Physiological and biomechanical responses to blood flow restricted walking in young adults

Thomas P Walden

Applying blood flow restriction (BFR) during low- to moderate-intensity walking can increase cardio-respiratory and muscular adaptations compared to unrestricted walking at the same speed. Such adaptations may convey additional health benefits typically restricted to aerobic exercise at higher intensities, such as jogging. However, due to safety concerns around extended periods of occlusion, prescription guidelines for BFR-walking utilise reduced exercise times (~10 min/session) or an interval-based approach to accumulate time spent within the BFR condition. When implemented, both methods see exercise programs prescribed for durations below the recommended weekly minimums for low-intensity (150 min). One way to reduce weekly exercise duration without compromising the associated chronic benefits is to increase the prescribed intensity (e.g., vigorous-intensity [75 minutes weekly]). For BFR to be considered an appropriate exercise modality to prescribe at a reduced weekly exercise duration, it must first provide similar acute physiological responses to a vigorous-intensity exercise such as jogging at similar or lower levels of perceived exertion and discomfort. Study one compared the acute cardiovascular, respiratory, and perceptual responses of BFR-walking to Jogging. Participants' comfortable walking and jogging speeds were determined during the familiarisation session using an RPE-based method. Exercise intensity was then prescribed at comfortable walking speed (6.0 ± 0.3 km∙h-1 [slow walk]), 120% of Comfortable walking speed (7.2 ± 0.3 km∙h-1 [fast walk]) and comfortable jogging speed (9.1 ± 0.7 km∙h-1). Both walking conditions were performed with (60% arterial occlusion pressure) and without BFR. All experimental sessions required participants to exercise for 10min. Heart rate (+8.0%, p<0.01) and left cardiac work index (+5.0%, p<0.01), oxygen consumption (+31.1%, p<0.01), minute ventilation (+19.2%, p<0.01) and respiratory rate (+7.8%, p<0.01) were all higher during the jogging sessions compared to BFR-walking sessions of both intensities. Notably, perceived exertion (+55%, p<0.01) and discomfort (+354%, p<0.01) ratings were higher during the BFR sessions compared to all unrestricted sessions (walking and jogging). As BFR-walking at either intensity was unable to induce similar acute physiological responses to jogging while eliciting higher perceptual responses, BFR-walking cannot be considered an effective alternative for jogging in active young individuals. Due to consistent discrepancies in the ventilatory data collected during study one, it was apparent that additional analyses were warranted before future research implements large doses of BFR-walking across a training program. More specifically, study two explored measures of fraction of expired oxygen and carbon dioxide collected during study one to identify differences between the BFR and unrestricted walking sessions. The fraction of expired oxygen and carbon dioxide are of interest as they underpin predictive calculations done by the metabolic cart. For example, a metabolic cart calculates variables such as respiratory exchange ratio and oxygen consumption under the assumption that carbon dioxide formed during energy metabolism is efficiently returned to the lungs. As the application of BFR reduces total systemic blood flow, these calculations could become compromised. During BFR sessions, exhaled carbon dioxide was lower (-5.5%; p<0.01) and exhaled oxygen was higher (+1.4%; p<0.01) compared to unrestricted sessions. As carbon dioxide produced during energy metabolism within the lower limbs is not returned to the lungs, all calculated measures will not accurately represent the amount of oxygen consumed during aerobic BFR exercise. Therefore, any measure calculated using indirect calorimetry during aerobic BFR exercise should be interpreted with caution. Anecdotal observations during study one suggested changes in gait kinematics during BFR-walking sessions, which had not been previously reported in the literature. Therefore, study three investigated the potential BFR-induced kinematic changes during walking and accompanying acute physiological, metabolic, and perceptual responses. Walking speeds were prescribed as a percentage of each individual's walk-to-run transition speed calculated during the familiarisation sessions (70% walk-to-run transition speed [5.0 ± 0.3 km∙h-1] and 90% walk-to-run transition speed [6.4 ± 0.4 km∙h-1]). During the BFR sessions, participants were asked to walk for as long as was comfortable, with sessions capped at 20min. Unrestricted sessions of the equivalent intensity were matched for time. Participants' gait kinematics were recorded during the final 10s of each minute. Participants were blinded to exercise duration, the session cap and when the recordings were taking place. Anterior trunk flexion (+37.0%, p<0.01) and knee flexion angle during stance phase (+16.8%, p<0.01) of BFR walking were increased relative to the unrestricted condition. In contrast, plantar flexion angle during push-off (-10.0%, p<0.01) and ankle joint velocity (-3.8%, p<0.01) were lower during BFR sessions. Heart rate (≥7.9%, p≤0.01), minute ventilation (≥10.2%, p≤0.01) and exercise-related sensations of exertion (≥33.1%, p≤0.01) and discomfort (≥98.6%, p≤0.01) were higher during BFR sessions, and blood lactate concentrations (+26.3%, p=0.019) were higher post-BFR sessions compared to the unrestricted equivalent, regardless of exercise intensity. Overall, changes in gait kinematics are evident during BFR-walking and changes which occur across the duration of the exercise period could be influenced by a combination of acute physiological, metabolic, and perceptual responses elicited by the application of BFR. The implementation of compensatory strategies during a BFR-walking task is suggestive of an increase in muscular fatigue. Practitioners should consider their patient's capacity to adopt compensatory strategies during BFR-walking and the biomechanical cues that herald on the onset of BFR-mediated fatigue. In conclusion, this thesis presents information highlighting the need for BFR-walking to be prescribed at a fast pace (20% above one’s comfortable walking speed) to provide additional cardiovascular, respiratory, and metabolic responses compared to the equivalent unrestricted walking. However, the compromise for implementing BFR-walking at a fast speed was a large increase in exercise-related sensations of exertion and discomfort, high systolic blood pressure and left cardiac work index, and multiple changes in walking technique. In addition, the efficacy of measures calculated by the metabolic cart, which play a critical role in the development of the current BFR prescription guidelines, are negatively impacted when exercising with BFR. The combination of findings discussed throughout this thesis suggests that BFR-walking is not a stand-alone modality that can replace a higher-intensity exercise such as jogging. Whether BFR-walking provides additional benefits when supplemented into a training program that consists of numerous exercise modalities such as unrestricted weightlifting or high-intensity interval training, requires further research. Furthermore, the increases in exercise-related sensations of exertion and discomfort, along with the changes in walking biomechanics, may limit the applicability of BFR-walking to some populations and requires consideration when adopting BFR in an older clinical population.

Physiological and biomechanical responses to blood flow restricted walking in young adults

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