pm$ 1.3	5.9 $\pm$ 1.3 ${}^{\text{a}}$	6.3 $\pm$ 1.0 ${}^{\text{a}}$	6.6 $\pm$ 1.2 ${}^{\text{a,b,c}}$
5	$<$ 0.001 (0.659)	5.4 $\pm$ 1.1	5.9 $\pm$ 1.2 ${}^{\text{a}}$	6.3 $\pm$ 1.2 ${}^{\text{a,b}}$	6.8 $\pm$ 1.4 ${}^{\text{a,b,c}}$
6	0.001 (0.620)	5.0 $\pm$ 1.4	5.6 $\pm$ 1.2 ${}^{\text{a}}$	6.1 $\pm$ 1.3 ${}^{\text{a,b}}$	6.5 $\pm$ 1.4 ${}^{\text{a,b}}$
7	$<$ 0.001 (0.690)	4.5 $\pm$ 1.2	5.3 $\pm$ 1.2 ${}^{\text{a}}$	5.8 $\pm$ 1.1 ${}^{\text{a,b}}$	6.2 $\pm$ 1.1 ${}^{\text{a,b}}$

${}^{*}$ Note: ${}^{\text{a}}$ denotes significantly greater than Set 1; ${}^{\text{b}}$ denotes significantly greater than Set 2; ${}^{\text{c}}$ denotes significantly greater than Set 3.

3. Results

3.1 Daily training loads, performance fatigability, and strength

For daily concentric, isokinetic forearm flexion training loads, there was no significant ( $p=$ 0.308) difference between the BFRRT (5.95 $\pm$ 0.9 Nm) and the non-BFRRT (5.58 $\pm$ 0.7 Nm) groups. In addition, for the daily concentric, isokinetic forearm extension training loads, there was no significant ( $p=$ 0.132) difference between the BFRRT (6.54 $\pm$ 1.1 Nm) and the non-BFRRT (5.84 $\pm$ 0.9 Nm) groups. For performance fatigability, there was no significant ( $p>$ 0.05) 4-way, 3-way interaction, or significant ( $p=$ 0.537) main effect for Group, but there was a significant ( $p=$ 0.013; $\eta_{p}^{2}=$ 0.299) Time $\times$ Trial (collapsed across Group and Muscle Action) interaction. Follow-up paired $t$ -tests indicated that at 0-wk and 2-wk the pretest MVICs (49.5 $\pm$ 5.3 Nm and 55.7 $\pm$ 4.2 Nm) were significantly ( $p<$ 0.001; $d=$ 1.3 and 2.0) greater than the posttest MVICs (43.9 $\pm$ 3.2 Nm and 47.8 $\pm$ 3.6 Nm, respectively) (Fig. 2). Individual percent change values (performance fatigability; collapsed across Group and Muscle Action) were calculated, and a $t$ -test indicated that there was a significant ( $p=$ 0.04; $d=$ 0.68) increase in performance fatigability from 0-wk (10.9 $\pm$ 5.0%) to 2-wk (14.1 $\pm$ 4.4%). For strength, there was no significant 3-way ( $p=$ 0.509) or any significant 2-way interaction ( $p<$ 0.05), but there was a significant main effect of Time ( $p<$ 0.001; $\eta_{p}^{2}=$ 0.80) and Muscle action ( $p=$ 0.031; $\eta_{p}^{2}=$ 0.234). For Time, the follow-up pairwise comparison (collapsed across Group and Muscle action) indicated that the 2-wk pretest MVIC (26.8 $\pm$ 2.7 Nm) was significantly ( $p<$ 0.001; $d=$ 1.1) greater than 0-wk pretest MVIC (23.7 $\pm$ 3.2 Nm). For Muscle action, the follow-up pairwise comparison (collapsed across Group and Time) indicated that the forearm flexion MVIC (26.5 $\pm$ 4.1 Nm) was significantly ( $p<$ 0.031; $d=$ 0.60) greater than the forearm extension MVIC (24.1 $\pm$ 3.9 Nm).

3.2 Ratings of perceived exertion

There was no significant ( $p=$ 0.593) 3-way interaction, but there was a significant ( $p<$ 0.001; $\eta_{p}^{2}=$ 0.721) Set $\times$ Training Session interaction. As a result, follow-up 1-way repeated measures ANOVAs (collapsed across Group) were performed for each Training Session (1–7). For each Training Session, there was a significant ( $p<$ 0.05) main effect across Set. In general, set 1 resulted in a significantly ( $p<$ 0.05) lower RPE than all other sets (Table 1). Alternatively, the significant 2-way interaction (Set $\times$ Training Session; collapsed across Group) was followed up with four (one per set), 1-way repeated measures ANOVAs. There were no significant ( $p>$ 0.05) main effect across Training Session for any set.

4. Discussion

The purpose of the present study was to examine the effects of low-load concentric, isokinetic, reciprocal forearm flexion and extension training, with and without BFR on perceptual responses, performance fatigability, and muscular strength. The primary findings of this study were that following short-term training: 1) There were no differences in the RPE values between the BFRRT and non-BFRRT groups; 2) performance fatigability increased in response to training, but there was no difference between groups or muscle actions; and 3) the BFRRT and non-BFRRT groups exhibited similar increases in forearm flexion and extension MVIC strength.

The current results indicated that there were no differences between the BFRRT and non-BFRRT groups for the perceptual responses across the short-term training. These findings are not consistent with a recent study [16] which examined multiple sets of dynamic constant external resistance (DCER) forearm flexion and leg extension muscle actions under four different conditions which included a training intensity of 15% 1RM with no BFR, 15% 1RM plus 40% of arterial occlusion, 15% 1RM plus 80% of arterial occlusion, or 70% 1RM with no BFR across 16 training sessions. Unlike the present study, the findings of this study indicated that low-load BFRRT reduced the perception of exertion to a greater extent than training without BFR. Several methodological differences, such as the use of a different tool for assessing the RPE, could account for the observed discrepancies. However, another study [17] reported that when unilateral leg extensions were performed at 15% 1RM to volitional failure, there were no differences in RPE values following one set with (40% or 80% of arterial occlusion) or without BFR. Thus, the current findings are in general agreement with those reported in [18] by demonstrating that there were no differences in the perceptual responses between BFRRT and non-BFRRT following short-term training of concentric, isokinetic, reciprocal forearm flexion and extension training.

In the current study, RPE increased across most of the sets during each training visit (Table 1). For all training visits, the mean RPE values after set 4 were significantly greater than those of set 1 and 2. These findings suggested that both training groups (BFRRT and non-BFRRT) experienced similar increases in sensations such as strain, intensity, discomfort, and fatigue by the end of set 4 [21]. It has been demonstrated that RPE values are influenced by metabolic stress via group III/IV muscle afferents [38]. This increase in metabolic stress has been proposed as potential mechanism for eliciting favorable muscle adaptations following BFR training [7, 13]. The current results, however, did not demonstrate any differences in RPE values, MVIC increases, or changes in performance fatigability between the BFRRT and non-BFRRT groups, which may have been indicative of similar degrees of metabolic stress between the two groups [38]. Pinto et al. [15] demonstrated that 3 sets of 10 repetitions of DCER in bilateral leg extension mode resulted in a significantly greater blood lactate (i.e. metabolic stress) response at an intensity of 65% 1RM with no BFR compared to 20% 1RM plus BFR. In addition, it has recently been suggested [39] that the BFR cuff pressure should be set above 40% of arterial occlusion to induce metabolic stress as indirectly indicated by increases in deoxyhemoglobin via near-infrared spectroscopy. Thus, perhaps the intensity (30% 1RM) and cuff pressure (40% of arterial occlusion) used in the current study for the concentric, isokinetic, reciprocal forearm flexion and extension training were not sufficient to elicit a difference in metabolic stress between the BFRRT and non-BFRRT group, which was reflected by the similar self-reported perceived exertion values.

The current investigation found that after completing 7 of the training prescriptions (i.e. 75 repetitions) of the low-load BFRRT or non-BFRRT there was an increase (collapsed across Group and Muscle Action) in performance fatigability (Fig. 2B). Previously, Mayhew et al. [29] reported that a 12-wk linear periodization non-BFRRT program resulted in a 35.5% increase in performance fatigability. The authors [29] attributed this effect to an interaction of training-induced neural, muscular, and metabolic adaptations. Given the short duration of the current study, however, it was hypothesized that neural factors (i.e. voluntary activation) specific to the mode (BFRRT and non-BFRRT) or muscle action (flexion and extension) primarily contributed to the increase in performance fatigability [40, 41]. Of note, this is the first investigation to examine performance fatigability following BFRRT in women. Previous studies [12, 35, 42], however, have reported a greater magnitude of performance fatigability following a single bout of BFRRT than non-BFR conditions. For example, Husmann et al. [12] examined performance fatigability following multiple sets (1 $\times$ 30, 3 $\times$ 15) of low-intensity (30% of maximum) unilateral leg extensions with 60% arterial occlusion versus non-BFR and reported a greater reduction in maximal voluntary moment following the BFR (45 $\pm$ 14%) than the non-BFR (24 $\pm$ 12%) condition. The current results were not consistent with those of Husmann et al. [12] and found no difference in performance fatigability before or after 2-wks of BFRRT or non-BFRRT. Husmann et al. [12] suggested that inconsistencies, such as the currently presented one, in BFR exercise-related investigations [42, 43, 44] may be due to methodological differences (i.e. cuff pressure, cuff application, exercise volume, etc.). Notable differences between the current study and Husmann et al. [12] were the cuff pressure, muscle group investigated, and sex of the subjects. The current study utilized previous cuff pressure recommendations of 40% of arterial occlusion [10, 35], whereas Husmann et al. [12] utilized 60% of arterial occlusion. Reis et al. [39] recently suggested, however, that 60% of arterial occlusion is the physiological threshold for tissue hypoxia and metabolite accumulation during low-intensity exercise, which Loenneke et al. [13] hypothesized were necessary for eliciting BFRRT related muscular adaptations. In addition, the current study examined a smaller amount of muscle mass (upper arm) than Husmann et al. [12] (leg extensors). Thus, it is possible that the cuff pressure and small amount of muscle mass utilized in the current study may not have induced enough metabolic stress to elicit a difference between the BFRRT and non-BFRRT groups. It remains debatable if the difference in the sex of the samples between the current study and Husmann et al. [12] contributed to the discrepancy in findings. It was recently demonstrated [31] that following low-load forearm flexion BFRRT, men and women exhibit similar degrees of performance fatigability (25.1 $\pm$ 22.9% vs. 24.5 $\pm$ 11.9%, respectively). Thus, it is most likely that cuff pressure and the muscle group utilized in the current study were responsible for the lack of difference in performance fatigability between the BFRRT and non-BFRRT groups. Future studies should continue to investigate the effects of BFRRT on performance fatigability in both men and women but utilize cuff pressures $\geqslant$ 60% of complete arterial occlusion.

The results of the current study indicated that there was no difference between the BFRRT and non-BFRRT groups for increases in maximal strength (MVIC). Following the 2-wks of BFRRT or non-BFRRT (6 training visits), there was a 13.1% increase (collapsed across Muscle Action and Group) in MVIC strength (Fig. 2A). Following 6–8 weeks of RT, it has been reported that there was no difference in the increase of strength [8, 45] between low-load unilateral leg extension with and without BFR. Fahs et al. [45] reported that following 6-wks of low-load (30% 1RM) unilateral DCER leg extension training, BFRRT and non-BFRRT conditions elicited an equal increase ( $\sim$ 18%) in 1RM. Jessee et al. [8] demonstrated that there was a 4.8% increase (collapsed across condition) in unilateral leg extension MVIC following 8-wks of DCER training at intensities of 15% 1RM with no BFR, 15% 1RM plus 40% of arterial occlusion, 15% 1RM plus 80% of arterial occlusion, or 70% 1RM with no BFR. Given the recreationally-active status of the subjects and short training period (2-wks) in the present study, it is not likely that an increase in hypertrophy (i.e. myofibril size) was responsible for the 13.1% increase in MVIC [40, 41]. Del Balso and Cafarelli [46] previously demonstrated that after only 2 isometric training sessions (6 sets of 10, 3–4 s MVICs of the plantar flexor muscles) there was approximately a 5.6% increase in MVIC (20% increase after 13 visits), which was attributed to increased muscle activation and increased neural drive. Thus, in the current study, it is likely that neural adaptations (i.e. voluntary activation, corticospinal excitability, etc.) led to the increase in MVIC to an equal extent for the BFRRT and non-BFRRT groups. Future studies, however, are needed to confirm this hypothesis due to previous conflicting reports [47]. Regardless of the precise mechanism (neural- vs. muscle morphology-related adaptation), the current study, in conjunction with previous results [7, 8, 45, 48], indicated that 2–8 weeks of either low-load concentric BFRRT or non-BFRRT resulted in equivalent increases in muscular strength (i.e. MVIC).

The current study was unable to determine if the strength gains were a result of hypertrophy or neural adaptations. This should be considered a limitation. Additionally, we did not record the work expanded in each individual repetition during training and hence it is unclear if the peak moment or work performed contributed more to RPE.

5. Conclusions

In summary, 2-wks of low-load concentric, reciprocal forearm flexion and extension training resulted in similar training-induced changes in perceptual responses, performance fatigability, and muscular strength between BFRRT and non-BFRRT. Future studies are needed to determine the work-RPE relationship as well as the underlying mechanisms related to strength adaptations following short-term training BFRRT training in women. However, the current findings may be used to reduce concerns of increased perceptual responses following BFRRT compared to non-BFRRT.

Author contributions

CONCEPTION: Joshua L. Keller, Ethan C. Hill, and Terry J. Housh.

PERFORMANCE OF WORK: Joshua L. Keller and Ethan C. Hill.

INTERPRETATION OR ANALYSIS OF DATA: Joshua L. Keller, Ethan C. Hill, Terry J. Housh, Cory M. Smith, John Paul V. Anders, Richard J. Schmidt, and Glen O. Johnson.

PREPARATION OF THE MANUSCRIPT: Joshua L. Keller, Ethan C. Hill, Terry J. Housh, Cory M. Smith, John Paul V. Anders, Richard J. Schmidt, and Glen O. Johnson.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Joshua L. Keller and Ethan C. Hill.

SUPERVISION: Terry J. Housh, Richard J. Schmidt, and Glen O. Johnson.

Ethical considerations

All procedures were conducted in accordance with the Declaration of Helsinki. The study was approved by Institutional Review Board (ID#: 20180918559EP). All subjects signed a written informed consent prior to testing.

Funding

Nebraska NASA Space Grant.

Footnotes

Acknowledgments

We would like to thank all the participants for their time and for volunteering to comply with the protocol of the study as well as the Nebraska NASA Space Grant for funding this study.

Conflict of interest

The authors have no conflict of interest and the results of the present study do not constitute endorsement of the product by the authors.

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