A narrative review of the effects of blood flow restriction on vascular structure and function

Blood flow restriction is growing in popularity as a tool for increasing muscular size and strength. Currently, guidelines exist for using blood flow restriction alone and in combination with endurance and resistance exercise. However, only about 1.3% of practitioners familiar with blood flow restriction applications have utilized it for vascular changes, suggesting many of the guidelines are based on skeletal muscle outcomes. Thus, this narrative review is intended to explore the literature available in which blood flow restriction, or a similar application, assess the changes in vascular structure or function. Based on the literature, there is a knowledge gap in how applying blood flow restriction with relative pressures may alter the vasculature when applied alone, with endurance exercise, and with resistance exercise. In many instances, the application of blood flow restriction was not in accordance with the current guidelines, making it difficult to draw definitive conclusions as to how the vascular system would be affected. Additionally, several studies report no change in vascular structure or function, but few studies look at variables for both outcomes. By examining outcomes for both structure and function, investigators would be able to generate recommendations for the use of blood flow restriction to improve vascular structure and/or function in the future.

A comparison of variability between absolute and relative blood flow restriction pressures

Recommendations are that blood flow restriction (BFR) be applied relative to arterial occlusion pressure (AOP) to provide a similar stimulus.

PURPOSE

Compare variability of the change in blood flow, shear rate, and discomfort between recommended relative pressures and an absolute pressure.

METHODS

During one visit, brachial arterial blood flow was measured in 91 participants using pulse-wave Doppler ultrasonography. After 5-min seated rest, AOP was measured. Following another 5-min rest, blood flow and discomfort were assessed twice before cuff inflation as controls (C1 and C2), then again with a cuff inflated to each BFR pressure (all measures separated by 1-min). Change scores from C1 to all subsequent measures were calculated (i.e., C2-C1; 40%AOP-C1; 80%AOP-C1; 100mmHg-C1). Variability of the changes were compared via pairwise modified Pitman-Morgan tests (α=.008).

RESULTS

Variance (95%CI) of the change for blood flow (mL/min), shear rate (1/sec), and discomfort (AU) had similar trends. C2-C1 differed from all conditions (all p<.001), 40%AOP-C1 differed from 80%AOP-C1 and 100mmHg-C1 (all p<.001), which did not differ (both p≥.117). Blood flow: C2-C1=469.79 (357.90, 644.07), 40%AOP-C1=1263.18 (962.34, 1731.80), 80%AOP-C1=1752.90 (1335.42, 2403.18), 100mmHg-C1=1603.18 (1221.36, 2197.92); Shear rate: C2-C1=6248.24 (4760.10, 8566.15), 40%AOP-C1=14625.30 (11142.06, 20050.95), 80%AOP-C1=22064.02 (16809.13, 30249.27), 100mmHg-C1=20778.76 (15829.98, 28487.21); Discomfort: C2-C1=0.07 (0.05, 0.08), 40%AOP-C1=2.03 (1.55, 2.78), 80%AOP-C1=4.26 (3.25, 5.84), 100mmHg-C1=4.50 (3.43, 6.17).

CONCLUSION

Contrary to previous suggestions, applying relative pressures does not necessarily guarantee a similar stimulus. It seems that higher pressures produce more variable changes even if the external pressure applied is made relative to each individual. This article is protected by copyright. All rights reserved.

The Basics of Training for Muscle Size and Strength: A Brief Review on the Theory

The periodization of resistance exercise is often touted as the most effective strategy for optimizing muscle size and strength adaptations. This narrative persists despite a lack of experimental evidence to demonstrate its superiority. In addition, the general adaptation syndrome, which provides the theoretical framework underlying periodization, does not appear to provide a strong physiological rationale that periodization is necessary. Hans Selye conducted a series of rodent studies which used toxic stressors to facilitate the development of the general adaptation syndrome. To our knowledge, normal exercise in humans has never been shown to produce a general adaptation syndrome. We question whether there is any physiological rationale that a periodized training approach would facilitate greater adaptations compared with nonperiodized approaches employing progressive overload. The purpose of this article is to briefly review currently debated topics within strength and conditioning and provide some practical insight regarding the implications these reevaluations of the literature may have for resistance exercise and periodization. In addition, we provide some suggestions for the continued advancement within the field of strength and conditioning.

The Impact of Ultrasound Probe Tilt on Muscle Thickness and Echo-Intensity: A Cross-Sectional Study

INTRODUCTION/BACKGROUND

To determine the influence of ultrasound probe tilt on reliability and overall changes in muscle thickness and echo-intensity.

MATERIALS AND METHODS

Thirty-six individuals had a total of 15 images taken on both the biceps brachii and tibialis anterior muscles. These images were taken in 2° increments with the probe tilted either upward (U) or downward (D) from perpendicular (0°) to the muscle (U6°, U4°, U2°, 0°, D2°, D4°, and D6°). All images were then saved, stored, and analyzed using Image-J software for echo-intensity and muscle thickness measures. Mean values (2-3 measurements within each probe angle) were compared across each probe angle, and reliability was assessed as if the first measure was taken perpendicular to the muscle, but the second measure was taken with the probe tilted to a different angle (to assume unintentional adjustments in reliability from probe tilt).

RESULTS

Tilting the probe as little as 2° produced a significant 4.7%, and 10.5% decrease in echo-intensity of the tibialis anterior and biceps brachii muscles, respectively, while changes in muscle thickness were negligible (<1%) at all probe angles. The reliability for muscle thickness was greater than that of echo-intensity when the probe was held perpendicular at both measurements (∼1% vs 3%), and the impact that probe tilt had on reliability was exacerbated for echo-intensity measurements (max coefficient of variation: 24.5%) compared to muscle thickness (max coefficient of variation: 1.5%).

CONCLUSION

While muscle thickness is less sensitive to ultrasound probe tilt, caution should be taken to ensure minimal probe tilt is present when taking echo-intensity measurements as this will alter mean values and reduce reliability. Echo-intensity values should be interpreted cautiously, particularly when comparing values across technicians/studies where greater alterations in probe tilt is likely.

Limb Occlusion Pressure: A Method to Assess Changes in Systolic Blood Pressure

Although often used as a surrogate, comparisons between traditional blood pressure measurements and limb occlusion assessed via hand-held Doppler have yet to be completed. Using limb occlusion pressure as a method of assessing systolic pressure is of interest to those studying the acute effects of blood flow restriction, where the removal of the cuff may alter the physiological response.

PURPOSE

We sought to determine how changes in limb occlusion pressure track with changes in traditional assessments of blood pressure.

BASIC PROCEDURES

Limb occlusion pressure measured by hand-held Doppler and blood pressure measured by an automatic blood pressure cuff were assessed at rest and following isometric knee extension (post and 5 minutes post).

MAIN FINDINGS

Each individual had a similar dispersion from the mean value for both the limb occlusion pressure measurement and traditional systolic blood pressure measurement [BF10: 0.33; median (95% credible interval): 0.02 (-6.0, 5.9) %]. In response to lower body isometric exercise, blood pressure changed across time. The difference between measurements was small at immediately post and 5 minutes post. The Bayes factors were in the direction of the null but did not exceed the threshold needed to accept the null hypothesis. However, at 5 minutes post, the differences were within the range of practical equivalence (within ± 4.6%).

PRINCIPAL CONCLUSIONS

Our findings suggest that changes in limb occlusion pressure measured by hand-held Doppler track similarly to traditional measurements of brachial systolic blood pressure following isometric knee extension exercise.

Assessing differential responders and mean changes in muscle size, strength, and the crossover effect to 2 distinct resistance training protocols

The objective of this study was to determine differences in 2 distinct resistance training protocols and if true variability can be detected after accounting for random error. Individuals (n = 151) were randomly assigned to 1 of 3 groups: (i) a traditional exercise group performing 4 sets to failure; (ii) a group performing a 1-repetition maximum (1RM) test; and (iii) a time-matched nonexercise control group. Both exercise groups performed 18 sessions of elbow flexion exercise over 6 weeks. While both training groups increased 1RM strength similarly (∼2.4 kg), true variability was only present in the traditional exercise group (true variability = 1.80 kg). Only the 1RM group increased untrained arm 1RM strength (1.5 kg), while only the traditional group increased ultrasound measured muscle thickness (∼0.23 cm). Despite these mean increases, no true variability was present for untrained arm strength or muscle hypertrophy in either training group. In conclusion, these findings demonstrate the importance of taking into consideration the magnitude of random error when classifying differential responders, as many studies may be classifying high and low responders as those who have the greatest amount of random error present. Additionally, our mean results demonstrate that strength is largely driven by task specificity, and the crossover effect of strength may be load dependent. Novelty Many studies examining differential responders to exercise do not account for random error. True variability was present in 1RM strength gains, but the variability in muscle hypertrophy and isokinetic strength changes could not be distinguished from random error. The crossover effect of strength may differ based on the protocol employed.

Blood flow restriction augments the skeletal muscle response during very low-load resistance exercise to volitional failure

The purpose of this study was to compare the acute muscular response with resistance exercise between the following conditions [labeled (% one-repetition maximum/% arterial occlusion pressure)]: high-load (70/0), very low-load (15/0), very low-load with moderate (15/40), and high (15/80) blood flow restriction pressures. Twenty-three participants completed four sets of unilateral knee extension to failure (up to 90 repetitions) with each condition, one condition per leg, each day. Muscle thickness and maximal voluntary contraction (MVC) were measured before (Pre), immediately after (Post-0), and 15 min after (Post-15) exercise and electromyography (EMG) amplitude during exercise. Pre to Post-0 muscle thickness changes in cm [95% CI] were greater with 15/40 [0.57 (0.41, 0.73)] and 15/80 [0.49 (0.35, 0.62)] compared to 70/0 [0.33 (0.25, 0.40)]. Pre to Post-0 MVC changes in Nm [95% CI] were higher with 15/40 [-127.0 (-162.1, -91.9)] and 15/80 [-133.6 (-162.8, -104.4)] compared to 70/0 [-48.4 (-70.1, -26.6)] and 15/0 [-98.4 (-121.9, -74.9)], which were also different. Over the first three repetitions, EMG increased across sets, whereas in the last three repetitions it did not. EMG was also different between conditions and was generally greater during 70/0. Repetitions decreased across sets reaching the lowest for 70/0, and for very low loads decreased with increased pressure. In trained participants exercising to failure, lower load and the application of restriction pressure augment changes in muscle thickness and torque. The EMG amplitude was augmented by load. Training studies should compare these conditions, as the results herein suggest some muscular adaptations may differ.

Can changes in echo intensity be used to detect the presence of acute muscle swelling?

OBJECTIVE

The purpose of this study was to examine acute changes in muscle thickness (MTH) and echo-intensity (EI), following four sets of biceps curls, when it is known that the change in MTH is due entirely to swelling.

APPROACH

Forty-nine resistance-trained men and women participated in this study. Individuals in the experimental group (n  =  23) visited the lab on two separate occasions. During the first visit, paperwork and one repetition maximum (1RM) strength were measured. During the second visit (3-5 d after visit 1), individuals performed four sets of biceps curls to failure using 70% of their 1RM. MTH and EI measurements were taken before and immediately following exercise using B-mode ultrasound. The ultrasound probe was equipped with a level to minimize the influence of probe tilt on the EI measurement. A time-matched control group (n  =  26) was included to account for measurement error for both MTH and EI. Results are presented as means (95% CI).

MAIN RESULTS

For MTH there was a group (Experimental versus Control) by time (Pre-Post) interaction (p   <  0.001). MTH increased in the experimental group (mean value change  =  0.44 (0.33-0.54) cm), but not in the control group (mean value change  =  -0.015 (-0.03-0.01) cm). For EI, there was no group by time interaction (p   =.074). In addition, there were no main effects for group (p   =  0.254) or time (p   =  0.314). The mean difference in the change in EI between groups was  -2.99 (-6.25-3.03) arbitrary units.

SIGNIFICANCE

EI cannot be used to detect exercise induced changes in muscle thickness.

Perceptual changes to progressive resistance training with and without blood flow restriction

The purpose was to examine changes in the perceptual responses to lifting a very low load (15% one repetition maximum (1RM)) with and without (15/0) different pressures [40% (15/40) and 80% (15/80) arterial occlusion pressure] and compare that to traditional high load (70/0) resistance exercise. Ratings of perceived exertion (RPE) and discomfort were measured following each set of exercise. In addition, resting arterial occlusion pressure was measured prior to exercise. Assessments were made in training sessions 1, 9, and 16 for the upper and lower body. Data are presented as means and 95% CI. There were changes in RPE in the upper body with condition 15/40 [-2.1 (-3.4, -0.850)] and 15/80 [-2.4 (-3.6, -1.1)] decreasing by the end of training. In the lower body, RPE decreased in condition 15/40 [-1.4 (-2.3, -0.431)] by the end of the training study. There was a main effect of time in the upper body with all conditions decreasing discomfort. In the lower body, all conditions decreased except for 15/80. For arterial occlusion pressure, there were differences across time in the 15/40 condition and the 15/80 condition in the upper body. Repeated exposure to blood flow restriction may dampen the perceptual responses over time.

A method to standardize the blood flow restriction pressure by an elastic cuff

Blood flow restriction training using a practical (non-pneumatic) elastic cuff has recently increased in popularity. However, a criticism of this method is that the pressure applied and the amount of blood flow restriction induced is unknown. The aim was to quantify blood flow following the application of an elastic cuff and compare that to what is observed using a more traditional pressurized nylon cuff. Thirty-five young participants (16 men and 19 women) visited the laboratory once for testing. In a randomized order (one condition per arm), an elastic cuff (5 cm wide) was applied to one arm and blood flow was measured following the cuff being pulled to two distinct lengths; 10% and 20% of the resting length based on arm circumference. The other arm would follow a similar protocol but use a pressurized nylon cuff (5 cm wide) and be inflated to 40% and 80% of the individuals resting arterial occlusion pressure. There was a main effect of pressure for blood flow with it decreasing in a pressure-dependent manner (High < Low, P < 0.001). The mean difference (95% CI) in blood flow between cuffs was -5.9 (-18.9, 7.0) % for the lower pressure and -4.0 (-13.2, 5.1) % for the higher pressure. When the relative changes for each cuff were separated by sex, there were no differences in the changes from Pre (P ≥ 0.509). The application of a pressure relative to the initial belt length, which is largely dependent upon arm circumference, appears to provide one method to standardize the practical blood flow restriction pressure for future research.

Acute skeletal muscle responses to very low-load resistance exercise with and without the application of blood flow restriction in the upper body

The purpose was to examine the acute skeletal muscle response to high load exercise and low-load exercise with and without different levels of applied pressure (BFR). A total of 22 participants completed the following four conditions: elbow flexion exercise to failure using a traditional high load [70% 1RM, (7000)], low load [15% 1RM,(1500)], low load with moderate BFR [15%1RM+40%BFR(1540)] or low load with greater BFR [15% 1RM+80%BFR(1580)]. Torque and muscle thickness were measured prior to, immediately post, and 15 min postexercise. Muscle electromyography (EMG) amplitude was measured throughout. Immediately following exercise, the 7000 condition had lower muscle thickness [4·2(1·0)cm] compared to the 1500 [4·4 (1·1)cm], 1540 [4·4(1·1)cm] and 1580 [4·5(1·0)cm] conditions. This continued 15 min post. Immediately following exercise, torque was lower in the 1500 [31·8 (20) Nm], 1540 [28·3(16·9) Nm, P<0·001] and 1580 [29·5 (17) Nm] conditions compared to the 7000 condition [40 (19) Nm]. Fifteen minutes post, 1500 and 1540 conditions demonstrated lower torque compared to the 7000 condition. For the last three repetitions percentage EMG was greater in the 7000 compared to the 1580 condition. Very low-load exercise (with or without BFR) appears to result in greater acute muscle swelling and greater muscular fatigue compared to high load exercise.

Effects of load on the acute response of muscles proximal and distal to blood flow restriction

To determine the effects of load and blood flow restriction (BFR) on muscular responses, we asked 12 participants to perform chest presses under four different conditions [30/0, 30/40, 50/0, and 50/40, presented as percentage one-repetition maximum (1RM)/percentage arterial occlusion pressure (AOP)]. Muscle thickness increased pre- to post-exercise [chest: mean 0.29, 95% confidence interval (CI) 0.21, 0.37 cm; triceps: mean 0.44, 95% CI 0.34, 0.54 cm], remaining elevated for 15 min post-exercise. Electromyography amplitude was greater with 50% 1RM and increased over time for the first three repetitions of each set of chest presses. The last three repetitions differed across time only. AOP increased from pre- to post-exercise, augmented by BFR [30/0: mean 31, 95% CI 18, 44 mmHg; 30/40: mean 39, 95% CI 28, 50 mmHg; 50/0: mean 32, 95% CI 23, 41 mmHg; 50/40: mean 46, 95% CI 32, 59 mmHg). Tranquility decreased and physical exhaustion increased from the pre- to post-condition, with both parameters returning to the baseline 15 min post-exercise level. In conclusion, load and BFR do not elicit meaningful differences in the acute response of chest press exercise taken to failure.

An investigation into setting the blood flow restriction pressure based on perception of tightness

OBJECTIVE

To determine whether the perceived tightness scale could be used to set sub-occlusive blood flow restriction pressures. A secondary aim was to determine variables that may impact individual ratings.

APPROACH

One hundred and twenty participants completed three separate conditions in one limb within the upper and lower body. Participants were asked to rate their perceived tightness for two of the three conditions, regarded as moderate pressure without pain (7/10) and intense pressure with pain (10/10). A third condition, arterial occlusion pressure, was completed that required no rating from participants. Order of conditions and limb assignment were randomized for each participant. Measurements for muscle and fat thickness along with limb circumference were completed on the tested limbs.

MAIN RESULTS

Order of conditions did not affect results in the upper or lower body. A condition effect was found for the upper body with the 7/10 rating lower than the arterial occlusion pressure [7/10: 132 (38) mmHg  <  Arterial Occlusion: 162 (24) mmHg  <  10/10: 202 (46) mmHg]. A condition effect was also found for the lower body with 7/10 condition [120 (33) mmHg] rating lower than arterial occlusion pressure [171 (28) mmHg] and 10/10 condition [178 (49) mmHg]. However, there was a non-significant difference between the arterial occlusion pressure and the 10/10 condition (difference of 7(-3, 18) mmHg, (P  =  0.159).

SIGNIFICANCE

Participants appear adept in their ability to rate sub-occlusive pressure based upon perceived tightness. Findings from this study provide some support for the utility of this method as a means for completion of practical blood flow restriction, whereby individuals tighten the cuff based upon their relative perceptual response.

Muscle Adaptations to High-Load Training and Very Low-Load Training With and Without Blood Flow Restriction

An inability to lift loads great enough to disrupt muscular blood flow may impair the ability to fatigue muscles, compromising the hypertrophic response. It is unknown what level of blood flow restriction (BFR) pressure, if any, is necessary to reach failure at very low-loads [i.e., 15% one-repetition maximum (1RM)]. The purpose of this study was to investigate muscular adaptations following resistance training with a very low-load alone (15/0), with moderate BFR (15/40), or with high BFR (15/80), and compare them to traditional high-load (70/0) resistance training. Using a within/between subject design, healthy young participants (n = 40) performed four sets of unilateral knee extension to failure (up to 90 repetitions/set), twice per week for 8 weeks. Data presented as mean change (95% CI). There was a condition by time interaction for 1RM (p < 0.001), which increased for 70/0 [3.15 (2.04,4.25) kg] only. A condition by time interaction (p = 0.028) revealed greater changes in endurance for 15/80 [6 (4,8) repetitions] compared to 15/0 [4 (2,6) repetitions] and 70/0 [4 (2,5) repetitions]. There was a main effect of time for isometric MVC [change = 10.51 (3.87,17.16) Nm, p = 0.002] and isokinetic MVC at 180^°/s [change = 8.61 (5.54,11.68) Nm, p < 0.001], however there was no change in isokinetic MVC at 60^°/s [2.45 (−1.84,6.74) Nm, p = 0.261]. Anterior and lateral muscle thickness was assessed at 30, 40, 50, and 60% of the upper leg. There was no condition by time interaction for muscle thickness sites (all p ≥ 0.313). There was a main effect of time for all sites, with increases over time (all p < 0.001). With the exception of the 30% lateral site (p = 0.059) there was also a main effect of condition (all p < 0.001). Generally, 70/0 was greater. Average weekly volume increased for all conditions across the 8 weeks, and was greatest for 70/0 followed by 15/0, 15/40, then 15/80. With the exception of 1RM, changes in strength and muscle size were similar regardless of load or restriction. The workload required to elicit these changes lowered with increased BFR pressure. These findings may be pertinent to rehabilitative settings, future research, and program design.

Magnetic resonance imaging-measured skeletal muscle mass to fat-free mass ratio increases with increasing levels of fat-free mass

BACKGROUND

To investigate the skeletal muscle mass to fat-free mass (SM-FFM) ratio in female and male athletes, as well as to examine the relationship between ultrasound predicted SM and magnetic resonance imaging (MRI)-measured SM.

METHODS

Seven female track and field athletes (female), 8 male collegiate swimmers (male-G1) and 8 male collegiate Olympic weightlifters (male-G2) volunteered. Whole-body SM volume was measured using MRI images obtained from the first cervical vertebra to the ankle joints. The volume of SM tissue was calculated and the SM volume was converted into mass units by an assumed skeletal muscle density. Muscle thickness was measured using ultrasound at nine sites and SM was estimated using an ultrasound-derived prediction equation.

RESULTS

Percent body fat was similar among the groups. FFM, MRI-measured SM and SM-FFM ratio were greater in Males-G2 compared to the other two groups and those variables of Male-G1 were higher than the Female group. There was an excellent correlation (r=0.976) between MRI-measured and ultrasound-predicted SM (total error=1.52 kg). No significant difference was observed between MRI-measured and ultrasound-predicted SM in the overall sample or within each group. The SM-FFM ratio was positively correlated (r=0.708) with FFM in female and male athletes.

CONCLUSIONS

We provide evidence for how the MRI-measured SM-FFM ratio changes with increasing levels of FFM and provide data that the ultrasound may be useful in estimating SM in athletes. Given the size limitations with MRI, both of these findings may be useful for future research investigating large sized athletes.

Very-low-load resistance exercise in the upper body with and without blood flow restriction: cardiovascular outcomes

It is proposed that, at very low loads, greater blood flow restriction (BFR) pressures might be required for muscular adaptation to occur. The cardiovascular and hyperemic response to very low loads combined with relative levels of BFR is unknown. Ninety-seven participants were recruited and assigned to 1 of 4 exercise conditions: 15% of 1-repetition maximum (1RM) without BFR (15/00), 15% 1RM with BFR at 40% of arterial occlusion pressure (AOP) (15/40), 15% of 1RM with BFR at 80% of AOP (15/80), and 70% of 1RM without BFR (70/00). Participants performed 4 sets of unilateral biceps curls. Blood pressure was measured before and after exercise; brachial artery blood flow was measured before exercise, following the second set, and 1 min following exercise. Systolic blood pressure increased following exercise in all conditions (+10 (11) mm Hg, P < 0.0005). Diastolic pressure increased in all but 70/00 (+2 (11) mm Hg, P = 0.107). Brachial artery blood flow increased following the second set of exercise in all but 15/80 (+43.4 (76.8) mL·min^-1, P = 0.348). One minute following exercise and cuff deflation, there were no differences in blood flow between conditions (P > 0.05). Similarly, artery diameter was increased in all conditions except 15/80 (+0.002 (0.041) cm, P = 0.853) following the second set, and increased in all conditions by 1 min following exercise (P < 0.05). In conclusion, exercise-induced hyperemia is blunted with increasing pressures of BFR. There is a modest increase in blood pressure at very low loads of resistance exercise in the upper body.

Perceptual and arterial occlusion responses to very low load blood flow restricted exercise performed to volitional failure

PURPOSE

Studies examining perceptual and arterial occlusion responses between blood flow restricted exercise and high load exercise often prescribe an arbitrary number of repetitions, making it difficult for direct comparisons. Therefore, the purpose of this study was to compare these protocols when performed to volitional failure.

METHODS

Individuals completed four exercise conditions varying in load and pressure: (i) 15% 1RM; no restrictive pressure, (ii) 15% 1RM; 40% arterial occlusion pressure, (iii) 15% 1RM; 80% arterial occlusion pressure, and (iv) 70% 1RM; no pressure. Four sets of knee extension exercises were performed until volitional failure (or until 90 repetitions per set) was completed.

RESULTS

A total of 23 individuals completed the study. While all conditions increased arterial occlusion pressure, the greatest increases (~30%) were observed in the blood flow restriction conditions. All lower load conditions resulted in greater RPE and discomfort than that of the high load condition, but only discomfort was increased further when adding blood flow restriction.

CONCLUSION

High load exercise will likely be perceived more favourably than lower load exercise to volitional failure; however, those who are incapable or unwilling to lift heavier loads may use blood flow restriction to help reduce the volume needed to reach volitional failure, although this will likely increase discomfort.

Determining Strength: A Case for Multiple Methods of Measurement

Muscle strength is often measured through the performance of a one-repetition maximum (1RM). However, we that feel a true measurement of 'strength' remains elusive. For example, low-load alternatives to traditional resistance training result in muscle hypertrophic changes similar to those resulting from traditional high-load resistance training, with less robust changes observed with maximal strength measured by the 1RM. However, when strength is measured using a test to which both groups are 'naive', differences in strength become less apparent. We suggest that the 1RM is a specific skill, which will improve most when training incorporates its practice or when a lift is completed at a near-maximal load. Thus, if we only recognize increases in the 1RM as indicative of strength, we will overlook many effective and diverse alternatives to traditional high-load resistance training. We wish to suggest that multiple measurements of strength assessment be utilized in order to capture a more complete picture of the adaptation to resistance training.

The affective and behavioral responses to repeated "strength snacks"

Background A training program consisting of only one-repetition maximum (1RM) training results in similar strength adaptations as traditional resistance exercise. However, little is known regarding the affective or behavioral responses to this type of training. Aim To examine the affective and behavioral response to either a traditional resistance exercise program or a biweekly 1RM-training program. Methods Participants were trained for 8 weeks (2× per week). The HYPER group completed four sets of 8-12 repetitions; the 1RM group (TEST) worked up to a single maximal repetition. Results The TEST group felt more revitalized and had an increase in positive engagement during their first visit, whereas the HYPER group showed an increase in feelings of physical exhaustion during their first and last visits. There were no pre to post differences for the change in behavior or self-efficacy between groups. Conclusion 1RM training appears to elicit a more favorable affective response, compared with HYPER training, which may ultimately improve adherence to resistance-type exercise.

Skeletal muscle mass in human athletes: What is the upper limit?

OBJECTIVES

To examine the amount of absolute and relative skeletal muscle mass (SM) in large sized athletes to investigate the potential upper limit of whole body muscle mass accumulation in the human body.

METHODS

Ninety-five large-sized male athletes and 48 recreationally active males (control) had muscle thickness measured by ultrasound at nine sites on the anterior and posterior aspects of the body. SM was estimated from an ultrasound-derived prediction equation. Body density was estimated by hydrostatic weighing technique, and then body fat percentage and fat-free mass (FFM) were calculated. We used the SM index and FFM index to adjust for the influence of standing height (ie, divided by height squared).

RESULTS

Ten of the athletes had more than 100 kg of FFM, including the largest who had 120.2 kg, while seven of the athletes had more than 50 kg of SM, including the largest who had 59.3 kg. FFM index and SM index were higher in athletes compared to controls and the percentage differences between the two groups were 44% and 56%, respectively. The FFM index increased linearly up to 90 kg of body mass, and then the values leveled off in those of increasing body mass. Similarly, the SM index increased in a parabolic fashion reaching a plateau (approximately 17 kg/m² ) beyond 120 kg body mass.

CONCLUSIONS

SM index may be a valuable indicator for determining skeletal muscle mass in athletes. A SM index of approximately 17 kg/m² may serve as the potential upper limit in humans.

Blood flow restriction and cuff width: effect on blood flow in the legs

Much of the literature examining blood flow restriction in the lower body uses cuffs of differing widths. It is currently unknown whether similar relative pressures using cuffs of differing widths elicit the same blood flow response.

PURPOSE

To examine the hemodynamic responses to relative pressures using two commonly used cuffs (10 and 12 cm).

METHODS

In a random order over two laboratory visits, one cuff was applied to the right proximal thigh of the participant (men = 17, women = 14), and arterial occlusion pressure (AOP) was measured. Ultrasound measures of blood flow, mean blood velocity, peak blood velocity and artery diameter were taken from the posterior tibial artery at rest and during the application of 10% increments of the AOP.

RESULTS

There was no significant difference between the 10- and 12-cm cuff relating to blood flow (-0·501 ml min-1 , SD 7·9, P = 0·728), mean blood velocity (-0·168 cm s-1 , SD 1·7, P = 0·590), peak blood velocity (0·586 cm s-1 , SD 11·7, P = 0·783) or artery diameter (0·003 cm, SD 0·02, P = 0·476). There was a main effect of pressure for blood flow (P<0·0005), mean blood velocity (P<0·0005), peak blood velocity (P<0·0005) and artery diameter (P = 0·005), with each decreasing with increasing pressures. Peak blood velocity increased to 60% of AOP before decreasing with increased pressure.

CONCLUSION

As long as relative pressures are applied, cuff width appears to have little to no effect on the blood flow stimulus during blood flow restriction at rest.

Differences in 100-m sprint performance and skeletal muscle mass between elite male and female sprinters

BACKGROUND

The sex difference in 100-m sprint performance between the world's best athletes is approximately 10%. We hypothesized that skeletal muscle mass (SM) relative to body mass may be a major factor contributing to this difference. The aim of this study was to examine the sex difference in absolute and relative SM and sprint performance in male and female sprinters.

METHODS

We analyzed the SM of male (N.=37) and female (N.=26) 100-m sprinters; the sample was divided into two subgroups within each sex according to personal best 100-m time: 10.00-10.90 s (M10; N.=22) and 11.00-11.70 s (M11; N.=15) for males and 11.00-11.90 s (F11, N.=14) and 12.00-13.50 s (F12, N.=12) for females. SM was estimated from ultrasound-measured muscle thickness (MT) using prediction equations.

RESULTS

There was an approximate 10% difference in 100-m sprint time between sexes, whereas absolute and relative values of SM for female sprinters were 70-71% and 79-84% of the male sprinters, respectively. No differences were observed within each male/female subgroup for fat-free mass, absolute and relative SM, excepting that leg SM index of M10 was higher than M11. The 100-m time was not different (0.27 s, P=0.051) between M11 and F11 subgroups, but absolute and relative values of SM and MT were higher and percent body fat was lower in the M11 than in the F11 subgroup.

CONCLUSIONS

Our results suggest that differences in muscle mass may not play a large role in determining successful performance in elite male and female sprinters.

Blood flow in humans following low-load exercise with and without blood flow restriction

Blood flow restriction (BFR) in combination with exercise has been used to increase muscle size and strength using relatively low loads (20%–30% 1-repetition maximum (1RM)). In research, the range of applied pressures based on a percentage of arterial occlusion pressure (AOP), is wide. The purpose of the study is to measure the blood flow response before exercise, following each set of exercise, and postexercise to low-load elbow flexion combined with no restriction (NOBFR), 40% of AOP (40BFR), and 80% of AOP (80BFR). One hundred and fifty-two participants volunteered; 140 completed the protocol (women = 75, men = 65). Participants were counter-balanced into 1 of 3 conditions. Following AOP and 1RM measurement, ultrasound was used to measure standing blood flow at rest in the right brachial artery. Participants performed 4 sets of elbow flexion at 30% 1RM. Blood flow was measured between sets and at 1 and 5 min postexercise. Blood flow decreased following inflation, with no difference between conditions (p < 0.001). Men had greater blood flow than women in all conditions at all time points (p < 0.001). Resting hyperemia decreased with pressure (NOBFR > 40BFR > 80BFR, p < 0.001). Blood flow increased from rest to after set 1 regardless of condition. Following cuff deflation, blood flow increased in both the 80BFR and 40BFR conditions. The reduction in hyperemia during BFR is pressure-dependent. Contrary to previous investigations, blood flow was increased above baseline following exercise.

A critical review of the current evidence examining whether resistance training improves time trial performance

A number of reviews have concluded that resistance training is beneficial for improving sports performance despite the inclusion of studies which do not actually measure a performance outcome (i.e. a timed trial). The purpose of this review was to examine only those studies which would allow us to infer the benefits of resistance training on improving time trial performance. Of the nine studies meeting all inclusion criteria only three demonstrated an additive effect of adding resistance training to the current activity-specific training being performed. These three studies demonstrated improvements in either 5 or 10 km time trial among recreationally skilled athletes (i.e. non-elite level time). Previous reviews have included studies which did not include: (1) performance outcomes; (2) control groups; and/or (3) equal volumes of activity-specific exercise among the resistance training and control groups. Presently, there is little evidence that adding resistance exercise to a sport-specific training program will augment time trial performance. While it is difficult to perform such long-term studies assessing the effects of resistance training among time trial athletes, the statement that resistance training is efficacious for improving time trial performance should be tempered until sufficient evidence is presented to support such claims.

Are higher blood flow restriction pressures more beneficial when lower loads are used?

The application of blood flow restriction during low-load resistance exercise has been shown to induce muscle growth with high or low restriction pressures, however, loads lower than 20% one-repetition maximum (1RM) remain unexplored. Fourteen trained individuals completed six elbow flexion protocols involving three different loads (10%, 15%, and 20% 1RM) each of which was performed with either a low (40% arterial occlusion) or high (80% arterial occlusion) pressure. Pre- and post-measurements of surface electromyography (sEMG), isometric torque, and muscle thickness were analyzed. An interaction was present for torque (p < 0.001) and muscle thickness (p < 0.001) illustrating that all increases in pressure and/or load resulted in a greater fatigue and muscle thickness. There was no interaction for sEMG (p = 0.832); however, there were main effects of condition (p = 0.002) and time (p = 0.019) illustrating greater sEMG in the 20% 1RM conditions. Higher blood flow restriction pressures may be more beneficial for muscle growth when very low loads are used.

Do metabolites that are produced during resistance exercise enhance muscle hypertrophy?

Many reviews conclude that metabolites play an important role with respect to muscle hypertrophy during resistance exercise, but their actual physiologic contribution remains unknown. Some have suggested that metabolites may work independently of muscle contraction, while others have suggested that metabolites may play a secondary role in their ability to augment muscle activation via inducing fatigue. Interestingly, the studies used as support for an anabolic role of metabolites use protocols that are not actually designed to test the importance of metabolites independent of muscle contraction. While there is some evidence in vitro that metabolites may induce muscle hypertrophy, the only study attempting to answer this question in humans found no added benefit of pooling metabolites within the muscle post-exercise. As load-induced muscle hypertrophy is thought to work via mechanotransduction (as opposed to being metabolically driven), it seems likely that metabolites simply augment muscle activation and cause the mechanotransduction cascade in a larger proportion of muscle fibers, thereby producing greater muscle growth. A sufficient time under tension also appears necessary, as measurable muscle growth is not observed after repeated maximal testing. Based on current evidence, it is our opinion that metabolites produced during resistance exercise do not have anabolic properties per se, but may be anabolic in their ability to augment muscle activation. Future studies are needed to compare protocols which produce similar levels of muscle activation, but differ in the magnitude of metabolites produced, or duration in which the exercised muscles are exposed to metabolites.

The acute muscular response to two distinct blood flow restriction protocols

The purpose of this study was to determine acute physiological and perceptual responses to two commonly implemented blood flow restriction protocols. Using a within-subject design, 15 participants (age ∼25) performed four sets of unilateral elbow flexion with each arm. One arm exercised using a 3-cm elastic cuff inflated to 160 mmHg, whereas the other arm exercised using a 5-cm nylon cuff inflated to 40% of the individual's arterial occlusion pressure. While both protocols elicited increases in acute muscle thickness [pre: 4.5 (0.2) cm, post: 5.0 (0.2) cm; p < 0.001] and electromyography amplitude [first 3 reps: 55 ( 5 ) %MVC; last 3 reps: 87 ( 10 ) %MVC], there were no differences between conditions. Both protocols produced decreases in post-exercise strength (pre: 70 Nm, post: 51 Nm; p < 0.001) with no difference between conditions. The nylon protocol resulted in more repetitions during sets 2 [13 ( 2 ) vs. 9 ( 4 ); p = 0.001] and 3 [10 ( 2 ) vs. 7 ( 4 ); p = 0.05], while producing lower levels of discomfort following each set (average 3 vs. 4; p < 0.05). In conclusion, both protocols produced similar acute responses thought to be important for promoting muscle growth. However, the use of arbitrary pressures may place some individuals under complete arterial occlusion which may increase the potential risk of an adverse event.