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An overview of the issues: physiological effects of bed rest and restricted physical activity


Section Editor(s): Convertino, Victor A. Writing Group Chair

Medicine & Science in Sports & Exercise: February 1997 - Volume 29 - Issue 2 - p 187-190
Basic Sciences: Symposium: Physiological Effects of Bed Rest and Restricted Physical Activity: an Update

Reduction of exercise capacity with confinement to bed rest is well recognized. Underlying physiological mechanisms include dramatic reductions in maximal stroke volume, cardiac output, and oxygen uptake. However, bed rest by itself does not appear to contribute to cardiac dysfunction. Increased muscle fatigue is associated with reduced muscle blood flow, red cell volume, capillarization, and oxidative enzymes. Loss of muscle mass and bone density may be reflected by reduced muscle strength and higher risk for injury to bones and joints. The resultant deconditioning caused by bed rest can be independent of the primary disease and physically debilitating in patients who attempt to reambulate to normal active living and working. A challenge to clinicians and health care specialists has been the identification of appropriate and effective methods to restore physical capacity of patients during or after restricted physical activity associated with prolonged bed rest. The examination of physiological responses to bed rest deconditioning and exercise training in healthy subjects has provided significant information to develop effective rehabilitation treatments. The successful application of acute exercise to enhance orthostatic stability, daily endurance exercise to maintain aerobic capacity, or specific resistance exercises to maintain musculoskeletal integrity rather than the use of surgical, pharmacological, and other medical treatments for clinical conditions has been enhanced by investigation and understanding of underlying mechanisms that distinguish physical deconditioning from the disease. This symposium presents an overview of cardiovascular and musculoskeletal deconditioning associated with reduced physical work capacity following prolonged bed rest and exercise training regimens that have proven successful in ameliorating or reversing these adverse effects.

Submitted for publication February 1995.

Accepted for publication December 1995.

Address for correspondence: Victor A. Convertino, Ph.D., Physiology Research Branch, Clinical Sciences Division, AL/AOCY, 2507 Kennedy Circle, Brooks Air Force Base, TX 78235-5117

Physiology Research Branch, Clinical Sciences Division, Brooks Air Force Base, TX 78235; Department of Health and Kinesiology, Texas A & M University, College Station, TX 77843; and Laboratory for Human Environmental Physiology, Life Sciences Division, NASA, Ames Research Center, Moffett Field, CA 94035

Historically, the research emphasis in exercise physiology has been placed on the study of physiological mechanisms underlying limitations of physical work performance and adaptations associated with increasing work capacity through exposure to exercise training regimens. Indeed, if exercise is repeated by ambulatory subjects for weeks or months, the capacity to perform work is increased while general strain on body systems is diminished. For instance, when exercising at the same absolute work rate following endurance training, heart rate and blood lactate are lower while stroke volume, cardiac output, blood volume, and muscle blood flow are increased compared with the pretrained state. These responses to exercise after training are manifestations of increased physiological capacity to perform physical work. However, less attention has been given to the lower end of the activity continuum, i.e., to the deconditioning induced by chronically-reduced activity and its implications for rehabilitation in diseased patients. Since it is reasonable that alterations in physiological responses to exercise following deconditioning should be opposite of those observed during training, understanding mechanisms of exercise training in ambulatory subjects has contributed significantly to providing direction for investigation of the physiological adaptation associated with chronic exposure to bed rest with many practical applications for health maintenance and for athletic performance.

The adverse effect of prolonged bed rest deconditioning in humans, with its accompanying physical inactivity, was known to Hippocrates who reported loss of strength and exercise performance after prolonged bed rest and inactivity(7). Few well-designed bed rest studies were performed prior to World War II which stimulated interest in the possible adverse effect of inactivity and deconditioning in the treatment of hospitalized patients(21,35). Shortly after the war, Taylor et al.(46,47), Deitrick et al. (18), and Whedon et al. (48) conducted their now classic human bed rest studies concerning blood volume, work performance, and metabolic function. With the advent of manned space flight in the early 1960's, there was significant stimulation of research using prolonged bed rest where some physiological responses to microgravity acclimatization could be simulated(12,27,32,38,41). In the recumbent position, there is loss of most hydrostatic pressure in the vasculature below the heart, virtual elimination of longitudinal compression on the spine and long bones of the lower extremity, reduced muscular force on virtually all bones, and reduced total energy utilization (i.e., reduced exercise). Confinement to bed rest may also lead to socio-psychological changes if patients or subjects move into new isolated surroundings and associate with different people in a more psychologically stressful environment.

Confinement to bed is not an uncommon treatment for patients with chronic disease or injury. The resultant physiological adaptation includes diminution in work capacity(3,4,6,8-10,13-15,17-19,22,25,29,30,42,43,45,47) associated with impairment of cardiovascular(3,9,10,14-15,17,34,36,39,42-45,47), hematologic (10,11,26), musculoskeletal(1,2,5,10,23-25,33,37), metabolic (4,18,42,45), thermoregulatory(16,31), immune (28), neuroendocrine and psychological (19) functions. Health care professionals may perceive the diminution in physical work capacity in chronically ill patients as a result of their infirmity; thus exercise is not commonly used for patients as an integral part of their treatment. However, a reasonable explanation for the reduced work capacity in patients is absence of regular daily physical activity that can present the physiological challenge necessary to maintain the capacity for many normal life functions upon reambulation. This bed rest-induced debilitation is apparent when comparing functioning of the trained athlete, who performs moderate to high intensity work with little strain, with that of the individual who has been confined to bed and cannot sustain even a minimal work output. Maximal exercise testing provides a tool to quantify bed rest deconditioning by assessing maximal capacity of physiological mechanisms underlying the most inherent functional response of the body - human movement.

Measurement of physiological responses to exercise in healthy subjects allows for distinction of deconditioning induced by bed rest to be separated from effects induced by the infirmity. Assumption of the horizontal body position provides an opportunity to examine the impact of reduced hydrostatic pressure gradient within the cardiovascular system, unloading of force on the muscles and bones, and lower energy utilization on physiological reserve capacity. Although most nonclinical investigations have involved young, healthy subjects 18-25 yr old(6,8,15,26,31,33,39,43,45,47), bed rest experiments conducted over the past decade have included older subjects and longer durations of bed rest similar to those of patients recovering from long-term illness(5,13,14,17,25,34,36,37). It is apparent that bed rest deconditioning, independent of disease effects, contributes significantly to the reduced reserve capacity to perform physical work that may be largely obviated by regular physical activity during periods of relative immobility. For example, reduction in resting and exercise stroke volume and cardiac output during bed rest occurs without any apparent pathologic change in ventricular function (12,36). Likewise, loss of skeletal muscle mass and subsequent strength reduction occur during bed rest in healthy subjects with normal functional force-velocity relationships (12). The premise that bed rest deconditioning can be partly explained independent of disease underscores the importance of early intermittent ambulation and physical activity that might significantly limit many debilitating effects during hospital confinement.

In addition to the clinical application of findings from bed rest experiments, these results from deconditioining have implications for understanding mechanisms of adaptation to sedentary life styles. Although bed rest does not completely model sedentary living habits, many physiological changes that occur during bed rest are qualitatively similar to those observed during detraining (40,44). Thus, bed rest may be a helpful short-term research tool for examining deleterious effects of sedentary living on exercise performance and for developing and testing the effectiveness of exercise countermeasures. Such countermeasures have included the successful application during bed rest of acute exercise to enhance blood pressure regulation and orthostatic stability (20), daily endurance exercise to maintain aerobic capacity(11,12,25,39), specific resistance exericses to maintain the integrity of musculoskeletal structure and function(1,24,25,33), and post-bed rest exercise to enhance recovery (17,43).

The following reports provide a history and update of physiological consequences of prolonged bed rest confinement. Convertino presents an overview of cardiovascular function with special emphasis on maximal oxygen uptake and functional working capacity. Bloomfield discusses structural alteration in the musculoskeletal system during bed rest and its impact on body function. Finally, Greenleaf provides data from an extensive experiment with evidence that specific high-intensity exercise training interventions during bed rest can attenuate many adverse effects of deconditioning. Emphasis of these papers is placed on the time course and magnitude of physiological changes; the degree to which these physiological changes can be prevented, counteracted, or attenuated; specific interventions (countermeasures) applied in an experiment that proved effective for attenuating or reversing the changes; and the implications of superimposed chronic illness or injury.

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