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Walking in Simulated Lunar Gravity WILLIAM LETKO AND AMOS A. SPADY, JR. Langley Research Center, NASA SUMMARY Experience in aircraft and in ground-based simulators indicates that man will be able to walk in lunar gravity with no apparent difficulty. The metabolic measurements reported herein indicate that the energy cost of locomotion in simulated lunar gravity is considerably less than in 1 g, as was found in earlier studies. The simulation technique used was unimportant for level walking, but generally had a large effect on the metabolic data obtained while ascending grades. Increases in the loads carried generally had a relatively small and inconsistent effect on metabolic costs. Changing the walking surface from a hard, smooth surface to one of sandy soil caused a large increase in the metabolic rate at the higher locomotion rate. INTRODUCTION Man is destined to play an increasingly im- portant and varied role in extraterrestrial activities, and for this reason his capabilities and requirements will have to be established to assure mission success. The need for adequate knowl- edge of man's capabilities was dramatically demonstrated by the Gemini flights. The severe energy demands of the attempted extravehicular activities were unexpected and resulted in a curtailment of the activities. Earth simulation of the Gemini mission provided a solution to the early problems and resulted in successful ex- travehi<hilar activities in later flights. So, too, Earth simulations must be depended upon to evaluate the demands of man's other anticipated space activities. One of the activities being studied by simula- tion is man's locomotive capability and the energy expenditure of self-locomotion in reduced gravity. Some studies of man's ability to walk in reduced gravity from 0.10 to 1.0 g have been studied in aircraft (refs. 1 and 2) and in ground-based simulators (ref. 3), for example. The imminence of the lunar mission has focused attention on man's capabilities in lunar gravity. A comprehensive study of locomotion in lunar gravity is being carried out, therefore, by Garrett AiResearch Corp. under contract with the NASA Langley Research Center. The inclined-plane simulation technique developed at Langley and the gimbal-vertical simulator of AiResearch are being used to simulate lunar gravity. In the study, man's locomotion char- acteristics and the metabolic cost of walking, running, and loping at velocities ranging from 2 to 12.8 km/hr are being determined. The effects of walkway-surface composition and grade, as well as effects of load carried, on the energy cost of locomotion have been determined for subjects wearing pressurized Gemini-4C suits. This paper reviews some of the results of the study. SIMULATION TECHNIQUES A number of simulation techniques, such as parabolic flight in aircraft, water immersion, the inclined-plane and gimbal-vertical suspension simulators, have been used to simulate reduced- gravity conditions. Each type of simulation has certain features and limitations which dictate its use for some types of activities and preclude its use for others. The inclined-plane and gimbal-vertical suspen- 347
348 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION sion simulators, used with a treadmill, have been found to be the most practical for the measure- ments of steady-state energy expenditure during walking, running, and loping in lunar gravity. The inclined-plane simulator (fig. 1) was de- veloped at Langley and is reported on in reference 4. The subject is supported on his side by a series of cables attached to an overhead trolley and monorail system. The monorail runs parallel with an inclined walkway upon which the subject can walk, run, and otherwise perform. The walkway is displaced from beneath the monorail so that the subject's long-body axis is inclined 9.6Â° from the horizontal, thus resulting in 1/6 g on the subject's feet. This suspension system, of course, restricts the subject's movements es- sentially to a single plane. This restriction does not appear to be serious, however. Subjects who have experienced 1/6 g walking both in air- craft with six degrees of freedom and in the inclined-plane simulator with three degrees of freedom have felt that the simulations are nearly identical. TROLLEY GANTRV STRUCTURED rYOKE ASSEMBLY WITH AIR PADS FIGURE 1. â The inclined-plane reduced-gravity simulator. The gimbal-vertical suspension simulator, as the name implies, supports the subject in an upright position (fig. 2), in contrast to the in- clined-plane simulator. A stalled turbine starter is used to support five-sixths of the sub- ject's weight for lunar-g simulation. The dynamic characteristics of this system were such that the simulation was degraded, especially FIGURE 2. âThe gimbal-vertical suspension simulator. for large up-and-down motions. For walking and running on the level, the vertical motions are relatively small and the simulation was not seriously affected. An important feature of the vertical simulator is that its configuration makes it readily adaptable for tests with a depositable soil surface. REDUCED-GRAVITY WALKING Both aircraft studies and ground-based simu- lations have indicated that man is capable of locomotion at g-levels as low as 0.10 g with no apparent difficulties. Aircraft, of course, pro- vide the proper g-level for the total human orga- nism, including the vestibular system, which cannot be achieved in Earth simulators. How- ever, its use is limited because the g-level can be maintained only for a very short time. As has been mentioned, experience in aircraft and that in ground-based, inclined-plane, 1/6-g simulation are subjectively very similar. Some vertical floor-reaction forces generated by one foot during walking in reduced g in an aircraft (ref. 2) and in a rotating-vehicle simulator (fig. 3) are compared in figure 4. In the rotating- vehicle simulator, a cable suspension system similar to that of the inclined-plane simulator was used. Figure 4 presents the ratio of the floor reaction force to the subject's Earth weight plotted against foot contact time. The data are presented for g-levels of about 0.17, 0.44, and 1.0.
WALKING IN SIMULATED LUNAR GRAVITY 349 FIGURE 3. â NASA-Langley rotating space station simulator. The figure shows that, generally, similar changes occur in the floor-reaction patterns with reduction in g-level for walking at low speeds in the aircraft and in the ground-based simulator. It should be pointed out that the walking speed was not closely controlled in these investigations, and for the data presented it varied from about 3 to 4 ft/sec. The data are presented to indicate general trends, and additional floor-reaction data, such as fore- and-aft shear and lateral shear, need to be com- pared for a more complete comparison of aircraft and ground-based simulator walking. These data were not available for the ground-based simulator. AIRCRAFT (REF. 21 GROUND SIMULATOR (FIG. 3) USING CABLE SUSPENSION OF SUBJECT 1.0 1.2 1.4 FIGURE 4. â Vertical floor-reaction force patterns for a single foot, low-speed walking. ENERGY EXPENDITURE As has been pointed out, the imminence of the lunar missions has resulted in an emphasis on the evaluation of walking in lunar g, especially the examination of the metabolic costs that will be incurred. The following data are concerned, therefore, with the energy costs of self-locomotion in simu- lated lunar gravity. The data are the average for six subjects, attired in G-4C suits pressurized to 3.7 psi. Figure 5 presents the metabolic costs of walking and running at rates from about 2 to 13 km/hr on a level, smooth, hard surface in simulated lunar gravity. Two sets of data, one obtained using the inclined-plane, and the other the gimbal-vertical suspension simulators, are presented. The results from both simulators show that the metabolic costs increase linearly with locomotive rate. The data from the gimbal- vertical simulator were generally only slightly higher than those obtained with the inclined plane. Thus, it appears that either technique would provide reasonably reliable results for locomotion on level, hard surfaces. METABOLIC RATE, kcal/rnin 6 8 10 LOCOMOTION RATE: km/hr FIGURE 5. â Comparison of metabolic rates for locomotion in 1 g and 1/6 g. For comparison purposes, also shown in figure 5, is the I-g metabolic cost for locomotion in the G-4C suit at rates up to 3 km/hr. The data indicate that the metabolic cost for l-g locomotion at 3 km/hr (highest that could be attained in 1 g) is twice that in 1/6 g. This result is, of course, similar to results obtained in earlier investiga- tions (ref. 4). The effect of grade on energy expenditure is
350 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION shown in figure 6 for two locomotive rates, 2 and 6 km/hr. Data obtained with the inclined-plane and with the gimbal-vertical suspension simu- lators are shown. Ascending a slope causes increases in the energy expenditure as expected. Ascending grades of 30Â° requires about twice the energy of level locomotion, as indicated by the inclined-plane results. The gimbal-vertical simulator results generally indicate substantially higher energy expenditures than those obtained with the inclined plane. Steady-state metabolic data were not obtained for the 30Â° slope in the vertical simulator, except at very low walking rates of 1 km/hr, because the subject's heart rate exceeded 180 beats/min before steady-state conditions could be obtained. The reason for the difference in energy expenditures obtained with the two simulators is uncertain to date, but may be established with the examination of additional data. INCLINED PLANE SIMULATOR METABOLIC RATE, kcal/fliin SIMULATOR - INCLINED PLANE -GIMBAL VERTICAL - 2 km/hr - I 1 i 7.5 15 30 GRADE, deg FIGURE 6. â Effect of grade on metabolic rate. The inclined-plane simulator was used to obtain metabolic costs of subjects carrying loads of 75, 240, and 400 Earth pounds while walking in simulated lunar g. This is equivalent to lunar weights of 12.5,40.0, and 66.6 pounds. The data are presented in figure 7 and show only a small and inconsistent effect of backpack weight for almost the entire range of locomotive rate used. A possible explanation for this is that the in- creased traction provided by the additional weights compensated for the cost of carrying these weights. METABOLIC RATE, kcsl min MOID 0 2 4 6 8 10 12 Id LOCOMOTION RATE, km hr FIGURE 1. âEffect of load carried on metabolic rate. Some preliminary data were obtained for sub- jects walking on a sandy soil deposited on the treadmill used with the gimbal-vertical simula- tor, as shown in figure 8. For comparison pur- poses, data obtained with a smooth, hard sur- face are also presented on the figure. The data show little effect of surface on energy expenditure at the low locomotive rates. Above 4 km/hr, however, the metabolic cost of walking on the sandy soil increases very rapidly, and at 8 km/hr the metabolic cost is about 1.5 times that for the smooth surface. This again points up the importance of simulating as closely as possible the conditions that will be encountered in extra- terrestrial missions. GIMBAL VERTICAL SIMULATOR METABOLIC RATE, kcal/ min /SANDY SOIL HARD SURFACE 2468 LOCOMOTION RATE, km/hr 10 FIGURE 8. â Effect of walkway surface characteristics on metabolic rate.
WALKING IN SIMULATED LUNAR GRAVITY 351 REFERENCES 1. ROBERTS, JAMES F.: Walking Responses Under Lunar and Low Gravity Conditions. AMRL-TDR-63-112. Aero- space Medical Research Laboratory, Wright-Patterson AFB, Nov. 1963. 2. BEEBE. D. E.: Force Analysis of Walking at Reduced Gravity. Thesis, School of Engineering of the Air Force Institute of Technology, Aug. 1964. 3. WORTZ. E. C.; AND PRESCOTT, E. J.: The Effects of Sub- DISCUSSION Tang: Based on your experiment, could you speculate whether a man carrying six times his own weight on the lunar surface will walk just as on the Earth? Letko: As I pointed out, 600 pounds of Earth weight would be 100 pounds on the Moon. Tang: I mean if he carries six times his body weight well distributed on his body and walks on the lunar surface, this would be the equivalent of walking on the Earth because as far as the weight on his feet is concerned, it would be the same. Six times his body weight on the lunar surface would be equivalent to his body weight on the Earth. Letko: I believe some experiments were made in a l/6-g simulator in which enough weight was added to the subject to bring him back to Earth weight and indicated that he could gravity Traction Simulation on the Energy Cost of Walking. Aerospace Med., vol. 37, Dec. 1966, pp. 1217-1222. 4. KUEHNEGGER, W.; ROTH. H. P.; AND THIEDE. F. C.: A Study of Man's Physical Capabilities on the Moon. Vol. Ill âWork Physiology Research Program. Doc. No. NSL 65-153. NASA CR-66119. Northrop Space Laboratories, 1965. walk, but the metabolic costs would be increased. Dietlein: How is the metabolic rate determined? Letko: By indirect calorimetry. Dietlein: Oxygen consumption plus CO2 production? Letko: Yes. Dietlein: You did not just measure oxygen? Letko: No. Dietlein: Do you have any data on the metabolic cost of a man righting himself if he fell down repeatedly, say, on the lunar surface? Letko: No, I do not, but we would like to conduct some experiments in which a total mission is simulated and deter- mine what the metabolic costs would be for the whole mission profile.