Comparative Effects of Vigorous-Intensity and Low-Intensity Blood Flow Restricted Cycle Training and Detraining on Muscle Mass, Strength, and Aerobic Capacity

Abstract

Kim, D, Singh, H, Loenneke, JP, Thiebaud, RS, Fahs, CA, Rossow, LM, Young, K, Seo, D-i, Bemben, DA, and Bemben, MG. Comparative effects of vigorous-intensity and low-intensity blood flow restricted cycle training and detraining on muscle mass, strength, and aerobic capacity. J Strength Cond Res 30(5): 1453–1461, 2016—Traditional high-intensity aerobic training has been shown to improve muscle protein synthesis and aerobic capacity; however, recent research indicates that low-intensity aerobic training with blood flow restriction (BFR) may have similar effects. The purpose of this study was to compare the effects of vigorous-intensity (VI) cycling vs. low-intensity cycling with BFR (LI-BFR) on muscle mass, strength, and aerobic capacity after training and subsequent detraining. Thirty-one physically active subjects were assigned to one of 3 groups: VI (n = 10, 60–70% heart rate reserve [HRR]), LI-BFR (n = 11, 30% HRR with BFR at 160–180 mm Hg), and no exercise control (n = 10, no exercise). Subjects in VI and LI-BFR cycled 3 times per week for 6 weeks (total 18 sessions). Body composition, muscle mass, strength, and aerobic capacity were measured pre, post, and after 3 weeks of detraining. A group × time interaction (p = 0.019) effect for both knee flexion and leg lean mass was found. For both VI and LI-BFR groups, knee flexion strength was significantly increased between pre and post (p = 0.024, p = 0.01) and between pre and 3 week-post (p = 0.039, p = 0.003), respectively. For the LI-BFR group, leg lean mass was significantly increased between pre and 3 week-post (p = 0.024) and between post and 3 week-post (p = 0.013). However, there were no significant differences between groups for any variables. The LI-BFR elicits an increase in the knee flexion muscle strength over time similar to the VI. An increase in the leg lean mass over time was seen in the LI-BFR, but not in VI and CON.

Introduction

Cycle training, which is generally used for improving cardiovascular and respiratory responses, may also induce changes to skeletal muscle caused by improvements of glycolytic capacity, actomyosin ATPase, and the ability to oxidize glycogen and fatty acids (⁹). Moreover, cycle training has been used for a broad range of populations leading to improvement of mixed muscle protein synthesis and aerobic capacity (³²). During cycle exercise, a risk of muscle damage is lower because of primarily concentric muscle contraction compared with running, and muscle protein synthesis is enhanced after acute cycling or chronic cycling (²⁶). Another benefit of cycle training is that it minimizes the loading stress on joints because body weight is supported by the seat of the bicycle ergometer, thus reducing the weight bearing placed on the joints compared with other exercises (⁸).

Previous studies that used blood flow restriction (BFR) training with low-load resistance exercise reported muscle hypertrophy and increased muscle strength compared with low-load resistance exercise without BFR (^4,33). Additionally, previous low-load BFR resistance training studies have reported similar muscle hypertrophy and strength gains as traditional high-intensity resistance training (^7,14), but the mechanism of muscle hypertrophy and strength gain induced by low-intensity exercise with BFR is not certain. One potential mechanism is that low-load exercise with BFR leads to an acidic intramuscular environment inducing stimulation of chemosensitive group III and IV afferents resulting in the additional recruitment of type II fibers (²⁰), which may be important for marked increases in muscle hypertrophy.

Safety is an important issue with this type of exercise. A study showed dramatic decreases in maximum isometric torque (MVC) and increases in a muscle damage marker (tetranectin) after 1 bout of BFR resistance exercise (5 sets to failure) compared with free flow exercise (³⁴); however, it should be noted that the MVC values at other time points were not different between the BFR and free flow groups except at immediate posttesting and tetranectin may not be an appropriate muscle damage marker (¹⁷). In addition, there was no muscle damage after low-intensity BFR exercise and no prolonged decrements in torque, prolonged muscle soreness, prolonged muscle swelling, or indirect markers of muscle damage in previous BFR studies (¹⁷).

Not only low-load BFR resistance training but also low-intensity walk training with BFR has been shown to induce increases in muscle strength (^3,28) and aerobic capacity (³) in various populations compared with low-intensity walk training without BFR. Moreover, 1 study reported muscle hypertrophy and increased maximal oxygen uptake after low-intensity cycle training with BFR compared with a group that cycled at the same intensity without BFR for 8 weeks (¹). However, previous studies that have used BFR most often investigated muscle strength and hypertrophy after aerobic training (walking and cycling) at same intensity without BFR. No studies have compared the muscular and aerobic capacity responses of low-intensity aerobic training combined with BFR to vigorous-intensity (VI) aerobic training. Thus, it is not known whether low-intensity cycle training with BFR would have the same benefits as VI cycle training in terms of changes in muscle mass, strength, and maximal oxygen uptake in a physically active college-aged population.

Furthermore, when untrained people undergo traditional endurance training at submaximal or maximal intensity for several weeks, exercise performance, i.e., aerobic capacity, is increased because of improved stroke volume (²²), but once the training stimulus is removed (detraining), many of the adaptations become partially or entirely diminished (²¹). Generally, detraining after endurance training typically results in decrements of capillary density, arterial-venous oxygen difference, oxidative enzyme activity, force production, and muscle cross-sectional area (mCSA), but no changes to glycolytic enzyme activities, muscle fiber distribution, or muscle strength performance (²¹). A previous study reported positive changes in muscle strength and mass after low-load with BFR resistance training and the slightly attenuated but still greater than the baseline values after a detraining period similar to high-load resistance training (³⁵). However, to date, no study has investigated the detraining response after low-intensity aerobic training in combination with BFR.

Thus, the purpose of this study was to investigate the effects of VI and low-intensity cycling with BFR (LI-BFR) on muscle mass, strength, and aerobic capacity with training and subsequent detraining. We hypothesized that the low-intensity cycling with BFR would result in increases in total lean mass, leg lean mass, mCSA, muscle strength, and aerobic capacity similar to the VI cycling group.

Methods

Experimental Approach to the Problem

In this study, a pre-post test comparison group design was used. The total length of this study was eleven weeks, including 1 week of pretesting, 6 weeks of training (3 times a week, total 18 sessions), 1 week of posttesting, and 3 weeks of detraining. The pretest with familiarization was accomplished 3–7 days before the first training session, and the posttest was completed within 2–9 days after the training period and after 3 weeks of detraining. In each testing session, the participants underwent testing early in the morning after having fasted for 8 hours. During the training and detraining period, subjects were asked to maintain their normal diet and physical activity. Participants were randomly assigned to VI cycling (60–70% heart rate reserve [HRR]) without BFR training group (VI), to a low-intensity cycling protocol (30% HRR) with BFR (160–180 mm Hg) (LI-BFR) group or to a nonexercise control (CON) group. Thigh mCSA, muscle strength, body composition, and maximal oxygen uptake were determined at 3 time points (pre, post, and 3 weeks-post training).

Subjects

Thirty-one healthy college-aged males (22.4 ± 3.0 years, range: 19–30 years) participated in this study. Subjects were physically active and had not engaged in a regular endurance or resistance exercise program for at least 4 months before this study. All participants completed an informed consent, health status questionnaire, and physical activity readiness questionnaire before the pretesting period, and then were familiarized with the study measurements. The study was approved by the University's Institutional Review Board for human subjects. Based on a sample size estimation calculated from data of Abe et al. (²), the number of subjects for each group ranged from 10 to 15 for all outcome measures to achieve a statistical power >0.80 with an alpha level of p ≤ 0.05. A nonprobability sampling technique was used because subject recruitment involved voluntary participation. Baseline physical characteristics of subjects are shown in Table 1. There were no significant baseline differences between the groups for any variable. The study conforms to the Code of Ethics of the World Medical Association (approved by the ethics advisory board of Swansea University) and required players to provide informed consent before participation.

Table 1: Physical characteristics.*†

Training Protocol

During training sessions, a polar heart rate belt across the chest was worn and a polar heart rate monitor displayed the heart rate of subjects. A standardized warm-up (5 minutes) on a stationary cycle ergometer (model 828 E; Monark, Vansbro, Sweden) preceded each training session. Subjects in the VI group then performed 20 minutes of cycling at 60% HRR for the initial 3 weeks followed by cycling at 70% HRR for remaining 3 training weeks. Subjects in the LI-BFR group performed 20 minutes of cycling at 30% HRR for 6 weeks. Heart rate reserve was calculated by subtracting the resting heart rate (HRrest, measured by a polar heart rate monitor) from estimated maximal heart rate (HRmax, 220—age) and then multiplying by percentage (%) intensity and finally adding HRrest to the result. The Borg rating of perceived exertion (RPE; 6–20 scale) was determined by every subject at end of all training sessions. Subjects in the LI-BFR group wore elastic cuffs (width: 5 cm, KAATSU master; Sato Sports Plaza, Tokyo, Japan) at the proximal ends of the thighs with initial pressure set at 40–60 mm Hg. The pressure in the cuffs was then increased incrementally by 20 mm Hg from 120 to 160 mm Hg, inflated for 30 seconds and deflated for 10 seconds, until the training pressure of 160 mm Hg was reached. During the training periods, the training pressure started at 160 mm Hg and was increased by 20 mm Hg after the initial 3 weeks so that the final arbitrary training pressure was 180 mm Hg (weeks 4–6) for each subject in the LI-BFR group (²). The subjects in the control group were asked not to participate in any type of structured exercise throughout the entire training period.

Muscle Strength

Concentric isotonic 1 repetition maximum (1RM) testing was performed to measure maximum strength for knee extension and knee flexion (MD110 Leg Extension and MD 118 Seated Leg Curl; Cybex, Medway, MA, USA). Before each 1RM test, subjects performed a warm-up at 50% of their estimated maximal strength for each exercise. During 1RM testing, the load was incrementally increased until subjects reached the maximum load that could be lifted through a full range of motion 1 time. One repetition maximum was attained within 4–6 attempts with a minute rest period between attempts.

Muscle Cross-Sectional Area

The mCSA of the right thigh was obtained by a peripheral quantitative computed tomographic (pQCT) scanner XCT 3000 with software version 6.00 (Stratec Medizintechnik GmbH, Pforzheim, Germany). All pQCT were measured by a trained pQCT technician, whose coefficient of variation (%CV) was 0.8% for the mCSA. A quality assurance (QA) test was completed before each testing session using a calibration phantom provided by the manufacturer. Peripheral quantitative computed tomography was assessed with a 2-mm slice at 50% of the length of the femur for determination of mCSA. A region of interest was drawn around the total CSA scan and analyzed for mCSA using the manufacturer's threshold driven software along with smoothing filter F01F06U01. Specifically, mCSA was determined by subtracting total fat area from the total CSA scan leaving just muscle and bone area. Next, the total bone area was subtracted from the muscle and bone area leaving total mCSA.

Body Composition

Dual-energy x-ray absorptimetry (DXA) (Lunar Prodigy encore software version 10.50.086; GE Medical Systems, Madison, WI, USA) was used to assess regional bone-free lean body mass (BFLBM) and fat mass of the limbs and total body. All DXA scans were conducted by 2 technicians and their %CV are 1.24 and 1.54% for percent body fat; 1.16 and 1.67% for total fat mass; and 0.64 and 1.03% for BFLBM. The DXA was calibrated daily after the QA procedures of the software. Before each test, height was measured by a wall mounted stadiometer (PAT #290237; Novel Products, Rockton, IL, USA), and weight was measured by an electronic scale (BWB-800; Tanita Corporation of America, Inc., Arlington Heights, IL, USA). The same technician performed each DXA scans. The subject's anteroposterior thickness as measured at the umbilicus by a straightedge ruler placed on the scan table, which was measured to set the scan mode (standard mode: between 13 and 25 cm or slow mode: over 25 cm).

Aerobic Capacity (V[Combining Dot Above]O₂peak)

A cycle ergometer (Model 828 E; Monark) and an indirect spirometry (Trueone 2400 metabolic measurement system; Parvomedics, Sandy, UT, USA) were used to determine V[Combining Dot Above]O₂peak. Before testing, gas and flowmeter calibration were performed. During the test, expired air was analyzed to measure O₂ consumption and CO₂ production. The graded exercise test (GXT) consisted of 7 stages from 25 to 325 W and increased by 50 W after each 3-minute stage with a pedaling rate of 50 rpm. Thus, subjects performed warm-up at 25 W for 1 minute. The same work-load was maintained in the first stage for 3 minutes, and then the work-load was increased by 50 W every 3 minutes until exhaustion. The GXT test was stopped if subjects achieved maximal effort by reaching a respiratory exchange ratio of 1.1 or greater, or started pedaling below 50 rpm or achieved estimated heart rate maximum (HRmax, 220—age). After finishing the GXT test, the subjects completed a 2–3 minutes of cool down. Also, heart rate (HR) was recorded during the entire testing period and during recovery using a polar heart-rate monitor (T31-coded transmitter; Polar Electro Inc., Lake Success, NY, USA).

Statistical Analyses

Mean and SDs were computed for each dependent variable. Baseline comparisons between the 3 groups (VI, LI-BFR, and CON) for each variable were evaluated by 1-way analysis of variance (ANOVA). Between group comparisons over time were made using a 2-way group (VI, LI-BFR, and CON group) × time (pre, post, and 3 week-post test) repeated-measures ANOVA. If there were any significant differences, 1-way ANOVAs across group and time were used to determine where the difference was. The data were analyzed by SPSS 18.0 (SPSS Inc., Chicago, IL, USA). All statistical analyses used a 0.05 level of significance. Family-wise error rate was kept constant by Bonferroni correcting for the number of comparisons made. Additionally, effect size was calculated by subtracting the pretest mean from the posttest mean and then dividing by pretest SD (³⁰).

Results

Physical characteristics of subjects are shown in Table 1. There were no significant differences between the groups for any variable. A description of exercise protocols is presented in Table 2. Rating of perceived exertion is shown in Figure 1.

Table 2: Description of exercise protocols for each group.*

Figure 1: Rating of perceived exertion (RPE: 6–20 scale); *p ≤ 0.05 group effect; **p < 0.01 group effect. LI-BFR = low-intensity cycling with BFR; VI = vigorous-intensity.

Muscle Strength

For knee extension, there was no significant group × time interaction (p = 0.228) or a group (p = 0.704) main effect for strength, but muscle strength significantly increased by time (p < 0.001) across all groups. On average, knee extension increased 4.74% (PRE-POST) and 8.29% (PRE-3 POST) in the VI group, 5.95% (PRE-POST) and 5.58% (PRE-3 POST) in the LI-BFR group, and 3.63% (PRE-POST) and 4.22% (PRE-3 POST) in the CON group. In the knee flexion muscle strength, there was a significant group × time interaction (p = 0.019) effects. For both VI and LI-BFR groups, knee flexion muscle strength significantly increased from pre to post (p = 0.024 and p = 0.01) and from pre to 3 weeks-post (p = 0.039 and p = 0.003), respectively. Knee flexion muscle strength in the CON group remained unchanged from pre to post and 3 weeks-post time periods, and there were no group differences at pre, post, and 3 weeks-post (Table 3). On average, knee flexion increased 7.15% (PRE-POST) and 6.93% (PRE-3 POST) in the VI group, 7.06% (PRE-POST) and 8.90% (PRE-3 POST) in the LI-BFR group, and 1.30% (PRE-POST) and 1.20% (PRE-3 POST) in the CON group.

Table 3: One repetition maximum muscle strength for knee extension and flexion.*†

Body Composition

There was no significant group × time interaction (p = 0.232) or group (p = 0.976) main effect for BFLBM, but the BFLBM significantly increased by time (p = 0.048) across all groups. On average, BFLBM increased 0.71% (PRE-POST) and 0.36% (PRE-3 POST) in the VI group, 2.00% (PRE-POST) and 1.80% (PRE-3 POST) in the LI-BFR group, and 0.18% (PRE-POST) and 0.72% (PRE-3 POST) in the CON group. There was a significant group × time interaction (p = 0.023) effect for leg lean mass. For the LI-BFR group, the leg lean mass was significantly increased from pre to 3 weeks-post (p = 0.024) and from post to 3 weeks-post (p = 0.013), but there were no group differences at pre, post, and 3 weeks-post. The leg lean mass in both VI and CON groups remained unchanged across time. On average, leg lean mass increased 0.44% (PRE-POST) and −0.41% (PRE-3 POST) in the VI group, 1.15% (PRE-POST) and 2.80% (PRE-3 POST) in the LI-BFR group, and 0.89% (PRE-POST) and −0.20% (PRE-3 POST) in the CON group. There was no significant group × time interaction, group, or time main effect for percent fat (p = 0.720, 0.280 or 0.114), total fat mass (p = 0.732, 0.570 or 0.145), or leg fat mass (p = 0.892, 0.589, or 0.287), respectively (Table 4).

Table 4: Body composition and muscle cross-sectional area (CSA).*†

Muscle Cross-Sectional Area

There was no significant group × time interaction (p = 0.639) or group (p = 0.642) main effect, but the mCSA significantly increased by time (p < 0.001) across all groups (Table 4). On average, mCSA increased 2.24% (PRE-POST) and 1.55% (PRE-3 POST) in the VI group, 2.45% (PRE-POST) and 2.67% (PRE-3 POST) in the LI-BFR group, and 1.43% (PRE-POST) and 1.21% (PRE-3 POST) in the CON group.

Aerobic Capacity (V[Combining Dot Above]O₂peak)

There was no significant group × time interaction (p = 0.081), group (p = 0.500), or time (p = 0.356) main effect for the V[Combining Dot Above]O₂peak (Table 5). On average, V[Combining Dot Above]O₂peak increased 5.25% (PRE-POST) and 6.68% (PRE-3 POST) in the VI group, 1.96% (PRE-POST) and −1.23% (PRE-3 POST) in the LI-BFR group, and −1.17% (PRE-POST) and −2.57% (PRE-3 POST) in the CON group.

Table 5: V[Combining Dot Above]O₂peak.*†

Rating of Perceived Exertion

The RPE in the VI group was slightly higher than the LI-BFR group although the training period and there were group differences at exercise session 3, 13, 17, and 18. The RPE is shown in Figure 1.

Discussion

This study is the first study to compare the effects of VI (60–70% HRR) cycling and low-intensity (LI-BFR, 30% HRR) cycling with BFR on body composition, mCSA, muscle strength, and V[Combining Dot Above]O₂peak. This study showed that the LI-BFR group had increases in knee flexion muscle strength across time similar to the VI group. Furthermore, the leg lean mass as determined by DXA in the LI-BFR group was increased across time with no change in mCSA assessed by pQCT, whereas both leg lean mass and mCSA in the VI and CON groups remained unchanged although the work-load (30% HRR) and RPE (mean: 11.5 ± 0.5) in the LI-BFR group were lower than the VI group (60–70% HRR and mean: 12.5 ± 0.5, respectively).

Contrary to the results of our study showing no significant change in mCSA, Abe et al. (¹) showed mCSA and muscle strength significantly increased after 8 weeks of low-intensity BFR cycle training (total 24 sessions) at 40% V[Combining Dot Above]O₂max (average 59% HHR for BFR and 42% HHR for non-BFR group) compared with a non-BFR cycle training group. In addition, Ozaki et al. (²⁷) reported mCSA and isokinetic strength of knee extension and flexion in elderly after walking with BFR for 10 weeks at 45% HRR were improved. Interestingly, the magnitude of increase in the mCSA in the LI-BFR of this study was approximately 2.5% (similar to the VI group: 2.3%) similarly to improvements observed in previous BFR studies (3.4% in the BFR cycling (¹) and 3.2% in the BFR walking (²⁷)). In addition, a review article reported that although cycle training in both young and old populations elicits muscle hypertrophy, the rate of muscle hypertrophy is slower than resistance exercise and longer training periods are required to induce muscle hypertrophy (²⁶). Thus, the longer training period and greater number of training sessions might influence better muscle hypertrophy responses.

In this study, knee extension and knee flexion increased similarly in the LI-BFR group (5.95 and 7.15%, respectively) compared with the VI group (4.74 and 7.06%, respectively). These changes were slightly lower than the knee extension isometric strength changes (7.7%), but were greater than knee flexion isometric strength changes (3.3%) after low-intensity cycling with BFR as reported by Abe et al. (¹). However, the increments in this study were much lower than the isometric knee extension and knee flexion strength increases (8.7 and 15%, respectively) from 10 weeks of BFR walk training in elderly (²⁷). It can be postulated that the magnitude of improvement of muscle mass and strength in this study were less than aforementioned BFR studies because intensity, frequency, and duration in the previous BFR studies were greater than this study. Thus, muscle hypertrophy and strength gain might be influenced by the volume of exercise, exercise intensity, and length of the training period. In addition, another important difference is that elderly subjects likely were weaker and had less muscle at baseline; thus, they may have had more opportunity for improvement because of the lower initial fitness levels.

Consistent with the results of our study showing no changes in % body fat, fat mass, and BFLBM after either training regimen, Nishida et al. (²⁴) showed that % body fat and lean body mass by hydrostatic weighing were not different after 6 weeks of cycle training at their lactate threshold (60 minutes, 5 times per week). The short training period and lower intensities might have influenced the results similar to the findings in our study. However, Carter et al. (⁶) reported significant decreases in percent fat and fat mass in both males and females after 7 weeks cycle training at 60% V[Combining Dot Above]O₂peak (60 minutes, 5 times per week). Thus, the intensity and volume of exercise in our study may have been insufficient to elicit reductions in % body fat. In this study; however, the leg lean mass in the LI-BFR group increased after training, but not in the HI and CON group. The mechanism of BFR muscle hypertrophy is unsure, but there are several possible hypotheses. Generally, When BFR is combined with low-load resistance exercise, type II fibers (fast twitch fibers) are recruited possibly because of an accumulation of metabolites in muscle; however, Loenneke et al. (¹⁸) reported that increases in muscle mass and strength from low-intensity walking with BFR are lower than low-intensity resistance exercise with BFR likely due to the lower accumulation of metabolites. Thus, the increases in muscle strength and mass from aerobic exercise with BFR may be caused by other mechanisms. Acute cell swelling after BFR exercise increases intracellular metabolites and is able to lead to increases in intracellular volume resulting in the activation of a volume sensor and subsequently stimulation of the mammalian target of rapamycin (mTOR) and mitogen-activated protein-kinase pathways (¹⁶).

In this study, aerobic capacity did not significantly improve for any group. Hawley (¹²) showed that general high-intensity endurance training improves aerobic capacity. However, previous studies investigating low-intensity walking and cycling with BFR training have also reported increased muscle volume, strength (^1,2), and maximal oxygen uptake (^1,3) in young and old populations. However, our findings are not in line with previous investigations. Potential reasons for the discrepancy may be due to the use of HRR to set the intensity of exercise. Heart rate reserve might not be appropriate for BFR exercise because of the observed increase in HR with the application of BFR in the absence of exercise due to the decrease in venous return (¹³). Ozaki et al. (²⁵) reported cycle exercise with BFR at rest, 20, 40, 60% V[Combining Dot Above]O₂max increased HR and reduced SV compared with cycle exercise without BFR. Moreover, the 30% HRR in this study of intensity was less than the intensity of other BFR cycle training study (average 59% HRR) resulting in improvement of aerobic capacity (¹). Furthermore, RPE is strongly related to exercise intensity (intensity parameters as lactate threshold and HR) (³¹). In this study, RPE in the LI-BFR (∼12 ± 0.5) group was lower than the VI group (∼13 ± 0.5) which might indicate that the LI-BFR intensity might have been too low to improve aerobic capacity. Also, in our study, V[Combining Dot Above]O₂peak in the VI group also did not significantly increase although V[Combining Dot Above]O₂peak in the VI group increased 5.25% (PRE-POST) and 6.68% (PRE-3 POST). Per the American College of Sports Medicine recommendation, moderate (40–59% HRR) or vigorous (60–89% HRR) intensity of aerobic exercise is required for health benefits (¹⁰). Mangione et al. (¹⁹) showed V[Combining Dot Above]O₂max at high-intensity and low-intensity group in elderly with knee osteoarthritis was not improved (5.7 and 3.05%, respectively) after 10 weeks cycle training (70 and 40% HRR, 25 minutes, 3 times per week). However, Harber et al. (¹¹) reported muscle mass and aerobic capacity in both young and old men improved (16 and 12%, respectively) after 12 weeks cycle training at high-intensity (60–80% HRR, 20–45 minutes, 3–4 times per week, total 42 sessions). Although this study used similar training intensities as previous studies, discrepancies may be due to differences in the duration of exercise sessions, length of the training period, or different populations tested for the VI group.

Usually, after detraining, lean body mass, muscle mass, and muscle strength are diminished because of the loss of a training stimulus (²¹). Furthermore, previous BFR studies have shown that muscle mass and strength after a detraining were slightly decreased but still sufficiently maintained (^23,35). In this study, however, the BFLBM in the LI-BFR group did not decrease after a 3-week detraining period and the leg lean mass, mCSA, and knee flexion strength slightly increased from post to 3 weeks-post. The maintenances of muscle mass and strength gains in the LI-BFR group are similar to findings from previous studies (^23,36) although measurement error may also be involved.

In this study, there are several potential limitations. Using a common restrictive pressure (160–180 mm Hg) for reducing blood flow was a limiting factor. A common pressure has been widely used for BFR studies (^1,3,27,29,37); however, a uniform pressure for subjects of different sizes of thigh might not be appropriate. Loenneke et al. (¹⁵) observed that the amount of restricted blood flow differs between subjects when the same pressure is used, and that arterial occlusion is largely dependent on the size of thigh. This study used resting brachial blood pressure to determine the restricted pressure, which is not a good predictor of arterial occlusion pressure of the thigh. Therefore, the use of a common pressure may have affected the outcomes of this study, with some subjects receiving more or less of a stimulus depending on the size of their limbs.

Another limitation may involve the total volume of exercise completed in this study, which may be important for improvements in muscle strength and hypertrophy. In this study, although the volume of exercise between VI and LI-BFR groups was not matched, the knee flexion muscle strength in the LI-BFR group increased similar to the VI group over time and the leg lean mass in the LI-BFR group was increased over time, but not in the VI group. In previous studies, low-load resistance exercise with BFR has shown similar muscular increases in strength as high-load resistance exercise with similar exercise volume between exercises (^7,14). The volume of exercise completed during cycle training might be important, and muscle strength and mass in the LI-BFR group might have been greater if exercise volume was matched; however, there is limited research comparing exercise volume between VI and low-intensity cycling with BFR. Another possible limitation was the fitness status of our subjects. The subjects in our study were recreationally active although they had not engaged in a regular resistance or endurance training program during the previous 6 months. If sedentary subjects had been used; perhaps, significant improvements would have been observed after the training period. Moreover, muscle hypertrophy in each region of a muscle may not be homogeneous; thus, proximal, central, or distal regions of each muscle have different rate of muscle hypertrophy after training (⁵); therefore, using multiple sites to assess mCSA may be required for better interpretation of muscle hypertrophy. In addition, leg lean mass assessed by DXA may be a better indicator for muscle hypertrophy because the leg lean mass contains the entire muscle mass in the upper and lower leg, but mCSA contains only 2-mm site of leg. All participants were instructed not to perform any other type of exercise during the training or detraining period. However, their diet and daily physical activity were not continuously monitored during the training and detraining period. Therefore, it is possible that some participants may not have followed these procedures, which could have influenced the % body fat, fat mass, BFLBM, and muscle strength results.

In conclusion, low-intensity cycling with BFR did not have better responses in BFLBM, knee extension muscle strength, V[Combining Dot Above]O₂peak, and fat mass compared with the VI and CON groups. However, the knee flexion muscle strength change for the LI-BFR group was similar to the change for the VI group. Moreover, the leg lean mass change assessed by DXA for the LI-BFR group was significantly increased but in the VI group had no change despite a lower intensity and lesser volume of exercise being performed by the LI-BFR group. The responses in the LI-BFR group showed a tendency to mimic the responses seen in the HI group after a 6-week training period, and these gains seem to be maintained during 3 weeks of detraining. Thus, LI-BFR cycling may be used to initiate similar changes in muscle size and strength as high-intensity cycling.

Practical Applications

It is not advisable for various types of populations to perform high-intensity exercise for muscle hypertrophy and strength gains due to the high mechanical stress as placed on joints and possible cardiac problems, but LI-BFR exercise has been shown results in similar muscular responses as high-load resistance exercise. In addition, cycle training is safe for most populations because of lower mechanical stress as placed on joints. According to the findings of this study, cycle training with BFR may improve leg muscle mass and strength when the appropriate length of the training program is used. Also, this type of exercise may be useful for elderly to improve muscle mass and strength and/or athletes who are in rehabilitation to maintain or improve their performance.

Acknowledgments

The authors thank all participants in this study for their time and participation. The authors also appreciate their colleagues for their assistance and help with this research project.

No conflicts of interest, financial or otherwise, are declared by the authors.