The bones of the lower extremity are particularly at risk during spaceflight (21,41). Countermeasures to bone demineralization in space not only are relevant to long-term missions but also are essential for the successful return to daily activities in 1g. For crew members spending 4 to 6 months on the International Space Station, decreases in areal bone mineral density (BMD) have been shown to reach rates of 0.9%·month−1 at the spine and 1.5%·month−1 at the hip. The loss in spine integral volumetric BMD has been documented to be at a rate of 0.9%·month−1. At the hip, integral, cortical, and trabecular volumetric BMD have been lost at rates up to 1.5%·month−1, 0.5%·month−1, and 2.7%·month−1, respectively (15). These losses are substantially higher than those associated with postmenopausal bone loss on Earth where, for instance, declines of 2.6% in spine trabecular volumetric bone mineral density may be expected over an entire year (22).
A variety of measures have been suggested and tried in an attempt to counteract bone loss induced by either spaceflight or extended bed rest. Broadly speaking, these countermeasures can be classified as addressing either the cause (i.e., weightlessness) or the effect (bone demineralization). Examples of the former include lower body negative pressure devices (10) that attempt to reintroduce some of the effects of gravity. Anabolic hormones, thiazide diuretics, and nutritional supplements and other pharmaceuticals (2) are examples of attempts to prevent demineralization. These countermeasures generally have shown limited potential in overcoming the problem of bone loss. Of added concern is the fact that, in some astronauts/cosmonauts, there may be only a 50% restoration of bone after 9 months of reentry into a 1g environment (32).
The benefits of exercising the cardiovascular and musculoskeletal systems are accepted widely on Earth. Such studies also have indicated that a relatively small number of appropriate weight loading can help delay or reverse bone and muscle deterioration. For instance, a series of studies performed by personnel in the Bone Research Laboratory at Oregon State University showed impressive increases in BMD during exercises that involved increased impact forces (either through jumping-type exercises or through the use of weighted vests during 1g locomotion): (i) jumping at ground reaction forces of eight times body weight caused significant increases in hip and lumbar spine bone mass in prepubescent children (8), (ii) premenopausal women who trained three times per week by completing 100 jumps and 100 repetitions of lower body resistance had significantly higher greater trochanteric BMD than control subjects (39), (iii) plyometric jump training in adolescent girls resulted in a significant increase in bone mineral content of the greater trochanter (3.1% vs 1.9%) versus control subjects (40), and (iv) long-term exercise using weighted vests prevented hip bone loss in postmenopausal women (33).
Given these successes of Snow’s group (33) in showing the benefits of plyometric-type exercises to bone health in subjects who range in age from children to postmenopausal women, it is tempting to speculate that countermeasure devices for astronauts should include impact loading. However, this approach is beset with issues that range from the ideal “dosing” of exercise in microgravity (including saturation of bone’s osteogenic potential and possible enhancement provided by insertion of rest periods (11) to limitations on vibrations that can be transmitted to the surrounding space structure (18).
Challenges With Exercise in Microgravity
The need for exercise devices (Fig. 1) was realized from the earliest National Aeronautics and Space Administration (NASA) missions. The Mark I exerciser was added for the second and third Skylab missions and used for a number of arm and leg exercises. It consisted of a rope that was wound around a drum that would rotate as the crew member applied tension to the rope. An asbestos brake restrained the rotation and thereby added to the level of exertion required to perform the exercise. Advantages of this system were its small mass and size: 5.49 kg including the two handles and an overall envelope of approximately 20 × 27 × 20 cm (17). Problems included (i) the polybenzimidazole rope, originally selected for its flame retardant characteristics, was proven to be weak and required replacement by a nylon rope and (ii) both the original and spare recoil springs broke after approximately 60 d of use (17). These types of mechanical failures were an issue on many subsequent orbital missions.
If a device as simple as the Mark 1 exerciser was prone to technical difficulties, then it is not surprising that more recent devices (with thousands of moving parts) have proven difficult to maintain. Logs kept by the International Space Station crew for Expedition 1 (31) make mention of “lots of little hardware problems” faced by crew members using the Treadmill Vibration Isolation System (TVIS), including complications with the user interface, a broken keypad not responding to inputs, problems with starting and restarting the belt, the machine not recognizing the PC cards used to store individual data, broken wires between the isolator bars and the TVIS, rivnuts found spinning freely once the outer cover was removed, cracking slats leading to eventual mechanical failure, belt tensioners being too tight, washers being of different sizes, chafing on the bungee sheaths, and a broken slat that put the machine out of order.
The situation with the cycle ergometer was equally problematic (31): a wrong connector put the machine out of order for more than a month, electronics were unreliable, the ergometer was stuck at maximum resistance, and original fasteners and screws were very difficult to remove.
These challenges not only detract from the quality of the exercise that crew members can achieve but also encroach on the time available for exercise. If one system experiences technical difficulties, usage of another system can increase; however, in some cases, there were simultaneous concerns (34) with all the major exercise systems: (i) the Cycle Ergometer with Vibration Isolation and Stabilization System (hard disk failure), (ii) the Treadmill Vibration and Isolation System (operations restricted to speeds less than 6 to 7 mph), and (iii) the Interim Resistive Exercise Device (canister supplies approaching estimated end of life).
General engineering challenges that must be taken into account when designing exercise devices for astronauts include the size of the device and the loads it transfers to the surrounding space vehicle. While the first of these does not have a large effect on current manned space missions, the size and mass of exercise equipment are major issues when space for food and medical supplies is of critical importance (e.g., in a mission to Mars where supplies cannot be replenished). In terms of vibration isolation, the engineering challenges are significant; for instance, a transient frequency of 0.4 Hz can be associated only with accelerations less than 0.8μg; at 10 Hz, these accelerations caused by forces must be less than 50μg (18).
The most obvious challenges facing planners of exercise countermeasure systems include (i) there are severe limitations on collecting appropriate data during space missions, (ii) the duration of most NASA missions has been less than 1 month, (iii) study sample sizes are very small, and (iv) all 1g simulations of weightless conditions have limitations on the degree to which they mimic true microgravity. Bed rest often is used as a 0g analog, and even in this situation, there are problems with subject compliance and/or maintaining consistent protocols (14).
While resistive exercise during bed rest has been shown to prevent bed rest-induced muscle atrophy (1), it seems to be more difficult to combat bed rest-induced bone loss in the same way. The best results so far have been achieved in a study by Shackelford et al. (30), in which resistive exercise was able to maintain bone mass in the calcaneus and in the lumbar spine during 17 wk of bed rest. However, there seemed to be still some bone loss at some regions of the hip (e.g., trochanter), a region that is of key importance to humans at risk for falling.
Opportunities Related to Exercise in Microgravity
Lanyon (16) emphasizes that load bearing is an important, “if not the most important,” functional influence on bone mass and architecture. Mechanical strains applied to bone in daily life and exercise act as stimuli to the normal bone remodeling response (19). Strain rate also has been shown to affect the remodeling response (13,29). More specifically, it has been suggested that a certain range of activity is needed to maintain bone density (the “dead zone” or “lazy zone”) (20). This range must be exceeded for bone density to increase, while falling below the range results in bone loss. Collectively, these findings offer opportunities for designers of exercise countermeasure systems.
Work done by Whalen and Carter (38) quantified the relationship between “bone health” and biomechanical loading as follows:
where S is a measure of benefit to bones, N denotes the number of cycles of loading the bone experiences, Q is the peak force applied to the bone, and M is a constant unique to each bone.
Genc et al. (11) have modified this equation by including provisions for loading caused by simply standing upright in a 1g gravitational environment. While their approach is more inclusive in terms of accounting for biomechanical loading, there remain opportunities for future investigators who wish to examine the relationship between cumulative loading on Earth and bone health in different anatomical locations.
Whether or not the effects of static “exercises” such as standing are included in models of bone health, it is clear that activities with a high peak force, such as running or jumping, are likely to be more beneficial for bone than exercises with lower peak forces, such as cycling or rowing (6). The engineering opportunity here is to design a system for creating these forces while simultaneously keeping the size, number of moving parts, and mass low. One potential system (Fig. 2) is the “Dynamic Exercise Countermeasure Device” (DECD), designed to fit within a 18 × 10 × 10-inch volume when collapsed and stowed.
There are two methods of operating the DECD, one intended for resistive exercise, the other intended for aerobic exercise. In the first method, intended for resistive exercise, a spring is attached between the baseplate and the handlebar. The user then does a squatting exercise; the lower body is strengthened by opposing the tension in the spring while the upper body is strengthened by working to hold the user in place. The second phase of exercise targets aerobic exercise. The spring is disconnected from the baseplate and handlebar, and the user repeatedly makes a jumping action, rapidly extending his or her legs as if to jump. The jumping motion provides aerobic exercise, but furthermore, it also supplies an impact force between the kickplate and the baseplate. When the user makes a jumping motion and his or her legs reach full extension, the momentum of the kickplate carries it further down the guide wire, and the spring connecting the two plates causes the kickplate to recoil and results in an impact force that is transmitted to the lower extremity. In this configuration, no impact forces are transmitted to the surrounding space vehicle structure — these forces are all “self-contained” between the astronaut’s feet and the baseplate.
A mathematical model was used to predict forces under the feet. This initially was based on a spring-mass system; however, in this kind of model, it is theoretically possible to get nonphysiological leg velocities if the mass of the kick- and baseplates is too low. For this reason, the model was modified to obey force versus leg extension speed described by Vandervoort et al. (35), where the applied force (AF) must satisfy the following:
The input for the model was based on work done by Vrijkotte (36). She collected data for the subjects jumping in 1g and in simulated zero gravity and showed a sinusoidal-force-versus-time relationship for the push-off phase. The peak AF was 1,550 N, and the duration lasted 0.4 s. These force input values were used until such time that the predicted velocities (of the feet pushing against the base- and kickplates) exceeded Vandervoort’s criteria, at which time the force was adjusted accordingly. This approach allowed for the determination of kinetic energy of the masses and thereby peak forces when the base- and kickplates impacted each other (Fig. 3).
A large body of data has been amassed regarding the ability of high-impact exercise to have a positive effect on the BMD in the lower extremities (7). More recently, various studies have examined resistive exercises that are coupled with vibration (4) (Table).Together with findings from groups using plyometric exercise protocols, these studies provide a compelling argument for incorporating impact forces into any exercise countermeasure protocol for long-duration missions in a microgravity environment. While there are likely to be many approaches for achieving high-impact forces, the DECD (Fig. 2) is a simple compact system that is predicted to result in impact forces in excess of 5,000 N. These force magnitudes compare well with studies showing osteogenic benefits of impact forces that are eight times body weight (8).
The challenges with using any mechanical system, as demonstrated by virtually every device developed thus far, relate to technical failures, need for spare parts, and time required for maintaining exercise equipment. From an operational point of view, there are size and mass limitations and technical issues with isolating vibrations from the surrounding spacecraft.
With this variety of challenges, there are obviously a range of research opportunities. These include (i) obtaining a more comprehensive understanding of the relationship between cumulative skeletal loading and bone mass, (ii) parsing out the effects caused by low-frequency high-impact loads versus high-frequency low-magnitude forces, (iii) developing universal exercise systems that benefit multiple physiological systems (bone/muscle/cardiovascular), (iv) developing subject-specific models that predict who will derive optimal benefit from the limited exercise options available to crew members, and (v) understanding which findings obtained in 1g using 0g analogs are applicable to true microgravity environments.
Funding from the National Space Biomedical Research Institute for the design of a DECD is acknowledged.
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