The American College of Sports Medicine recommends training at intensities of at least 70% of one's 1 repetition maximum (1RM) to receive increases in skeletal muscle hypertrophy (3). Recently, training at low intensities (20% 1RM) with moderate vascular occlusion has resulted in muscle hypertrophy (25). Such results make moderate vascular occlusion a promising technique to a variety of populations, particularly those who cannot withstand high mechanical stress on the joints. Moderate vascular occlusion has resulted in increases in muscle cross-sectional area (1,2,14,19-21,25), muscular strength (1,2,5,14,15,19-21), and muscular endurance (9,20) without significant elevations of any known markers for muscle damage (myoglobin, lipid peroxide, creatine kinase) (1,20).
Blood-flow occlusion training is postulated to exert beneficial adaptation through a variety of mechanisms (13). One mechanism is through an accumulation of metabolites such as lactate. Whole-blood lactate (WBL) (7,17), plasma lactate (6,13,15), and muscle cell lactate (10,11) have all been shown to significantly increase over control in response to exercise with blood-flow restriction. Lactate accumulation is significant, because growth hormone (GH) has been shown to be stimulated by an acidic intramuscular environment (18). Victor and Seals (22) indicated that a low pH stimulated sympathetic nerve activity through a chemoreceptive reflex mediated by intramuscular metaboreceptors and group III and IV afferent fibers (22). Consequently, this same pathway has been shown to play an important role in the regulation of hypophyseal secretion of GH (8,22).
The occlusive stimulus is typically produced by a KAATSU Master Apparatus or modified blood pressure cuffs, which makes this mode of training available to few because these devices are expensive and require a high level of skill to operate (12). These specialized apparatuses tend to be expensive; thus, a need exists for the development of a practical mode of applying an occlusive stimulus in the hope that more of the population can receive the positive adaptations associated with such training.
It has been postulated previously that elastic knee wraps, when applied to the proximal portion of the target muscle, might elicit a stimulus similar to that seen with the KAATSU Master Apparatus (12). Elastic knee wraps are inexpensive and easy to apply; thus, a pilot study was conducted with the aim of investigating whether or not intermittently applying knee wraps provide an adequate occlusive stimulus to significantly increase WBL accumulation over a control condition. Because of the novelty of using elastic knee wraps as a mode to restrict blood flow, the study erred on the side of caution, with knee wraps intermittently applied and rest intervals set at 150 seconds between sets.
Experimental Approach to the Problem
To examine the effects of intermittent vascular occlusion with elastic knee wraps on the knee extensors, the primary objective was to compare the WBL responses to 4 sets of low-intensity bilateral leg extension exercise (30% 1RM) with and without an occlusive stimulus. Heart rate and RPE were also investigated. Elastic knee wraps were used because they were easy to obtain, affordable, and practical.
Twelve, recreationally active, healthy persons with no known symptoms of impaired endothelial function or known risk factors for cardiovascular or metabolic diseases took part in this study. Subjects were recruited by word of mouth across campus. Their strength levels were considered relatively steady state, and no dramatic changes in their strength could be expected during the time course of the study (3 weeks). Subjects were permitted to train their quadriceps as usual up to 48 hours before each testing trial. Alcohol was restricted 24 hours before each test, and caffeine was restricted 12 hours before each test. Subjects were informed about the procedures and potential risks of the tests before their informed consent was obtained. The institutional review board at Southeast Missouri State University approved the study.
The subjects were tested for their bilateral knee extensor strength using a selectorized leg extension machine (FL-100, Flex Fitness Incorporated, Murrieta, CA, USA). Before testing, subjects performed low-intensity aerobic exercise on a treadmill, walking at 2.5 mph for 10 minutes to warm up their leg musculature. After treadmill walking, subjects were instructed to perform 8-10 repetitions. After a rest period of 90 seconds, weight was increased, and subjects were instructed to perform 4-6 repetitions. After a rest of 90 seconds, weight was increased, and subjects were instructed to perform 1 repetition. Weight was progressively increased until 1RM was determined. All 1RMs were achieved within 5 attempts. One repetition maximum was defined as the maximum weight that could be lifted through a controlled, full range of motion (ROM) with approximately a 1 second concentric and 1 second eccentric phase. Full ROM was visually defined as completing a repetition from the starting angle of 90° to a full lockout of 180°.
Occlusion and Control Testing Procedures
Testing trials were separated by at least 6 days and no more than 8 days. The exercise protocol required subjects to perform bilateral leg extensions at ∼30% of their 1RM for 4 sets, with a goal set at 30 repetitions for the first set followed by 3 sets of 15 repetitions with 150 seconds of rest between all sets. A repetition was counted if the movement was completed through a controlled, full range of motion with approximately a 1 second concentric and 1 second eccentric phase. The OCC trial was completed first, so subjects who were unable to meet the repetitions for a particular set, were matched for total repetitions completed for that particular set in the CON trial. For the OCC trial, 76-mm-wide knee wraps (Red-Line, Harbinger, Napa, CA, USA) were applied to the upper thigh, as depicted in a review article by Loenneke and Pujol (12). The same investigator applied the elastic knee wraps in each trial to maximize intrarater reliability. Upon completion of each set, knee wraps were removed immediately and reapplied before the next set. Control trial methodology was the same as in the OCC trial, except that knee wraps were not applied.
Lactate, Heart Rate, and Ratings of Perceived Exertion
Whole-blood lactate was measured using a handheld analyzer (Lactate Plus, Nova Biomedical Corporation, Waltham, MA, USA). Fingertip WBL samples (ca. 0.7 μl by volume) were collected before the start of the exercise bout, immediately after each set, and 3 minutes into recovery using the manufacturer guidelines for testing. The subjects' fingers were cleaned with alcohol solution before testing. Fingertips were punctured with a lancet, and the first drop of blood was wiped off to decrease the chance of contamination. The finger was lightly squeezed to obtain a second drop of blood, and when the drop appeared, the end of the test strip was touched to the blood drop until the test strip was filled. Heart rate was monitored and transmitted to an attachment worn on the wrist (FS1, Polar Electro, Lake Success, NY, USA). Heart rate was measured before the start of the exercise bout, immediately after each set, and 3 minutes into recovery. Ratings of perceived exertion were determined after each set of exercise using the standard Borg scale (4).
All values are expressed as mean ± SE. Whole-blood lactate, HR, and RPE data were analyzed using repeated-measures analysis of variance to determine significant differences between OCC and CON. When significance was found, Fisher's least significant difference post hoc test was used to determine pairwise differences. Significance was set at p ≤ 0.05.
Subjects' characteristics were age 21.2 ± 0.35 years, height 168.9 ± 2.60 cm, and body mass 71.2 ± 4.16 kg. Table 1 presents mean WBL, HR, and RPE values for each set of exercise. Whole-blood lactate, HR, and RPE significantly increased with exercise regardless of the training condition (p = 0.001). There were no significant WBL differences between OCC and CON (n = 10), although there was a nonsignificant trend for WBL to be higher with OCC (6.28 ± 0.66 vs. 5.35 ± 0.36 mmol·L−1; p = 0.051). Whole-blood lactate for 2 subjects were excluded from analysis because of incomplete data at 2 time points. Heart rate levels were significantly different between OCC and CON (n = 11) for sets 2-4 with no difference 3 minutes postexercise (p = 0.29). Mean HR was 128.86 ± 4.37 b·min−1 for OCC and 119.72 ± 4.10 b·min−1 for CON (p = 0.02). Heart rate data for 1 subject were incomplete; thus, that subject's HR was excluded from final analysis. Ratings of perceived exertion levels were significantly higher with OCC compared to CON after every set of leg extension (OCC 15.10 ± 0.31 vs. CON 12.16 ± 0.50, p = 0.001).
The current study was the first to investigate the effects of knee wraps as a practical mode of producing blood-flow occlusion. This pilot study demonstrated that knee wraps did not produce differences in WBL between OCC and CON but that there were significant increases in HR and RPE with practical occlusion.
This study found no difference in WBL between OCC and CON; although previous research has demonstrated that lactate is increased with blood-flow restriction (7,16-18,20), providing 1 mechanism by which GH is increased (8,22). Reasons for the disparity may include the small sample size, reperfusion in between sets, and the length of rest periods. Prior research used 30- to 60-second rest intervals (12), whereas this study used 150 seconds. The occlusion stimulus in past studies was applied and maintained throughout the exercise bout, whereas our investigation used intermittent occlusion. Intermittent occlusion with 150-second rest intervals likely allowed glycolytic fibers to fully recover between sets, making a rapid recruitment of fast twitch fibers unnecessary. Another possibility is occluding blood flow may result in a slower diffusion of lactate out of the muscle tissue resulting in a more pronounced intramuscular acidic environment and therefore a greater local stimulation of group IV afferents before its diffusion out of the cell (13). To illustrate, Reeves et al. (16) showed that although occlusion training resulted in a greater GH response than a nonoccluded control, there were no significant differences in blood-lactate concentrations between groups.
Heart rate and RPE were significantly increased over CON with practical OCC. Elevated HR has been shown previously to occur with occlusion training, attributable to decreased venous return (17). The current data indicate the subject's perceived exertion was greater with OCC, which is in disagreement with other findings that found no differences between training conditions (23,24). The discrepancy is attributable to differing protocols because Wernbom et al. (23,24) did not control for volume of work between groups. The goal of their study was to examine endurance capacity; thus, both groups were performing all-out sets. Ratings of perceived exertion indicate that subjects perceive exercise with practical occlusion to be of greater intensity than CON, despite the same external load and volume of work.
The repetition protocol was modeled off previous investigations using moderate blood-flow restriction, where 1 set of 30 repetitions was completed followed by 3 more sets of 15 repetitions (30-second rest between sets) (1). However, because of the novelty of knee wraps as an occlusion stimulus, the protocol for this study was tailored to err on the side of caution; thus, knee wraps were intermittently applied and rest intervals were set at 150 seconds between sets.
With this being the first investigation using knee wraps as a means of practical occlusion, limitations exist. One limitation was the small sample size; however, with this being preliminary research, the sample was adequate to allow for a greater understanding of the acute response to practical occlusion, specifically with exercise prescription, because the current study demonstrated that subjects could tolerate blood-flow restriction from knee wraps. In addition, although metabolic stress has been implicated to cause the release of favorable hypertrophic hormones, the only blood measurement taken in this study was WBL. Future investigations should measure the GH response to practical occlusion.
In conclusion, this pilot study demonstrated that there are no significant differences for WBL between OCC and CON, although there was a trend for higher levels with OCC (p = 0.051). However, this study did find significant increases for HR and RPE from an acute practical occlusion stimulus with knee wraps. The current study also found that subjects were able to tolerate practical occlusion with knee wraps, and future investigations should examine shortened rest periods, and a continuous occlusive stimulus throughout the exercise bout. The need for an affordable, effective means to receive the benefits of occlusion training exists, thus the need for further research in the field of practical occlusion.
Previous studies have found that the use of blood-flow occlusion will stimulate muscle hypertrophy (1,2,14,19-21,25). One mechanism for the stimulus of increased protein synthesis is a decrease in pH as a result of lactic acid production. In this study, low-intensity exercise with intermittent occlusion using elastic knee wraps did not increase metabolic stress more than CON. The reperfusion of the muscles between sets may moderate the effects of the occlusive stimulus and therefore not create adequate stimuli for protein synthesis.
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