The performance of in-flight exercise is the principal countermeasure of both the Russian and the American spaceprograms (Grigoriev et al., 1992; Davis et al., 1996). It is believed to be beneficial to the musculo-skeletal, cardiovascular, and immune systems, thereby preventing, at least partially, muscle atrophy, osteoporosis, orthostatic intolerance, vestibular disturbances and the deterioration of immunological parameters. The performance of in-flight exercises enables space-travellers to be capable of performing their duties during the flight, to be able to meet unexpected emergency situations, and to be able to function normally and without any extra risk after their return.

Musculo-skeletal problems are most likely to be the limiting factor during long-duration spaceflight (Bachl et al., 1993). In the previous two chapters, the spaceflight-related problems in muscle tissue and bone tissue were discussed, together with the present knowledge of exercises that are able to enhance these two types of tissue. In this chapter, first the current and some planned in-flight exercise devices will be presented, then published exercise protocols aimed at long-term spaceflight will be discussed, resulting in a new description of an exercise protocol that should be performed during long-term spaceflight.

In this paragraph several ways of exercises that have been used during spaceflight will be discussed and some proposed exercises will be presented. The different exercises can be divided into resistance exercises, ergometers and dynamometers.

Several resistance exercises have been performed during spaceflight. Isometric exercises are easiest to perform in space, and are prescribed during long-term spaceflights (Convertino, 1990; Bachl et al., 1993). Other resistance exercise devices, called "expanders" or "extensors", are generally made of springs, elastics or rope and pulley machines. A relatively simple device is the "Mini Gym", also known as "Exer Gym" or MK-1, that was used during the Skylab III and IV flights (see figure 2). Consisting of only a spring and a capstan, this device allows for isokinetic exercises of several muscle groups, also eliciting some eccentric contractions (Sawin et al., 1975; Bachl et al., 1993; Convertino, 1996). According to Convertino, the "Mini Gym" allows for high mechanical loading on muscles and bones. Exercises performed on other expanders and similar devices mainly train endurance, as these become unpleasant to use at levels required to induce high impacts (Convertino, 1996). However, the exact loads produced by the "Mini Gym" have never been tested. Until this has occurred, the effectivity of the "Mini Gym" remains in doubt.

The Russian spaceprogram also uses expanders and some resistance pulling device (Convertino, 1990). Another special feature of the Russian program is the TNK V-1, or "Penguin suit" (see figure 3). This is a costume with rubber bands woven into the fabric, extending from the shoulders to the hip and further to the feet. This applies a constant resistance to the muscles involved, which also allows for eccentric contraction. The axial loads a "Penguin suit" can provide is reported to be either 50 per cent of body weight on earth (Rambaut & Goode, 1985; Convertino, 1990) or 70 per cent (Bachl et al., 1993;

Convertino, 1996), but the exact loads have never been tested (Convertino, 1996). The cosmonauts wear the suit for eight to twelve hours a day, and also during the performance of any exercises.

Schwandt et al. (1991) presented two new concepts of resistive exercise devices that have not yet been used in space. The first device is based on dynamic inter-limb resistance, which can only be used for the legs. If one leg extends concentrically, the force that is produced is transmitted through a rope and pulley to the other leg, which contracts eccentrically resisting the force. The eccentric force is therefore limited by the strength of the contracting muscles. The second device allows for multiple resistive exercises. A full series of strength exercises can be performed against a pneumatic resistance with the subject lying or sitting in a work station.

Both these devices are expected to be beneficial during spaceflight, although the multiple resistive exercise device might be too large and too heavy to be used in space (Convertino, 1996). However, the eccentric contractions invoked by the dynamic inter-limb resistance device cannot be expected to elicit the changes in the muscle that are normally associated with eccentric contractions. The effectiveness of these type of contractions are probably due to the greater forces that can be produced (see paragraph 3.2.4). With the inter-limb resistance device, however, the magnitude of the eccentric forces is limited by those of the concentric forces. Therefore, this device does not offer any extra benefits compared to any regular device that allows for concentric exercises.

During spaceflight, both treadmills and cycle ergometers are often used. Rowing ergometers have been proposed as a favourable exercise device, as rowing involves whole body exercise without a gravitational component (Cavanagh et al., 1992; Castellsaguer, 1995). However, so far the equipment has been considered to be too heavy and too large to justify inclusion in the in-flight exercise programs (Bachl et al., 1993).

Cycle ergometers have been used as exercise devices for both the arms and legs, since the Salyut and Skylab flights in the early 1970s (Bachl et al., 1993). The present ergometer used in the Space Shuttle is designed by the European Space Agency (ESA) and allows for a power production of zero to 350 Watts with a pedalling frequency of 40 to 115 rotations per minute. To keep the exercising person in place, the person is supported by a restraining body harness (Convertino, 1996). Although a cycle ergometer is an efficient device to maintain aerobic capacity during spaceflight, it is of less importance for the musculo-skeletal system. Muscle atrophy is far better prevented by resistance exercises (Whittle, 1979), and cycling has no effects on BMD (Snow-Harter & Marcus, 1991).

As mentioned in paragraph 4.2.1, artificial gravity can be generated by continuous rotations, but this is energetically very costly. However, short daily exposures to some vertical force could be able to maintain the musculo-skeletal system during spaceflight (Vernikos et al., 1996). Creating artificial gravity by human muscle power is possible with only two bicycles (Convertino, 1996). The two exercising persons would circle around while cycling, which

could place a load equal to 1G on the body. The problem of coriolis forces still remains, but preliminary testing failed to invoke motion sickness problems (Convertino, 1996).

Treadmill exercises are believed to have the most benefits for both muscles and bones of the legs under weightlessness, because it places the highest possible loads on the human body and is also accompanied by eccentric muscle contractions, despite the lack of gravitational forces (Bachl et al., 1993). Treadmill exercise in space is also very effective because the tethering system ensures that besides the legs, all postural muscles are used for body stabilization. Posture keeping exercises are thought to increase the effectivity of training (Castellsaguer, 1995; Convertino, 1996).

To increase the loads on the body, and also to keep the exercising person on the treadmill, the exercising person is tethered to the treadmill with elastic straps, or bungee cords (figure 4). These straps apply mechanical loads on the exercising person of 60 to 70 per cent of terrestrial values (Hargens, 1994). Tightening the elastic straps can even produce mechanical loads equal to 2G (Convertino, 1996), but it has been reported that this is accompanied by extreme discomfort where the bungee cords compress the shoulder and pelvic regions (Hargens, 1994).

Treadmills that are used during spaceflight are either active (motor-driven) or passive (human powered). The first is in use by the Russian space agency, the second by NASA (Hargens, 1994; Convertino, 1996; Davis et al., 1996). The two types of treadmills produce the same maximal ground reaction force (GRF-max) but the active treadmill produces higher rates of change in force (dF/dt) than the passive treadmill, which might be accompanied by a greater osteogenic response (Davis et al., 1996). NASA uses the passive treadmill because it is light and small, but if dF/dt is found to be the most important stimulus for osteogenesis, an active treadmill must be used (Davis et al., 1996).

A final proposed modification of a treadmill, is the "Grasim" treadmill, which is a part of the European Manned Space Infrastructure scenario (Bachl et al., 1993). The "Grasim" is a circular platform. Once one leaves the centre of the platform, this is detected by sensors in the shoes and under the platform and the gravity one is subjected to will increase. No specifications were given by Bachl et al., but based on an accompanying picture, this increase in gravitational loading is presumably done by elastic straps, which elicits the same problems of discomfort as reported with the bungee cords attached to a normal treadmill.

Since October 1991, the first valid dynamometer that gives reproducible results is operative in space, namely in the space station Mir (Bachl et al., 1993). It is the Austrian made "Motomir", which can be used as both a training and an diagnostic device (Bachl et al., 1993; Baron et al., 1994). The "Motomir" allows for both arm & leg dynamometry and for both concentric and eccentric contractions, making it the perfect resistance training device. Presently, the ESA is also designing a dynamometer, which is set to fly in the Space Shuttle in 1998 (Castellsaguer, 1995; Castellsaguer, 1996). This dynamometer, named the Muscle Atrophy Research and Exercise System, or MARES, is also meant to be both a strength measurement device and a training device. MARES shall cover all possible motions of the wrist, elbow, shoulder, ankle, knee, hip and trunk, both as single-joint movements and as multi-joint movements. The training facility includes cycle ergometry, making the standard cycle ergometer superfluous, which is favourable from an operational point of view.

While the use of exercise protocols has been advocated for decades, the exact contents of such a protocol has been under debate for the same period of time. The few proposed exercise protocols that have been published in the readily available scientific literature, indicate that the views on what would be the most efficient protocol (with efficiency defined as eliciting the greatest responses in the least amount of time) are rapidly changing. Proposed protocols have been published by Tipton (1983b), Greenleaf et al. (1989) and Bachl et al. (1993). These propositions differ in muscles that should be exercised, the type of muscle contractions involved, and ergometers that should be used. These proposed protocols will be discussed in this paragraph.

Tipton (1983b) recommended high-intensity, physical impact exercises to enhance both muscle tissue and bone tissue. During short-term flights (shorter than three weeks), isometric and isokinetic exercises should be performed by the anti-gravity muscles. During long-term flights, the frequency of the performance of the exercises should be increased, and all major muscle groups should be trained. Cycle ergometer exercise was recommended for cardiovascular training, and treadmill exercise was only recommended "to fulfil the desire for movement that is present in most astronauts and cosmonauts".

Greenleaf et al. (1989) prescribed daily isotonic and isokinetic exercises for both the arms and the legs. The different in-flight duties of the crewmembers dictate which limbs should be emphasized while performing these exercises. The pilots, for example, should train their legs, while the people engaging in extra-vehicular activities should mainly train their arms. In addition to these resistive exercises, 30 minutes a day of cycle ergometry was recommended, at 70 to 100 per cent of ones maximum (whether this is maximal power output or oxygen consumption was not stated). These exercise prescriptions were meant for 15 to 180 days stays in microgravity.

Finally, Bachl et al. (1993) proposed daily concentric exercises of both the arms and the legs, at 60 to 80 per cent of maximal voluntary contraction. Expander exercises should be performed daily also, as well as twenty minutes of walking and running on a treadmill. The exercise prescription of Bachl and his group was meant for the Russian space station Mir, so a daily eight hour period of wearing the "Penguin suit" is also included. Once a week, the arms and legs should perform maximal exercises, including concentric, eccentric and isometric contractions. In the weeks preceding extra-vehicular activities or re-entry, extra exercises should be performed to be able to sustain the accompanying increased workloads.

None of these protocols fulfil all requirements of efficient in-flight exercise training for the musculo-skeletal system, as discussed in the paragraphs 3.3 and 4.4. Greenleaf et al. (1989) included only exercises of the arms and legs, while the muscles in the back are also affected (Convertino, 1990). Tipton (1983b) and Greenleaf et al. (1989) did not include eccentric contractions, while Bachl et al. (1993) recommends this type of contractions only once a week. Recent studies have indicated that eccentric contractions should be included in every training session, in order to elicit the greatest effects on muscle tissue (Berg & Tesch, 1994; Hortobagyi et al., 1996).

In addition, relatively few attention is given to losses in bone mineral density, despite the expectation of Tipton (1983b) that this would probably create the greatest problems during long-term spaceflights. Based on the tibial BMD data of one cosmonaut, which was reported to show "no decrease" after a 175 day flight, Bachl et al. (1993) expect that the exercise program during that flight elicited sufficient high-impact loads to eliminate any bone losses. Obviously, this conclusion is rather presumptuous, being based on only a single anatomical site of one subject. But terrestrial studies indicate that sufficient high-impact loading to stabilize or even increase BMD is possible (Chilibeck et al., 1995; Swezey, 1996). The challenge is to translate this kind of exercises into effective exercises in a microgravity environment.

Of all ergometers, the inclusion of a treadmill in an exercise protocol is most favourable, as it elicits the highest forces on the legs (see paragraph 5.1.2). However, the protocol as proposed by Bachl et al. (1993) was the only protocol that included this type of exercises.

Recent findings have made clear that vigorous exercise plays an important role in enhancing bone mass. The same kind of exercise also has strength enhancing effects on muscle tissue. Thus, it should be possible to create one specific exercise regime to battle the degenerative processes of the musculo-skeletal system. Such a regime should be minimally time consuming, but still effective to maintain the musculo-skeletal system at a desirable level, so crewmembers are able to participate in extra-vehicular activities and any emergency contingencies during flight and landing, and are not detrimented in their activities of daily life after return to earth. In order to achieve this, earth based research has shown that prescribed exercises should probably include intermittent, high peak strain (or high rate of change in strain) producing activities.

The similarities between muscle and bone tissue are not surprising. Bone mass and muscle mass have often been shown to correlate with each other (Schoutens et al., 1989; Gutin & Kasper, 1992; Chilibeck et al., 1995). This is thought to be caused by the mechanical pull of the muscle applied to the bone (Schoutens et al., 1989; Snow-Harter & Marcus, 1991). Therefore, it is not surprising that strength training and training associated with high impacts have both a muscle tissue and a bone tissue enhancing effect (Chilibeck et al., 1995). It is therefore expected, that these kinds of exercises are able to counteract both muscle atrophy and osteoporosis during spaceflight.

High resistance training, with sufficiently high impacts, can probably be performed under weightlessness using a "Mini Gym" or workstation-like device, as described by Schwandt et al., (1991). The exact magnitudes of the loads that these devices provide should be determined, so the effectivity of exercises performed on these devices will be beyond any doubt. Tethered treadmill running, especially with the application of LBNP, also seems to be promising to elicit the desired high impacts. Until it has been determined whether dF/dt or GRF-max is primarily responsible for the osteogenic responses to exercise, an active treadmill would be favourable above a passive one. Creating artificial gravity by two cyclists seems possible in a cycle ergometer, but is a very complex way of creating the same loading on the musculo-skeletal system as LBNP-treadmill running does.

Treadmill running mainly exercises the legs and places loads on the spine. For upper body exercises, resistance exercises could be beneficial. To create these exercises, the newly developed dynamometers are very useful. These allow for maximal eccentric contractions of various muscles and muscle groups, also of the lower body. The MARES, as being developed at ESA in The Netherlands also allows for cycle ergometry, which is beneficial both for the psychological distraction and for the occasional aerobic exercises. For the same reasons a rowing ergometer would be advisable, despite it’s size and weight. While performing exercises on these sophisticated devices, it should not be forgotten that the highest losses in muscle strength were associated with isometric exercises. These are the easiest exercises to perform, but just as important as exercise bouts on a dynamometer.

Longitudinal studies have shown that a strength training protocol of three days a week at 60 per cent of one repetition maximum until exhaustion increases BMD (Chilibeck et al., 1995). However, these studies were performed on earth, where a 1G strain on all weight bearing bones is continuous. Although the common posture that one attains under weightlessness does stretch some muscles (Dudley et al., 1992), thereby posing some strain on the skeleton, the quantification of these inherent loads are unknown (Rabin et al., 1993), but probably minimal. Whether a protocol that proved to be efficient on earth is also effective without such a minimal "back ground strain" is questionable.

A "back ground strain" might be possible to achieve during spaceflight by the use of "Penguin suits". Although the loads it applies to the body are not similar to 1G, they can still be sufficient to maintain some osteogenic activity. Research should be performed to determine the exact loads the "Penguin suit" generates, and whether these loads have some bone or tissue enhancing effect. Wearing "Penguin suits" while exercising adds extra impact to the loading of the musculo-skeletal system Although the loads of a "Penguin suit" have never exactly been quantified, it seems a very important exercise countermeasure during long-term spaceflight.

A major confounding factor in the determination of an in-flight exercise protocol based on the present scientifical knowledge, is the ignorance of the exact loads that in-flight exercise devices place on the spacetravellers. Once the magnitudes of the loads that devices like the "Mini Gym" and "Penguin suit" place on people are quantified, research can be performed directly whether these loads are sufficient to elicit physiological changes during long-term spaceflights.

Appropriate exercise protocols do seem to be a very effective tool in counteracting the degrading effects on the musculo-skeletal system under sheer weightlessness. But until it’s use has been fully accepted and understood, other precautions should also be considered before engaging in long-term spaceflight. An important precaution would be to achieve an appropriate pre-flight training status for future spacetravellers. The Russian space agency assigns full-time trainers to work individually with each future cosmonaut (Jennings & Bagian, 1996). It would be advisable for future spacetravellers to engage in high impact exercises long before their spaceflight starts. These kind of exercises increase BMD and a higher BMD at the beginning of spaceflight is favourable. Also, the muscle strength enhancing exercises that are planned during spaceflight should also be performed in the weeks before launch. This way, the "neural phase" of strength training is likely to have passed, and the muscle enhancing effects of these strengthening exercises are effective from the first day of performance of these exercises.

Sufficient strain can be produced by both strenuous aerobic and strength training, but also by squeezing a simple tennis ball. Which regimen is the most effective, is unknown. The results of studies that have included combinations of different regimens hint that a combination might be most effective, but this has not been proven yet (Gutin & Kasper, 1992). In any case, in-flight exercise protocols can only be described in general terms, as the precise individual exercises are different between each individual, depending on their abilities and in-flight work requirements.

The requirements of an efficient exercise protocol as discussed in this literature study are based both on facts and on currently accepted theories. As previously mentioned, the ideas about the contents of an in-flight exercise protocol during long-term spacemissions rapidly change over time. It would be presumptuous to believe that the proposed exercise protocol could be a final draft. However, the assumed effectiveness of the proposed exercises is based on strong evidence, and it is believed that long-duration spaceflights, with a duration of up to three years, are physiologically possible, when these exercises are adhered to.