Although the importance of aerobic exercise in disease prevention and maintenance of a healthy lifestyle has been extensively demonstrated [1-4], it was recently reported by the American Heart Association (AHA) that approximately 30% of the adult population within the United States does not engage in regular aerobic exercise [2]. The most commonly reported reason why adults did not engage in regular exercise was due to a "lack of time" within their daily routine [5, 6]. In order to best integrate exercise into a time constrained schedule many have turned to high-intensity interval training (HIIT) due to the advantageous training outcomes reported in a relatively short duration (2-4 week) [7, 8]. In addition, the exercise volume is significantly reduced (~80-90%) during HIIT sessions compared to traditional "continuous" cardiovascular exercise sessions [8, 9] thus decreasing the time spent exercising [8]. However, the exercise intensities used during HIIT sessions ("all-out effort" [9, 10] or near maximal intensities [11, 12]) may become a deterrent or may not be appropriate for certain populations. An exercise technique known as blood flow restriction (BFR) exercise may be an acceptable alternative approach for these populations as it utilizes low exercise intensities. BFR exercise has been shown to concurrently increase muscle hypertrophy [13, 14], muscle strength [13] and peak oxygen uptake (VO2pk) [14, 15] subsequent to low-intensity (i.e., walking, cycling) cardiovascular training programs. The combination of BFR (i.e., decreased exercise intensity) and interval training (i.e., decreased exercise volume) is both intriguing and a unique alternative solution that could potentially be applicable to a variety of populations. This alternative exercise approach (i.e., BFR interval training) addresses many commonly cited barriers for exercise retention (i.e., time constrained schedules, high exercise intensities).
Therefore, the primary purpose of this dissertation was to determine the results of a short duration (2 weeks) BFR low-intensity interval training (BFR-LIIT) program on aerobic capacity and skeletal muscle strength (chapter 5). However, before the primary purpose could be investigated many secondary aims needed to be examined, including i) determining the effect of occlusion duration on the microvascular oxygenation and neuromuscular activation during exercise (chapter 3) and ii) determining the acute physiological responses (oxygen uptake, microvascular oxygenation, neuromuscular activation) to BFR used in cardiovascular exercise models (constant load, chapter 4; interval, chapter 5).
The effects of occlusion duration were examined as healthy subjects performed isometric knee extension contractions at different sub-maximal intensities under control (CON, no occlusion), immediate occlusion (IO) and pre occlusion (PO) conditions. During the IO condition the occlusion pressure (130% of the resting systolic blood pressure, 130% SBP) was applied immediately prior to exercise while the occlusion pressure (130% SBP) was applied five minutes prior to exercise in the PO condition. Varying the occlusion duration did not affect the neuromuscular activation of the exercising musculature (p > 0.05), although activation did significantly increase with increasing sub-maximal exercise intensities. However, PO elicited greater microvascular deoxygenation (deoxy-[Hb+Mb]), as assessed by near-infrared spectroscopy) compared to CON at all exercise intensities (p < 0.05), whereas the deoxy-[Hb+Mb] was only greater during PO compared to IO at the lowest exercise intensity tested (20% maximal voluntary contraction, MVC). Furthermore, IO resulted in greater deoxy-[Hb+Mb] compared to CON only at low exercise intensities (20% MVC, 40% MVC). In conclusion, although occlusion duration did significantly affect neuromuscular activation, BFR techniques influenced microvascular oxygenation the most during low-intensity exercise.
Many investigations have observed an increased neuromuscular activation with BFR resistance exercise [16-19], however, the peripheral responses (i.e., neuromuscular activation, microvascular oxygenation) to BFR cardiovascular exercise (i.e., cycling) has yet to be determined. Therefore, healthy subjects performed bouts of heavy (above estimated lactate threshold, >LT) constant cycling exercise with and without BFR. No difference in oxygen uptake (VO2) was observed (p > 0.05) despite a greater deoxy-[Hb+Mb] response during the beginning and end of BFR exercise compared to control (CON) exercise (p < 0.05). Unlike previous BFR resistance training investigations [16-19], BFR cycling exercise resulted in significantly lower neuromuscular activation during the end of exercise. Additionally each exercise condition elicited an increase in blood lactate concentration (from 20 watt baseline cycling to immediately post-exercise), however, plasma vascular endothelial growth factor receptor 2 was not significantly affected subsequent to any exercise condition. These results may suggest that the perturbation caused by BFR during low-intensity cycling exercise may have a greater localized affect within the exercising muscle, similar to previous investigations [20-23].
Lastly, healthy subjects completed a short duration BFR low-intensity interval training (BFR-LIIT) program on a cycle ergometer. The subjects performed 8-12 intervals at 40% VO2pk during six exercise sessions across two weeks. During the BFR-LIIT sessions continuous bilateral occlusion was applied to the proximal thigh at an occlusion pressure of 130% SBP. Significant increases in the estimated LT and knee extensor strength (isometric, eccentric) were observed following BFR-LIIT. However, no changes were detected in VO2pk and oxidative phosphorylation capacity at the level of the mitochondria (assessed from the phase II oxygen uptake time constant).
Collectively all of the investigations suggest that the perturbation induced by BFR techniques during cardiovascular exercise has a greater localized affect within the exercising musculature. Furthermore, we suggest that exercise volume is more heavily relied upon to induce significant training stimuli during BFR exercise since the exercise intensity is reduced. This could explain the lack of increase in VO2pk (3.3%) following BFR-LIIT as a low exercise volume (interval exercise, 2 weeks) was combined with low-intensity exercise. Therefore, the findings within this dissertation would not recommend the use of BFR during short duration (2 weeks), low volume (interval) exercise programs if the training objectives include significant peak cardiovascular adaptations (VO2pk). Future investigation into an appropriate dose response of BFR low-intensity exercise and exercise volume is required to explain previous reports of increases in VO2pk subsequent to BFR training [14, 15]. However, rapid improvements in muscle strength and sub-maximal aerobic capacity (estimated LT) were observed with BFR-LIIT that may have considerable applicability to certain populations.