Impact of altered muscle perfusion through lower body negative pressure on acute cardiovascular, physiological and molecular responses to resistive leg press exercise

Kurzfassung

Space flight is associated with various physical challenges, among them the loss of muscle mass and strength which particularly affects postural and lower limb muscles and is yet to be adequately tackled by countermeasures. Beyond the lack of the accustomed gravitational load, reduced blood supply through the headward fluid shift astronauts experience, resulting in lower availability of oxygen and nutrients, is another likely contributing factor to the deconditioning of lower limb muscles. Adequate solution for this issue might therefore be found in the combination of an appropriate form of exercise with lower body negative pressure (LBNP), which generates a fluid shift toward the lower limbs, and can thus counteract the microgravity-induced blood volume reduction in the lower limbs and produce physiological responses equivalent to orthostasis independently of body position or the presence of gravity. Combination of LBNP and exercise should expectedly produce musculoskeletal loads and the cardiovascular stimulus needed to maintain gravity-like adaptation and has been proposed as a countermeasure against deconditioning during long-duration space flight. Ground-based simulations of deconditioning related to space flight, such as bed rest, have shown benefits of combining LBNP and exercise for counteracting the loss of muscle strength and endurance. However, utilizing such countermeasures in long-duration space flight would require the corresponding equipment to be suitable for use under microgravity conditions and an exercise stimulus powerful enough to induce appropriate muscle responses. Resistance exercise is known to maintain the strength and prevent atrophy of chronically unloaded lower limb muscles, even showing potential for promoting their hypertrophy, and it has previously been suggested that an ideal countermeasure program for astronauts should include high-intensity concentric and eccentric exercise. In addition, adaptive responses to resistance training can be enhanced by reduced contraction velocity, and training with a constant load could not prevent isokinetic strength losses induced by bed rest. Present thesis addresses these issues by exploring the benefits of LBNP applied during intense, slow-paced resistance exercise with varying force on a novel, robotically controlled leg press (RCL) developed at the German Aerospace Center in Cologne, which operates without any gravity-dependent energy storage. It was hypothesized that LBNP would enhance the blood volume and oxygen availability in the leg muscles, consequently stimulating oxidative energy metabolism and potentially benefiting muscle energy levels and performance. These physiological effects were expected to be accompanied by acute molecular responses involving the energy sensor AMPK and angiogenic factors. Two studies with healthy male subjects were conducted within the present thesis. First study involved 9 subjects performing a series of 15 slow-paced concentric (4 s) and eccentric contractions (4 s) in a permutated crossover study design, without or with 40 mmHg LBNP and 4 s pause between repetitions. Beyond exploring the effects of LBNP, a further aim of this study was to investigate possible benefits of additional relaxation periods between repetitions for muscle perfusion. Exercise was performed with a varying load, starting with a force corresponding to 6% of the one-repetition maximum (1-RM) at knee flexion and gradually increasing to 60% of 1-RM in the first half of the individual range of motion, thereafter remaining constant until full extension. Non-invasive measurements were used to characterize acute cardiovascular responses and leg muscle oxygenation and performance. As continuous exercise proved to have more favorable effects, it was applied during a second, parallel group study with 18 subjects, additionally investigating acute molecular responses of the energy sensor AMPK in the vastus lateralis and circulating angiogenic factors.53 Results of the crossover study revealed that LBNP superimposed on exercise induces various acute physiological and cardiovascular responses, which are substantially influenced by the exercise mode (i.e., presence or absence of relaxation periods between contractions). Heart rate increase and stroke volume decrease were more apparent during continuous exercise under ambient pressure or LBNP compared to respective intermittent exercise. Furthermore, reduced cardiac output and elevated total peripheral resistance were only present with continuous exercise. LBNP facilitated the blood refill during low force periods of the contractions, detected as increase in the total hemoglobin and oxyhemoglobin content of the vastus lateralis, with a substantially higher margin between maximal levels under LBNP vs. control during continuous than during intermittent exercise. Elevated respiratory oxygen uptake and post-exercise lactate levels with LBNP indicated enhanced muscle energy turnover with possible contributions from oxidative as well as anaerobic metabolism. EMG amplitude increment was substantially higher during intermittent exercise with LBNP. In contrast, continuous exercise with LBNP showed a trend toward lower EMG amplitude increase compared to control, indicating possible benefits regarding muscle fatigue, which could potentially manifest during prolonged exercise. The outlined findings on continuous exercise were partially reproduced in the parallel group study, particularly regarding the increase in heart rate, oxygen uptake and total muscle hemoglobin. However, oxyhemoglobin levels were found to rather gradually decline during exercise with LBNP, accompanied by steadily rising deoxyhemoglobin content. Taken together with the elevated oxygen uptake and a trend toward lower post-exercise lactate levels, these findings indicated higher oxygen consumption and reliance on oxidative metabolism under LBNP. Higher EMG amplitude increment under LBNP nevertheless suggested insufficient energy provision and ensuing metabolically induced fatigue. LBNP enhanced the post-exercise reduction in MMP-2 and abolished the increase in endostatin as well as the reduction of P-AMPK, AMPK and their ratio observed under ambient pressure. Taken together, it was demonstrated that LBNP superimposed on continuous, slow-pace and highintensity leg press exercise enhances blood volume and oxygenation of the vastus lateralis, suggesting facilitated muscle perfusion. Furthermore, enhanced peripheral blood supply and higher oxygen exploitation due to LBNP modify the effects of intense resistance exercise, resulting in activation of the energy sensor AMPK and distinct regulation of angiogenic factors involved in muscle tissue remodeling and capillary growth. Adequate LBNP tolerance and exercise conditions addressing individual needs provided, combining resistance exercise with LBNP could stimulate oxidative energy provision and potentially facilitate muscle performance. LBNP might therefore promote beneficial structural adaptations of skeletal muscles during resistance exercise and contribute to astronauts achieving increased muscle strength and endurance with corresponding countermeasure activities during space flight. Continued research into simulated orthostasis remains of high significance for the optimization of countermeasures for muscle loss induced by space flight.