The effect of eccentric exercise with blood flow restriction on muscle damage, neuromuscular activation, and microvascular oxygenation

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1 The University of Toledo The University of Toledo Digital Repository Theses and Dissertations 2015 The effect of eccentric exercise with blood flow restriction on muscle damage, neuromuscular activation, and microvascular oxygenation Jakob Del Lauver University of Toledo Follow this and additional works at: Recommended Citation Lauver, Jakob Del, "The effect of eccentric exercise with blood flow restriction on muscle damage, neuromuscular activation, and microvascular oxygenation" (2015). Theses and Dissertations This Dissertation is brought to you for free and open access by The University of Toledo Digital Repository. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of The University of Toledo Digital Repository. For more information, please see the repository's About page.

2 A Dissertation entitled The Effect of Eccentric Exercise with Blood Flow Restriction on Muscle Damage, Neuromuscular Activation, and Microvascular Oxygenation by Jakob Del Lauver Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Exercise Science Dr. Barry W. Scheuermann, Committee Chair Dr. Suzanne Wambold, Committee Member Dr. Michael A. Tevald, Committee Member Dr. David L. Weldy, Committee Member Dr. Patricia R. Komuniecki, Dean College of Graduate Studies The University of Toledo May 2015

3 Copyright 2015, Jakob Del Lauver This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author.

4 An Abstract of The Effect of Eccentric Exercise with Blood Flow Restriction on Muscle Damage, Neuromuscular Activation, and Microvascular Oxygenation by Jakob Del Lauver Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Exercise Science The University of Toledo May 2015 Despite the fact that numerous previous investigations have demonstrated the robust effects of BFR resistance training in producing muscle hypertrophy and improved strength, the underlying mechanisms responsible for such effects remain poorly understood. Limited work has been done examining the effects of contraction type on the acute responses to BFR resistance training. In addition, while BFR has been demonstrated to induce similar improvement in muscle mass and strength, it is unknown what effect it will have on the muscles response when subsequent high-intensity exercise is performed. Purpose: The purpose of this dissertation was to examine the effect that eccentric muscle actions with BFR have on microvascular oxygenation and neuromuscular activation, as well as to examine the effects that eccentric actions with BFR has on attenuating muscle damage during a subsequent bout of maximal exercise. Methods: Participants where healthy, recreationally active and/or sedentary college aged males. Participants were randomly assigned to one of three groups, 30% of maximal torque (LOW), 30% of maximal torque with BFR (LBFR), or control (CON). Participants in both LOW and LBFR groups performed a precondition bout of low-intensity eccentric exercise consisting of four sets (1x30, 3x15) of eccentric exercise of the knee extensors iii

5 (quadriceps). The LBFR performed the exercise bout with the addition of BFR, with an external occlusion pressure corresponded to 130% of the resting systolic blood pressure. Surface electromyography (semg) and near-infrared spectroscopy (NIRS) was used to record the neuromuscular activation and microvascular oxygenation of the knee extensors during the low-intensity precondition exercise (LOW, LBFR). Ninety-six hours after the precondition exercise, all participants performed a bout of maximal eccentric exercise consisting of six sets of ten repetitions, this was done in an attempt to induce muscle damage. Following the maximal eccentric exercise bout, all participants reported back 24, 48, 72, and 96 hours later to assess muscle damage and function. Muscle damage and function was assessed via changes in maximal voluntary isometric contraction (MVIC) torque, maximal voluntary concentric torque, range of motion (ROM), muscle soreness, and mid-thigh circumference. Results: There was a significant difference in neuromuscular activation of the vastus medialis (VM-RMS-root mean squared) between LOW and LBFR (p = 0.05). During set-2 LBFR resulted in greater activation of the VM- RMS (LOW 47.7 ± 11.5% MVIC, LBFR 67.0 ± 20.0% MVIC) compared to LOW, as well as during set-3 (LOW 51.4 ± 8.6% MVIC, LBFR ± 20.5% MVIC). There was a significant difference in deoxyhemoglobin signal (deoxy-[hb+mb]) between LBFR and LOW during set-2 (LBFR 13.1 ± 5.2 µm, LOW 6.7 ± 7.9 µm), set-3 (LBFR 14.6 ± 6 µm, LOW 6.9 ± 7.4 µm), and set-4 (LBFR 13.8 ± 5.9 µm, LOW 7.8 ± 8.5 µm). Total hemoglobin concentration [THC] was significantly higher during LBFR compared to LOW (p = 0.03). Immediately post maximal eccentric exercise all groups showed a significant decrease in MVIC torque (LOW 74.2 ± 14.1%, LBFR 75 ± 5.1%, CON 53 ± 18.6%), with LOW and LBFR resulted in a smaller decrease compared to CON. At 24, 48, 72, and 96 hours post maximal eccentric exercise both LOW and LBFR did not iv

6 demonstrate a significant force deficit from baseline. Conclusion: The results of the current investigations suggest that the LBFR condition resulted in increased metabolic stress, as shown by the greater deoxy-[hb+mb] response and increased blood volume (increased [THC]), as well as greater neuromuscular activation compared to LOW. In spite of the considerable differences observed in the preconditioning exercise stimulus, there was no difference observed in the protective effect inferred by LOW and LBFR preconditioning. However, both LOW and LBFR resulted in a significant protective effect. Further studies are necessary to determine the mechanisms responsible for the protective effect conferred by both low-intensity and low-intensity with BFR eccentric actions against maximal eccentric actions induced muscle damage. v

7 This dissertation is dedicated to my loving parents. Thank you for all always supporting me and letting me find my way.

8 Acknowledgements I would first like to thank my parents for their love and support throughout my life and especially during these last few years. Second, I would thank my brother Chance, sister-in-law Diedre and the rest of my family for all their love and support. I would not be where I am today without your love, encouragement, and guidance. I would like to thank my mentor, Dr. Barry Scheuermann for taking in an orphaned graduate student. Thank you for your support and guidance throughout my doctoral studies and during the completion of this dissertation project. I would also like to acknowledge Dr. Suzanne Wambold, Dr. Michael Tevald, and Dr. David Weldy thank you for help and willingness to serving on my dissertation committee. Next, I would like to give a special thanks to Trent Cayot for all his help and friendship during this adventure. Lastly, I would like to thank all the faculty members and fellow graduate students in the Department of Kinesiology. These individuals include Sophie Lalande, Tim Rotarius, Drew Misko, Aaron Ward, Erin Garmyn, and Shinichiro Sugiura. This experience has truly been an amazing one and I will never forget it. vi

9 Table of Contents Abstract iii Acknowledgements... vi List of Figures... xii List of Abbreviations... xiii List of Symbols... xv 1. Introduction Introduction Blood Flow Restriction References Literature Review Blood Flow Restriction Mechanisms of BFR Hypoxic Intramuscular Environment Enhanced Metabolic Stress BFR and Muscle Fiber Recruitment BFR Cellular Swelling Eccentric and Concentric Muscle Contractions with BFR vii

10 2.4 Muscle Damage and BFR Safety of Blood Flow Restriction Exercise Skeletal Muscle Physiology Eccentric Muscle Contractions Force Velocity Relationship Eccentric Muscle Contractions: Muscle Activation Eccentric Muscle Contraction: Metabolic and Cardiovascular Responses Eccentric Exercise Training Eccentric Muscle Contractions: Muscle Damage Repeated Bout Effect and Skeletal Muscle Damage Repeated Bout Effect and Preconditioning Eccentric Muscle Contraction Velocity and Repeated Bout Effect Underlying Mechanisms of the Repeated Bout Effect Neural Theory Mechanical Theory Cellular Theory References The Effects of Blood Flow Restriction on Tissue Oxygenation and Muscle Activation during Eccentric Actions Introduction viii

11 3.1.1 Purpose Hypothesis Methods Subjects Procedure Exercise Protocol Low-intensity Group Low-intensity with BFR Group Measurements Techniques Isokinetic Dynamometer Surface Electromyography (semg) Maximal Voluntary Isometric Contractions Near-Infrared Spectroscopy (NIRS) Assessment of Peak Eccentric Torque Signal Processing Statistical Analysis Results Torque Results Surface Electromyography Near-infrared spectroscopy ix

12 3.7 Discussion References Low-intensity Eccentric Blood Flow Restriction and the Attenuation of Muscle Damage Introduction Purpose Hypothesis Methods Subjects Protocol Low-intensity Group (LOW) Low-intensity with BFR Group (LBFR) Maximal Eccentric Contraction: Muscle Damaging Protocol Recovery Sessions Measurements Techniques Isokinetic Dynamometer Range of Motion Muscle soreness Mid-thigh circumference Maximal Voluntary Isometric Contractions x

13 4.3.6 Assessment of Peak Torque Signal Processing Statistical Analysis Results Discussion References General Conclusion References Appendix A Appendix B xi

14 List of Figures 3-1 Neuromuscular activation of the vastus lateralis Neuromuscular activation of the vastus medialis Deoxygenation status of the vastus lateralis Total hemoglobin concentration of the vastus lateralis Change in MVIC torque Change in concentric maximal torque Change in joint angle Muscle soreness xii

15 List of Abbreviations 1RM. One repetition maximum AMP.Adensosine monophosphate ACSM..American college of sports medicine ANOVA...Analysis of variance ASIS.Anterior superior iliac spine ATP...Adenosine triphosphate BFR...Blood flow restriction CK.Creatine Kinase Deoxy-[Hb+Mb]...Deoxygenated hemoglobin E-C Excitation-contraction EMG..Electromyography GH.Growth hormone HR.Heart rate HSP...Heat shock protein MPF..Mean power frequency MRI..Magnetic resonance imaging MVIC...Maximal voluntary isometric contraction NIRS.Near infrared spectroscopy %Ox...Oxygen saturation oxy-[hb+mb] Oxygenated hemoglobin PCr Phosphocreatine Pi...Inorganic phosphate semg Surface electromyography xiii

16 RMS.Root mean squared ROM.Range of motion SD.Standard deviation SSC...Stretch-shortening cycle [THC] Total hemoglobin concentration VAS..Visual analog scale VO 2...Oxygen consumption VO 2max..Maximal oxygen consumption xiv

17 List of Symbols C Degrees Celsius cm..centimeter Hz..Hertz khz Kilohertz m Meter ml...milliliter ml/min Milliliter a minute mm.millimeter mmhg Millimeter of mercury N m Newton meter nm..nanometer μm.micromolar..degrees /s...degrees a second xv

18 Chapter 1 1. Introduction 1.1 Introduction Blood Flow Restriction It has been well established that a traditional resistance-training program is effective at promoting muscle hypertrophy and strength (Schoenfeld 2013). During resistance exercise, motor units are recruited according to Henneman s size principle, which states that under conditions of voluntary tension development, motor units are recruited in order of increasing size (Henneman 1957; Henneman, Somjen et al. 1965). Thus, small motor units associated with type I muscle fibers are typically activated initially during low-intensities, whereas larger motor units associated with type II muscle fibers are recruited as the intensity increases. During resistance training, in order to increase muscle mass and strength it is important to activate type II muscle fibers as these fibers exhibit a greater hypertrophy response compared to type I fibers (MacDougall, Sale et al. 1982; McCall, Byrnes et al. 1996; Pearson and Hussain 2015). Therefore, the American College of Sports Medicine (ACSM) recommends that an individual exercise at an intensity equivalent to 70% of their one repetition maximum (1RM) or greater in order to achieve muscular hypertrophy (Kraemer, Adams et al. 2002; American College of Sports Medicine., Thompson et al. 2010). However, the results of several recent 1

19 studies using low-intensity resistance training (20%-30% 1RM) combined with blood flow restriction (BFR) have shown significant improvements in muscle hypertrophy and strength despite the use of intensities below 70% 1RM (Takarada, Sato et al. 2002; Takarada, Tsuruta et al. 2004; Abe, Kearns et al. 2006; Madarame, Neya et al. 2008; Karabulut, Abe et al. 2010; Clark, Manini et al. 2011; Yasuda, Loenneke et al. 2012). BFR, which is also referred to as KAATSU training, involves decreasing blood flow to the active muscle by the application of a wrapping device, such as pressurized cuffs, while performing various exercises. These cuffs are generally applied to the limb(s) being exercised and are typically placed proximally on the limb, such as the inguinal crease for the lower extremities (Takarada, Sato et al. 2002; Laurentino, Ugrinowitsch et al. 2008) or distal to the deltoid for the upper extremities (Takarada, Takazawa et al. 2000; Pope, Willardson et al. 2013). These cuffs are then inflated to a pressure prior to exercise that occludes venous return while still allowing for some arterial inflow (Iida, Kurano et al. 2007). This reduced blood flow is thought to induce a hypoxic intramuscular environment that enhances the training stimuli and effect in the exercising muscle (Takarada, Takazawa et al. 2000; Takada, Okita et al. 2012). The exact mechanism responsible for the adaptations observed after BFR training are unknown, although several mechanism have been proposed and include enhanced metabolic stress due to hypoxia and/or preferential recruitment or reduced oxygen availability induced additional recruitment of fast twitch muscle fibers (Loenneke, Fahs et al. 2012; Wilson, Lowery et al. 2013). While several studies have demonstrated the effectiveness of BFR training at inducing improvements in muscle hypertrophy and strength, currently there are a limited 2

20 number of studies that have examined the acute responses during exercise with BFR. Further examination of these acute responses such as the microvascular oxygenation status and neuromuscular activation of the exercising muscle will provided further insight into the possible mechanics associated with BFR training. In particular, there has been limited examination of the effect of performing eccentric contractions with BFR. This is of interest as during traditional resistance-training, eccentric training has been associated with greater gains in muscular strength and mass compared to concentric training (Roig, O'Brien et al. 2009), provided that the training intensity was adjusted to account for the greater force generating capacity of eccentric contractions. However, initial investigations have suggested that the training stimulus resulting from eccentric contractions with BFR is not sufficient to induce the muscle hypertrophy and strength increase commonly associated with BFR (Yasuda, Loenneke et al. 2012). However, the exercise intensity utilized previously have been based on maximal voluntary isometric contraction force for both concentric and eccentric intensity, thus potentially resulting in an relative eccentric intensity less than the relative concentric intensity. Currently few investigations have examining the effect of eccentric muscle actions with BFR resistance exercise on neuromuscular activation (Wernbom, Jarrebring et al. 2009; Yasuda, Loenneke et al. 2012) and furthermore, there are no known investigations examining the microvascular oxygenation responses during eccentric actions with BFR. The examination of these responses is warranted as they are believed to play a role in the mechanisms associated with BFR. Therefore, the aim of the following investigations were to further examine the effect that eccentric muscle actions 3

21 with BFR have on microvascular oxygenation and neuromuscular activation (Chapter 3), to provide insight into the acute responses to eccentric exercise with BFR. The relatively low-intensity that can be utilized with BFR training to elicit adaptations may make it a particularly attractive alternative for patient populations where high joint loads may be contraindicated, such as those recovering from injury or those with degenerative joint disease. In addition, BFR has been suggested to be a valuable training method that could be utilized in periodization programming with athletes. However, many important practical questions remain about how to best implement BFR into a resistance-training program. One such question is whether BFR exercise has any effect on the responses to a subsequent bout of high-intensity exercise, such as the repeated bout effect commonly seen in traditional resistance training programs. An unaccustomed bout of high-intensity exercise has been demonstrated to result in significant muscle soreness and damage, however when that high-intensity bout is repeated there is less muscle soreness and damage. This has been coined the repeated bout effect or the protective mechanism. While training with BFR has been suggest to result in an increased metabolic stress compared to free flow resistance training, it is unknown if this difference in initial stress will influence the repeated bout effect. It is unknown if prior low-intensity resistance training with BFR will result in the attenuation of the muscle damage and/or soreness commonly observed after unaccustomed high-intensity exercise, thus does lowintensity resistance training with BFR provide a significant protective effect against muscle damage associated with high-intensity exercise (Chapter 4). 4

22 1.2 References Abe, T., C. F. Kearns, et al. (2006). "Muscle size and strength are increased following walk training with restricted venous blood flow from the leg muscle, Kaatsu-walk training." J Appl Physiol 100(5): American College of Sports Medicine., W. R. Thompson, et al. (2010). ACSM's guidelines for exercise testing and prescription. Philadelphia, Lippincott Williams & Wilkins. Clark, B. C., T. M. Manini, et al. (2011). "Relative safety of 4 weeks of blood flowrestricted resistance exercise in young, healthy adults." Scand J Med Sci Sports 21(5): Henneman, E. (1957). "Relation between size of neurons and their susceptibility to discharge." Science 126(3287): Henneman, E., G. Somjen, et al. (1965). "Excitability and inhibitability of motoneurons of different sizes." J Neurophysiol 28(3): Iida, H., M. Kurano, et al. (2007). "Hemodynamic and neurohumoral responses to the restriction of femoral blood flow by KAATSU in healthy subjects." Eur J Appl Physiol 100(3): Karabulut, M., T. Abe, et al. (2010). "The effects of low-intensity resistance training with vascular restriction on leg muscle strength in older men." Eur J Appl Physiol 108(1): Kraemer, W. J., K. Adams, et al. (2002). "American College of Sports Medicine position stand. Progression models in resistance training for healthy adults." Med Sci Sports Exerc 34(2): Laurentino, G., C. Ugrinowitsch, et al. (2008). "Effects of strength training and vascular occlusion." Int J Sports Med 29(8): Loenneke, J. P., C. A. Fahs, et al. (2012). "The anabolic benefits of venous blood flow restriction training may be induced by muscle cell swelling." Med Hypotheses 78(1):

23 MacDougall, J. D., D. G. Sale, et al. (1982). "Muscle ultrastructural characteristics of elite powerlifters and bodybuilders." Eur J Appl Physiol Occup Physiol 48(1): Madarame, H., M. Neya, et al. (2008). "Cross-transfer effects of resistance training with blood flow restriction." Med Sci Sports Exerc 40(2): McCall, G. E., W. C. Byrnes, et al. (1996). "Muscle fiber hypertrophy, hyperplasia, and capillary density in college men after resistance training." J Appl Physiol 81(5): Pearson, S. J. and S. R. Hussain (2015). "A Review on the Mechanisms of Blood-Flow Restriction Resistance Training-Induced Muscle Hypertrophy." Sports Med 45(2): Pope, Z. K., J. M. Willardson, et al. (2013). "Exercise and Blood Flow Restriction." J Strength Cond Res 27(10): Roig, M., K. O'Brien, et al. (2009). "The effects of eccentric versus concentric resistance training on muscle strength and mass in healthy adults: a systematic review with meta-analysis." Br J Sports Med 43(8): Schoenfeld, B. J. (2013). "Potential mechanisms for a role of metabolic stress in hypertrophic adaptations to resistance training." Sports Med 43(3): Takada, S., K. Okita, et al. (2012). "Low-intensity exercise can increase muscle mass and strength proportionally to enhanced metabolic stress under ischemic conditions." J Appl Physiol 113(2): Takarada, Y., Y. Sato, et al. (2002). "Effects of resistance exercise combined with vascular occlusion on muscle function in athletes." Eur J Appl Physiol 86(4): Takarada, Y., H. Takazawa, et al. (2000). "Effects of resistance exercise combined with moderate vascular occlusion on muscular function in humans." J Appl Physiol 88(6):

24 Takarada, Y., T. Tsuruta, et al. (2004). "Cooperative effects of exercise and occlusive stimuli on muscular function in low-intensity resistance exercise with moderate vascular occlusion." Jpn J Physiol 54(6): Wernbom, M., R. Jarrebring, et al. (2009). "Acute effects of blood flow restriction on muscle activity and endurance during fatiguing dynamic knee extensions at low load." J Strength Cond Res 23(8): Wilson, J. M., R. P. Lowery, et al. (2013). "Practical blood flow restriction training increases acute determinants of hypertrophy without increasing indices of muscle damage." J Strength Cond Res 27(11): Yasuda, T., J. P. Loenneke, et al. (2012). "Effects of blood flow restricted low-intensity concentric or eccentric training on muscle size and strength." Plos One 7(12). 7

25 Chapter 2 2 Literature Review 2.1 Blood Flow Restriction Recently, blood flow restriction (BFR) training has gained a substantial interest from investigators due to the apparent effectiveness of combining regular low-intensity exercise with BFR on muscular hypertrophy and strength. Blood flow restriction, which is also referred to as KAATSU training was first introduce by Dr. Yoshiaki Sato and he patented a KAATSU training device in Dr. Sato coined the term KAATSU as KA means additional and ATSU means pressure in the Japanese language, thus in English KAATSU means additional pressure. BFR involves decreasing blood flow to a muscle by the application of a wrapping device, such as pressurized cuffs. These cuffs are typically applied to the limb(s) being exercised and are generally placed proximal on the limb, such as the inguinal crease for the lower extremities (Takarada, Sato et al. 2002; Laurentino, Ugrinowitsch et al. 2008) or distal to the deltoid for the upper extremities (Takarada, Takazawa et al. 2000; Pope, Willardson et al. 2013). These cuffs are then inflated prior to exercise to restrict venous return while still allowing for arterial inflow, although the extent that arterial inflow is affected by the cuffing appears to be highly 8

26 variable and dependent on several factors including limb girth, mean arterial pressure and cuff width (Loenneke, Fahs et al. 2012). One of the key aspects of BFR training that has made it a particularly attractive alternative to more habitual training programs is the low relative intensity at which adaptations such as increases in muscle size and strength are stimulated. Traditionally, the American College of Sports Medicine recommends resistance training using intensities of 70% or greater of one repetition maximum (1RM) in order to achieve muscular hypertrophy and improved strength under normal conditions (American College of Sports Medicine., Thompson et al. 2010). However, numerous studies have demonstrated that low-intensity (20-30% 1RM) resistance training combined with BFR elicits similar increases in muscle size and strength compared to traditional (non-bfr) resistance training performed at much higher intensities (>70% 1RM) (Takarada, Sato et al. 2002; Takarada, Tsuruta et al. 2004; Abe, Kearns et al. 2006; Madarame, Neya et al. 2008; Karabulut, Abe et al. 2010). Loenneke et al. (2012) recently performed a metaanalysis to quantitatively identify which training variables result in the greatest strength and hypertrophy outcomes with lower body low-intensity training with BFR. They found a mean overall effect size for muscle strength for low-intensity BFR of 0.58 and 0.00 for low-intensity training and a mean overall effect size for muscle hypertrophy for lowintensity BFR of 0.39 and for low-intensity training. Loenneke et al. (2012) concluded that the findings of their meta-analysis confirmed previous ACSM recommendations that regular low-intensity resistance training does not provide an adequate stimulus to produce substantial increase in strength or muscle hypertrophy. However, when that same low-intensity exercise is combined with BFR a significant 9

27 increase was found, comparable to a previous meta-analysis using higher intensities with both strength and muscle hypertrophy (Loenneke, Wilson et al. 2012). In addition to resistance training studies, previous studies have also reported increased muscle cross-sectional area and strength after BFR plus walking (Abe 2009), BFR plus cycling (Abe, Fujita et al. 2010), and BFR plus body weight circuit training (Ishii 2005). Abe et al. (2009) examined the effects of low-intensity cycle exercise training with and without BFR on muscle size and maximum oxygen uptake (VO 2max ), a measure of overall cardiovascular fitness. Subjects in the BFR group trained 3 sessions per week at 40% of VO 2max for 15 minutes with BFR, while the control group trained 3 sessions per week at 40% of VO 2max for 45 minutes without BFR. Magnetic resonance imaging (MRI) measured thigh and quadriceps muscle cross sectional area and muscle volume increased by % and strength tended (not significant) to increase by 7.7% in the BFR training group (Abe, Fujita et al. 2010). They found no change in muscle size and strength in the control group. In addition, significant improvements in VO 2max and exercise time to exhaustion were observed in the BFR training group but not in the control group. The authors suggested that their results suggest that low-intensity short duration cycling exercise combined with BFR improves both muscle hypertrophy and aerobic capacity concurrently (Abe, Fujita et al. 2010). This finding is of particular interest, as concurrent improvements in aerobic capacity and muscle hypertrophy in response to a single mode of training have not been previously reported. However, subsequent studies that have also examined BFR during interval training have found that interval training with BFR did not provide any additive effect on VO 2max or sub-maximal cycling performance compared to control (non-bfr) interval training (Keramidas, 10

28 Kounalakis et al. 2012), implying that BFR exercise may be of benefit during some but not all exercise training modes. 2.2 Mechanisms of BFR Currently the proposed mechanism(s) through which BFR resistance training is thought to facilitate the increase muscular hypertrophy and strength include an hypoxic intramuscular environment, enhanced metabolic stress, alterations in muscle fiber recruitment, and cellular swelling (Loenneke, Fahs et al. 2012; Wilson, Lowery et al. 2013). While the mechanism(s) behind the benefits seen with BFR training have not been established it has traditional been thought to occur from the decreased oxygen available to the active muscles and the accumulation of metabolites due to venous occlusion (Loenneke, Fahs et al. 2012). However, some recent evidence has suggested that increases in muscular hypertrophy and strength following low-intensity exercise with BFR might also be induced through muscle cell swelling (Loenneke, Fahs et al. 2012). While there are many proposed mechanisms each with some evidence of support it is most likely a combination of these mechanisms that contribute to the effects observed following low-intensity BFR resistance training, with each warranting further examination Hypoxic Intramuscular Environment During sustained isometric and isokinetic muscle actions, there is a linear and inverse relationship to intramuscular pressure and in part to muscle blood flow (Sjogaard, Savard et al. 1988; Aratow, Ballard et al. 1993; Degens, Salmons et al. 1998). However, this blood flow occlusion seems to be dependent on the muscle fiber type, penation angle 11

29 of the muscle fibers, anatomical arrangement of arteries, and tension per unit. During intermittent isometric and isotonic contractions mean muscle blood flow is not decrease and can even increase (Sjogaard, Savard et al. 1988; Quaresima, Homma et al. 2001), most likely do to compensatory flow during the relaxation phases between contractions (Pereira, Gomes et al. 2007). However, during resistance exercise there appears to result in a state of restricted circulation caused by reduced venous outflow with no restriction to arterial inflow (Pereira, Gomes et al. 2007). Iida et al. (2007) demonstrated KAATSU suppresses the arterial blood flow to the thighs in a pressure dependent manner. They found that the application of 100 mmhg KAATSU decreased femoral arterial blood flow from ± 41.8 ml/min to ± 25.6 ml/min and arterial blood flow decreased to 63.8 ± 21.7 ml/min and 34.8 ± 16.2 ml/min (Iida, Kurano et al. 2007). These results suggest that the application of KAATSU restricts venous blood flow and causes venous pooling with the pressure reduction of arterial blood flow (Iida, Kurano et al. 2007). Therefore, the vascular occlusion associated with BFR exercise may result in alterations in muscle oxygenation and/or blood volume that are commonly of high-intensity exercise. Previous investigations have examined the tissue specific (muscle) oxygen availability and utilization responses in a variety of resistance exercise conditions (Tanimoto and Ishii 2006; Pereira, Gomes et al. 2007; Ganesan, Cotter et al. 2015). These previous investigations have utilized near infrared spectroscopy (NIRS) which is an optical techniques used to measure tissue concentrations of oxy- and deoxy-hemoglobin. Tanimoto et al. (2006) examined that changes in oxygenation of the vastus lateralis muscle during knee extension protocols commonly used in resistance training. Subjects 12

30 were assigned to one of three groups; low-intensity (~50% of 1-RM) with slow movement and tonic force generation (3s for eccentric and concentric actions, 1s pause, and no relaxing phase), high-intensity (~80% 1RM) with normal speed (1s for concentric and eccentric actions, and 1s for relaxing), or low-intensity with normal speed (same intensity as low-intensity with tonic force and same speed as the high-intensity). Tanimoto et al. (2006) found that EMG and NIRS analyses showed that one bout of lowintensity with slow movement caused sustained muscular activity and the largest muscle deoxygnation among the three types of exercise examined. In addition, they found the after 12 weeks of training that there was significant increases in muscle size and strength in the low-intensity slow speed with tonic force that were similar to the high-intensity results. The authors suggested that the larger deoxygenation and blood lactate concentration observed in the low-intensity slow speed group might have induced a greater restriction in blood flow which may be related to muscle hypertrophy (Tanimoto and Ishii 2006). These results and the results from other previous investigations (Azuma, Homma et al. 2000; Hoffman, Im et al. 2003) that the kinetics of intramuscular oxygenation and blood volume measured by NIRS are dependent upon the resistance training protocols (number of sets (Hoffman, Im et al. 2003), different intensities (Azuma, Homma et al. 2000), and exercise duration) and can vary for different muscles (Pereira, Gomes et al. 2007). With the increased attention given to BFR training during recent years the use NIRS techniques to examine muscle hemodynamics and oxygenation during BFR and compare those to similar non-bfr exercise, as well as high-intensity non-bfr can provide insight into process by which BFR training works. Ganesan et al. (2015) recently 13

31 utilized time resolved NIRS to quantify tissue oxy-hemoglobin and deoxy-hemoglobin, and oxygen saturation in the vastus medial is during knee extension with and without BFR. Subjects performed three sets of knee extension on a dynamometer under three different conditions; until fatigue without BFR, until fatigue with BFR, and same number of repetitions as fatigue BFR condition but without BFR. They found that BFR was associated with higher deoxy-hemoglobin concentration and significantly lower oxygen saturation during recovery periods between sets. Ganesan et al. (2015) also utilized a piecewise linear spline method and found a spike in the deoxy-hemoglobin before the onset of deoxy-hemoglobin clearance during recovery in the BFR condition thus causing the deoxy-hemoglobin clearance to begin at a higher concentration. In addition the oxyhemoglobin kinetics during recovery were also slowed during recovery from BFR (Ganesan, Cotter et al. 2015). While the results of this investigation by Ganesan et al. (2015) began to explain some of the effects of BFR on tissue oxygenation several factors should be consider when examining the results. Ganesan et al. (2015) identified that the time domain NIRS utilized was limited to acquisition time of ~3s/measurements and therefore there is a loss of sensitivity to transient changes. In addition, they did not compare their results to higher intensity non-bfr exercise, thus further examination of tissue oxygenation and muscle activation during BFR exercise is warranted Enhanced Metabolic Stress Metabolic stress such as that manifested during heavy resistance exercise which include decreased adenosine triphosphate (ATP), depletion of phosphocreatine (PCr), increased inorganic phosphate (Pi), increased adenosine diphosphate/atp ratio, increased adenosine monophosphate (AMP) production, decreased intramuscular ph, and an 14

32 accumulation of lactate have been suggested as a key stimulator of adaptations (Kraemer, Marchitelli et al. 1990; Goto, Ishii et al. 2005; Pope, Willardson et al. 2013). Metabolic accumulation has been demonstrated to stimulate a subsequent increase in anabolic growth factors, fast-twitch fiber recruitment, and increased protein synthesis through mammalian target of rapamycin (mtor) pathway (Kawada and Ishii 2005). In a recent review, Pope et al. (2014) stated that numerous studies have found that the ischemic (Takano, Morita et al. 2005) and hypoxic intramuscular environment associated with BFR protocols induces a greater rate of ATP hydrolysis, exaggerated PCr depletion (Suga, Okita et al. 2012), decreased ph (Suga, Okita et al. 2009; Suga, Okita et al. 2012), and an increased lactate response (Takarada, Nakamura et al. 2000; Takano, Morita et al. 2005; Reeves, Kraemer et al. 2006). Suga et al. (2012) examined the acute effects of multiple sets on intramuscular metabolic stress during two separate low-intensity plantar flexion exercise bouts. They examine two distinct BFR bouts which where intermittent pressure in which the pressure was released during rest intervals between sets and continuous pressure maintained throughout exercise and released after the final set, which where compared to a high-intensity bouts and a low-intensity bouts without BFR. Suga et al. (2012) found that with intermittent pressure there was a non-significant trend towards greater metabolic stress compared with the low-intensity bout without BFR. Only the bout where the pressure was continuously applied during BFR elicited similar metabolic stress similar to high-intensity bout without BFR (Suga, Okita et al. 2012). The dominant body of research supports a strong correlation between exerciseinduced metabolic stress and increased hypophyseal growth hormone (GH) secretion (Schoenfeld 2013). The absolute magnitude of these hormonal elevations is substantial, 15

33 for example Fujita et al. (Fujita, Abe et al. 2007) found that BFR increased post exercise GH levels 10 fold compared to low-intensity exercise without BFR while others have (Takarada, Nakamura et al. 2000) reported elevations of 290 fold over baseline. The post exercise elevations in GH are presumably mediated by increased lactate and hydrogen (H + ) buildup in the blood due to venous occlusion (Hakkinen and Pakarinen 1993; Gordon, Kraemer et al. 1994). A reduction in ph associated with metabolic accumulation also may potentiate GH release via chemoreflex stimulation that is mediated by intramuscular metaboreceptors and group III and IV afferents (Viru, Jansson et al. 1998; Loenneke, Wilson et al. 2010). However, changes in blood lactate do not always predict changes in GH. For example previous investigations have demonstrated that while occlusion training resulted in a greater GH response than a non-occluded control group there were no significant differences in blood lactate concentrations between groups (Reeves, Kraemer et al. 2006). Loenneke et al. (2010) have suggested that this disparity is due to that blood flow restriction there is 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 prior to its diffusion out of the cell. It has also been proposed that the additional intramuscular metabolites stimulate changes in GH as group III and IV afferents are sensitive to changes in adenosine, potassium (K + ), H +, hypoxia, and [AMP] (Loenneke, Wilson et al. 2010). In addition, an increase in these metabolites are thought to drive the exercise pressure reflex leading to an increase heart rate and blood pressure which has also been postulated to facilitate an increase in GH following occlusion training (Pierce, Clark et al. 2006; Loenneke, Wilson et al. 2010). Caution should be used when explaining GH roles in the adaptations seen after occlusion 16

34 training as there is no evidence that GH enhances muscle protein synthesis when combined with traditional resistance training (Yarasheski, Campbell et al. 1992); however occlusion training may be different. In addition, Ehrenborg and Rosen (2008) concluded in analysis of the literature that the major benefit of GH seems to be the stimulation of collagen synthesis, which might have a positive protective effect on ruptures of muscles and tendons and allow harder training with shorter recovery periods (Ehrnborg and Rosen 2008). Circulating GH is known to stimulate synthesis and secretion of insulin-like growth factor 1 (IGF-1). However, to date the results are unclear whether this occurs in response to occlusion training. An increase in IGF-1 following low-intensity resistance training with BFR has been reported previously by Abe et al. (2006) as well as Takano et al. (Takano, Morita et al. 2005). However, it has also been reported that there is no increase in IGF-1 following walking with BFR (Abe, Kearns et al. 2006). In addition, empirical support for the potential hypertrophic role of metabolic stress can be noted by examining the moderate intensity training regimens adopted by many bodybuilders, which are intended to heighten metabolic buildup at the expense of higher training intensities (Schoenfeld 2013). Schoenfeld (2013) stated that these routines have been found to induce significantly more metabolic stress than higher intensity regimens typically employed by power-lifters (Schoenfeld 2010) and that these training protocols can result in greater increases in muscle growth, although results are not consistent across studies (Campos, Luecke et al. 2002). 17

35 2.2.3 BFR and Muscle Fiber Recruitment During muscle contractions, muscle fibers are recruited according to the size principle (Henneman, Somjen et al. 1965). This principle states that small motor units that are associated with type I muscle fibers are activated first and as intensity increases (i.e. Force requirement), larger motor units consisting of type II muscle fibers are recruited until maximal tension is reached. It has been demonstrated that type II muscle fibers respond to resistive exercise stress with greater hypertrophy than type I muscle fibers (MacDougall, Elder et al. 1980; McCall, Byrnes et al. 1996), thus for hypertrophy to be maximized the activation of type II fibers is important. Since motor unit activation and fiber recruitment is relatively linear to force production (Moore, Burgomaster et al. 2004), many exercise programs are aimed at increasing muscle size and strength by performing repetitions at a target intensity of at least 70-95% 1 RM (American College of Sports Medicine., Thompson et al. 2010). However, Moritani et al. (1992) demonstrated that during 4 minutes of handgrip exercise at 20% of MVIC with BFR (200 mmhg) greater recruitment of fast twitch fibers was elicited compared to the same exercise performed without occlusion. Moritani et al. (1992) observed increases in motor unit firing rate and spike amplitude consistent with greater recruitment of fast twitch motor units. In addition, several studies utilizing electromyography (EMG) have demonstrated that during BFR exercise there is greater neuromuscular activation compared to free flow exercise (non-bfr) performed at the same intensity (Moritani, Sherman et al. 1992; Takarada, Takazawa et al. 2000; Yasuda, Brechue et al. 2009). However, previous investigations have also demonstrated no difference in neuromuscular activation between BFR and non-occlusion conditions (Wernbom, 18

36 Jarrebring et al. 2009; Kacin and Strazar 2011). Wernbom et al. (2009) examined the effect of three sets of isotonic knee extension exercise at a sub-maximal intensity of 30% MVC to volitional fatigue (3sets). The absolute occlusion pressure of 100 mmhg was applied at the beginning of the first exercise set and was released after the completion of the third exercise set. The occlusion cuff remained inflated while the subject received 45 seconds of intermittent recovery following the completion of each exercise set and therefore the number of repetitions performed to volitional fatigue dictated occlusion duration. Similar neuromuscular activation patterns between conditions were observed, with the exception that without BFR set 3 demonstrated greater activation during the eccentric phase (Wernbom, Jarrebring et al. 2009). In addition, current unpublished results from our laboratory have demonstrated no significant difference in neuromuscular activation during isometric exercise between BFR and control conditions. More specifically, according to these unpublished results, BFR occlusion durations (immediate occlusion or pre-occlusion 5 minutes before exercise) did not elicit a substantial impact on neuromuscular activation during isometric exercise at a variety of sub-maximal exercise intensities. In addition to EMG, Fujita et al. (2007) found that BFR resistance exercise compared to a worked matched control without BFR demonstrated increased S6 kinase1 (S6K1) phosphorylation, which primarily occurs in type II muscle fibers (Koopman, Zorenc et al. 2006), indirectly linking BFR to increased recruitment of fast twitch fibers (Fujita, Abe et al. 2007). Based on the current research there appears to be some evidence that BFR may enhance recruitment of higher threshold motor units. Given that fast twitch fibers have a substantially greater hypertrophic potential compared with 19

37 slow twitch fibers, this phenomenon would seem to be associated with at least some of the increase in muscle strength and hypertrophy associated with BFR training BFR Cellular Swelling One of the more novel mechanisms that might be involved in the hypertrophic responses to BFR training is that the increased metabolic stress results in an increase in intracellular fluid known as cell swelling. This phenomenon is thought to serve as a physiological regulator of cell function (Haussinger, Lang et al. 1994). It has been shown that hydration-mediated cell swelling results in an increase in protein synthesis and a decrease in proteolysis in a variety of different cells types, which include hepatocytes, osteocytes, breast cells and muscle fibers (Lang, Busch et al. 1998). With respect to muscle, it has been theorized that a stimulus associated with cell swelling may trigger proliferation of satellite cells and facilitate their fusion to hypertrophy myofibers (Dangott, Schultz et al. 2000). Also cell swelling is able to inhibit catabolism, shifting the protein balance towards anabolism (Loenneke, Fahs et al. 2012). Although mechanisms behind the potential anabolic effect of muscle swelling have yet to be fully determined it has been proposed that increased pressure against the cytoskeleton and/or cell membrane is perceived as a threat to cellular integrity. This causes the cell to initiate a signaling response that ultimately leads to the reinforcement of the muscle ultrastructure (Lang 2007; Schoenfeld 2013). Human research has also indicated that cell swelling can also positively affect metabolism through the sparing of protein and promotion of lipolysis (Berneis, Ninnis et al. 1999; Keller, Szinnai et al. 2003; Loenneke, Fahs et al. 2012). While currently there is a lack of research that has directly investigated whether cellular swelling enhances muscle growth, several authors have stated that it is 20

38 conceivable that the pooling of blood induced by BFR may be sufficient to cause a shift in the intracellular water balance, even without exercise (Loenneke, Fahs et al. 2012). In addition, resistance training has been shown to induce alterations in intracellular and extracellular water balance dependent upon the type of exercise and intensity (Sjogaard 1986; Schoenfeld 2013). Thus, it may be that BFR increases the intracellular to extracellular pressure gradient thereby increasing the flux of water into the cell helping to drive cellular swelling. Without BFR this pressure gradient is typically insufficient for rapid sustained water fluxes (Loenneke, Fahs et al. 2012). It also appears that this cell swelling phenomenon may be driven by metabolic accumulation as lactate accumulation has been shown to act as a primary contributor to osmotic changes in skeletal muscle (Sjogaard, Adams et al. 1985; Frigeri, Nicchia et al. 1998; Schoenfeld 2013). Schoenfeld (2013) recently stated that the intramuscular buildup of lactate has been shown to trigger volume regulatory mechanisms, and the acidic environment associated with exercise induced metabolite accumulation may magnify these effects. Another possible factor that has been speculated is the reperfusion of blood flow to the muscle following BFR (Loenneke, Fahs et al. 2012; Schoenfeld 2013). The restoration of blood flow following BFR could provide the stimulus for forcing fluid into the muscle cell from the altered pressure gradients and cellular channels and may explain how the fluid shift occurs (Loenneke, Fahs et al. 2012). Aquaporins, a family of transmemebrane water channel proteins, which are widely distributed in various tissues play an integral part in transcellular and trasepithelial water movement have been proposed to affect the long term hypertrophic response (Frigeri, Nicchia et al. 1998). Specifically, aquaporin 4 has been shown to be expressed in mammalian type II muscle fibers and appears to be 21

39 involved in the rapid equilibrium of osmotic gradients created from the intracellular accumulation of metabolites (Frigeri, Nicchia et al. 1998; Frigeri, Nicchia et al. 2004). Loenneke et al. (2012) proposed that why this metabolic accumulation may be small during BFR only (no exercise) that the increase in intracellular metabolites would be greater when BFR is combined with resistance training exercise, which would result in greater movement of water into the working muscle. While previous research has demonstrated the effectiveness of low-intensity BFR training at stimulation increases in muscle mass and strength, the exact mechanism(s) responsible for these adaptations are not completely understood. It appears however, that enhanced metabolic stress, fiber recruitment, and cellular swelling in combination or individually may play prominent roles during BFR training, however there are likely other mechanism contributing. It is reasonable to speculate that the decreased oxygen availability to the muscle and the accumulation of metabolites during BFR training may stimulate other possible mechanisms, such as the increase in high threshold fiber recruitment by the stimulation of group III and IV afferents (Loenneke, Fahs et al. 2012). Further research is warranted to continue to examine these possible mechanisms underlying the benefits seen with BFR training to advance the understanding of BFR training so that it may be utilized safely and effectively. 2.3 Eccentric and Concentric Muscle Contractions with BFR There are numerous differences between eccentric and concentric muscle actions but the two most important differences are the decrease in metabolic cost and the lower muscle activation during eccentric actions compared to concentric actions as discussed previously. Currently few studies (Umbel, Hoffman et al. 2009; Yasuda, Loenneke et al. 22