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Over the past 35 years, humans have been exposed to the environment of space for varying duration lasting from days to weeks to several months The unique effect of space is created when a spacecraft orbits the earth at a high velocity, creating a centrifugal force that virtually balances Earth’s gravitational pull The net effect is an environment in which there is almost no gravitational vector, hence the term microgravity
During the course of human exposure to this unique environment, a number of adaptations have been observed both inflight and post-flight These include, but are not limited to the following: inflight- vestubular and oculomotor disfunction, a reduction in the intrinsic strength and power output of skeletal muscle, bone atrophy, and changes in connective tissue function; post-flight- reductions in work capacity and aerobic metabolic potential during high intensity exercise, cardiovascular deconditioning, and balance and sensory motor disruption These deficits, both individually and in combination with one another, have had a significant negative impact on the functional and structural integrity of mammals, including humans, such that a major objective in both the NASA and Russian Space Programs has been to seek countermeasures to either fully ameliorate or minimize these deficits
Musculoskeletal Deconditioning during Space Flight
On Earth, the musculoskeletal system is continuously subjected to the effects of a constant gravitational pull When astronauts are exposed to the weightlessness of space flight, however, their reliance on leg musculature for movement and locomotion becomes practically nonexistent There is no need for active opposition to gravity or maintenance of customary terrestrial posture Very little muscular force is required by the upper extremities, which initiate most of the movements (Baldwin, 1996) A novel pattern of locomotion evolves that is appropriate and sufficient for directed movements in microgravity However, the functional load on the musculoskeletal system is insufficient to maintain normal physiological status of the skeletal and muscular systems, resulting in both significant structural, physiological and functional changes (Baldwin, 1996; Thomason et al, 1992; Bikle et al, 1997)
Muscular Deconditioning
The plasticity of human tissues allows them to adapt to the unique type of underloading, or disuse, that occurs during space flight (Booth and Gollnick, 1983; Stein and Gaprindahvili, 1994; Convertino, 1996; Convertino et al, 1997; Baldwin, 1996) Changes in skeletal muscle fibers are mainly manifested as a loss of muscle size, mass and volume, with a reduction in cross-sectional area occurring both in slow- and fast-twitch fibers (Baldwin, 1996; Tesch et al, 1990; Edgerton et al, 1995; LeBlanc et al, 1995) The degree of muscular atrophy and impaired muscle function is, however, heavily influenced by the function of the muscles on Earth, the duration of the flight, dietary intake, and the amount of exercise performed The postural muscles, or the weight-bearing muscles in the lower extremities, are the muscles that are affected the most from the absence of gravity (LeBlanc, 1995; Tesch et al, 1990; Dudley et al, 1992) Collectively, the alterations provide the underlying factors whereby individuals following prolonged space flight have reduced strength, power, and musculoskeletal endurance, as well as a reduced capacity of performing routine motor activities requiring both stability and fine motor skills (Desplanches, 1997)
Skeletal Deconditioning
On Earth, both gravity and the strain that muscular pull places on the skeleton are beneficiary stimuli in the regulation of protein synthesis and degradation the skeleton (Edgerton, 1996; Branca, 1999) In these terrestrial conditions, the postural muscles of the lower extremities and the trunk are continuously acting against the gravitational pull of the earth as a way of keeping balance in order to stay upright By doing so, they generate strain on the weight-bearing bones of the skeleton, thereby preserving the homeostasis of bone formation and bone resorption In weightlessness, however, not only is gravitational pull absent, there is also virtually no muscular activity necessary for postural reasons (Baldwin, 1996)
The increase in urinary calcium levels and the increase in collagen markers (Smith et al, 1998, 1999a, 1999b) indicates that the delicate balance between bone formation and resorption is disrupted and could become negative during space flight (Turner, 2000) Mineral losses are found particularly in the load-bearing sites (the lower extremities and lumbar vertebrae), while the upper extremities appear to be spared (Tipton and Hargens, 1996; Zernicke et al, 1990; Bikle et al, 1997)
Although the rate of bone loss appears to be somewhat less than the rate of the decrease in muscle mass in response to the unloading, urinary calcium levels remain above preflight levels for the entire duration of the flights Bone at these specific sites is lost at an estimated rate of one percent per month (LeBlanc et al, 1998; Smith et al, 1999) One major concern is that a loss of one standard deviation in bone mineral density gives rise to a twofold risk of spine fractures and a 25-fold increased risk in hip fractures (Lane et al, 2000) Other concerns include the possibility that once trabecular bone is lost, it may not be fully replaced (Garber et al, 2000); however, other investigators have stated that replacement is possible, but that recovery will require months to years (Bloomfield, 1997; LeBlanc et al, 1998)
Risk of Deconditioning
On the short flights of the Space Shuttle (<15 days), the relatively small deterioration or deconditioning of the muscoluskeletal system has not presented an immediate health or operational hazard to the crewmembers (Berg and Tesch, 1992) Space flights on board the ISS, on the other hand, will last up to 6 months Since bone loss and muscle atrophy are likely to continue throughout space flight, any reduction in musculoskeletal function, will theoretically impair the physical capabilities of the crew (Bloomfield, 1997; Edgerton et al, 1995) An integral task of some crewmembers on-board the ISS is the performance of extravehicular activities (EVA), required for building and maintaining the International Space Station (Powell et al, 1993) Sustained (upper body) muscular activity during an EVA is physically challenging for short-duration Shuttle missions, and could potentially be more challenging if the musculature is deconditioned during longer Space Station missions (Jennings and Bagian, 1996) Very high levels of muscular strength and endurance will probably not be required during routine work in and about the International Space Station, but sufficient performance capacity is needed in case of emergency situations, which may require high-intensity and/or long-duration activities
Even if physiological adaptations are not apparent during microgravity exposure, they could manifest themselves upon return to the gravitational challenge of Earth’s terrestrial environment The major concerns include osteoporotic fractures, decreased bone strength and decreased ability to maintain upright balance with sufficient flexibility (due to decreased muscle strength and performance) Muscular fitness, as well as bone strength, are necessary to ensure sufficient reserve capacity of the crew to optimize survival in case of emergency situations during re-entry and landing (Edgerton et al, 1995; Booth and Criswell, 1997) Unassisted emergency egress capability (post-landing) is required for shuttle bailout (LeBlanc et al, 1997) Crew members are expected to be able to egress the orbiter top windows, lower themselves to the ground with a rope "Sky Genie", and run or walk 1200ft from the potentially dangerous vehicle, all while wearing a hot and cumbersome escape suit (Jennings and Bagian, 1996; Lee et al, 2001) However, since these emergency situations have not manifested themselves, it is still uncertain as to what extent the physiological integrity of all astronauts during prolonged exposure to microgravity should be maintained
Countermeasures
The rationale for developing effective countermeasures for prolonged space flight should be based upon both operational and medical requirements designed to adequately address the health concerns of the crew (Bloomfield, 1997) Since the absence of gravity is thought to be responsible for the changes that lead to deconditioning, efforts to mitigate these changes have focused primarily on applying linear forces to the body, in an attempt to simulate Earth’s gravity (Convertino, 1996) The most direct approach would be to generate artificial gravity inside the spacecraft (Burton, 1997; Lackner et al, 2000) The gravitational field could be generated either passively by rotating the whole spacecraft, or a part of it Since this is not yet technically feasible, emphasis has been placed on generating an active force that’s simulates gravitational pull In the latter configuration, the astronauts would power a short-armed centrifuge that would create a centrifugal force (diPrampero et al, 1994 and 1997) Studies are currently being undertaken to test the feasibility of this concept
Other passive strategies that have been proposed as countermeasures to the physical deconditioning are pharmacological treatment, hormonal treatment, biochemical control, lower body negative pressure, and electrical muscle stimulation (Herbison and Talbot, 1985; references in Berg and Tesch, 1998) Unfortunately, no supportive data was found to demonstrate that any of these methods are sufficiently effective in maintaining muscle strength or mass during m g (Tesch and Berg, 1997) Due to ethical, technical, and economical issues, the US Space Program has not yet adopted these methodologies (Convertino, 1996)
Physical exercise to maintain the integrity of the musculoskeletal systems is well accepted in both the Russian and American Space Program (Convertino, 1996) However, in flight treadmill running, rowing, and cycling using a cycle ergometer primarily focus on maintaining aerobic capacity and possibly muscular endurance, with less attention given to preserving total musculoskeletal integrity (Convertino, 1990, Berg and Tesch, 1994 and1998, Tipton, 1983; Schwandt et al, 1991; Egerton et al, 1995)
In accordance with the concept of exercise specificity, exercise that simulates heavy resistance training, appears to be the most popular approach to increase musculoskeletal strength and power During this type of exercise, both bone and muscle tissue benefit simultaneously since the load placed on muscles also produces loads on the skeleton (Beachle and Earle, 2000; Turner, 2000; Chamay, 1972) Dynamic resistive exercise has been shown to promote significant increases in hip and spine bone mineral density (Colletti et al, 1989; Tsuzuku et al, 1998; Chilibeck et al, 1995; Tsuzuku et al, 1998), and hypertrophy (Tesch and Berg, 1990; Convertino, 1991), and muscular strength throughout the full range of motion (Dudley et al, 1991; Berg and Tesch, 1998) Since this type of training seems an effective way to induce muscle hypertrophy in a terrestrial setting, researchers have argued that this type of training should also be employed to combat muscle atrophy induced by long-term space flight (Tesch et al, 1990; Convertino, 1991; Kirby et al, 1992) Recently, the issue of strength training during space flight has been addressed by an integrated NASA task force (Task Force on Countermeasures Final report, 1997)
One way to study the effects of high-resistance strength training in microgravity is to simulate the effects of a microgravitational environment by means of bed rest In these bed rest studies, subjects are confined to a bed for a significant period in a 6 degrees head-down tilt Such studies (Bamman et al, 1997, 1998; Koryak, 1998; Ferrando et al, 1997) indeed confirm the potential benefits of resistance training during space flight in order to combat the adverse effects of long-term m g exposure
In ground-based studies, exercise protocols specifically aim at enhancing the musculoskeletal system by relying on constant gravitational pull, even when the exercises are not being performed Typically with this type of dynamic constant external resistance, or DCER exercise (Fleck and Kraemer, 1997), weight is raised and then lowered resisting against the gravitational pull With each repetition, this dynamic approach results in concentric and eccentric muscle actions, respectively (Berg and Tesch, 1998; Fleck and Kraemer, 1997) In exercise programs developed for space flight, where a gravitational "background strain" is absent, the capability of providing eccentric muscle actions is solely dependent upon the design and implementation of special equipment that can generate this type of resistance without relying upon a gravitational pull The non-terrestrial environment of space thus imposes unique challenges for the development of exercise equipment, since barbells or weight-stack machines obviously cannot be used during weightlessness It is therefore necessary to replace the gravitational force from a lifted weight (including body weight) with an alternative force or power source
The Houston division of Wyle Laboratories-Life Sciences and Systems supports the Medical Sciences Division at NASA’s Lyndon B Johnson Space Center in the development of countermeasures to combat the deleterious effects of weightlessness on the human body NASA tasked Wyle Laboratories to develop a Resistive Exercise Device (RED) that provides resistance for strength training on-board the International Space Station An interim RED (iRED) was developed and is currently being used for this purpose The device has been termed "interim" because it does not meet all of the scientific requirements for resistive exercise in space due to the limitation in power and space on-board the ISS The iRED has been designed to meet the minimal strength training needs of the crew, while keeping the device small and portable At later stages of ISS development, as logistics improve, the iRED will be replaced with more advanced devices
Aims of the Study
At this time, there are no ground-based data from human subjects to compare the effectiveness of training with this experimental weight training device to that of traditional free weights Therefore, the primary purposes of this study were as follows:
Changes in muscle mass due were determined by Magnetic Resonance Imaging (MRI) and Dual Energy X-ray Absorptiometry (DEXA) Changes in muscle strength were determined by means of one repetition maximum (1RM) sessions and by means of daily training records DEXA was also used to determine changes in bone mineral density (BMD)
The hypothesis is that training with an identical protocol on both the iRED and free weights would result in a smaller training response in the subjects training on the iRED when compared with the free weight group, due to the reduced eccentric loading provided by the iRED Furthermore, by increasing the total volume of work during training with iRED (6 training sets as apposed to 3 training sets), the effectiveness of the training program will be more similar to that of free weight training.