Physiological Mass Measurements in Skylab

                WILLIAM E. THORNTON AND JOHN ORD

NINE YEARS AGO while working on the Manned Orbiting Laboratory Project at the Aerospace Medical Division of the Air Force, we concluded that one of the first priorities in space medical research was to determine the cause and time course of the weight loss which always seemed to accompany space flight. It was obvious to us and to many others that a carefully controlled intake/output study with accurate daily mass measurements in-flight would be required. At that time, the insurmountable problem to such a study was the lack of an instrument for nongravimetric mass measurement. The first priority, then, was development of a mass-measurement device which did not depend on weight. Development was started and by 1966 we had built prototypes of the instruments flown on Skylab.As time went on, the Manned Orbiting Laboratory program had an unfortunate end, we had mass-measuring devices, and NASA had a planned in-flight balance study without a mass-measuring device so we formed a joint effort which was implemented on Skylab.

Gravimetric mass determination or weighing is such a simple and accurate process that no other methods have been developed or really needed since the Egyptians began using balances 5000 or more years ago. The only practical alternative to gravimetric attraction is some determination of the inertial property of mass. The method chosen to do this in 1965, and not necessarily the present method of choice, was the spring-mass oscillator constrained to linear motion.

                    Procedure

Theory.—Figure 19-1 is a functional illustration of the equipment and its motion. A sample mass is placed between two springs and constrained to linear motion in the longitudinal axes of the springs. If the mass is displaced from its rest position Xo to a new position "X" and mass assembly released, it will undergo essentially undamped natural oscillation at a frequency given by the well known relationship shown. If this period of oscillation is accurately measured by a high resolution timer, mass may be calculated. Rather than attempt a calculation based on machine quantities such as spring rates, a calibration which would have inevitable errors from gravitational effects, an in-flight calibration using precision masses was done.

Figure 19-2 is a plot showing a calibration record chosen at random from one of the small or specimen mass measuring devices used on Skylab and it simply shows that it follows the theoretical curve reasonably well. It really was chosen at random, for linearity is usually approximately 0.1 percent and normally no points can be found off the curve. With care and by using a modified calibration curve, accuracy of 0.01 percent, or better, can be obtained with solid masses.

This system is sensitive to any nonrigidity (slosh) in either sample or mounting and to any external or sample oscillation (jitter) if either of these effects are near the fundamental frequency of oscillation. Thus, in the case of some food, liquids, and the human body, special arrangements have to be made.

Two small instruments each with a capacity of 1 kilogram were flown—one was located in the Wardroom. All food was carefully weighed, analyzed, and identified preflight. Any package which was not totally consumed, and only six or so out of the thousands were not, was placed in the device and measured. A perforated elastic sheet holds the food package to it. Operation consists of turning the counter on, adjusting it to zero, and rotating and holding the lever which successively unlocks, displaces, and then releases the specimen tray.

The time for three periods of oscillation is then registered by the opto-electronic counter up to 10 seconds. This time is recorded and voice relayed to Earth where mass is calculated and suitable nutritional adjustments are made to meals for the next day.

There is a second and identical instrument in the Head on which all vomitus, of which there was only three or four samples, and all feces, collected in fecal bags, were measured. An on-board graphic conversion to mass measurement was made to allow proper setting of the fecal drying timers. All fecal samples were dried and returned to Earth in toto with recorded oscillation time periods for analyses.

Figure 19-3 shows a large or body mass measuring device with a capacity of 100 kilograms. A basal body mass was made by each crewman every morning after arising and voiding. The same type of clothing of known mass was worn each day and any extra objects were removed from the pockets. Although the human body is supposed to move as a single rigid structure below 1 cycle per second, this proved to be only approximately true; and it was necessary to reduce slosh to a minimum by folding the body into the most rigid configuration possible, and to reduce the period of 1 cycle of oscillation to 2 seconds. Straps are necessary under weightlessness to constrain the body to the seat.

The same timer and timing arrangement is used for both Body Mass and Specimen Mass-Measurement Devices. After strapping in, the seat is unlocked by cocking the displacement and release device on the large handle. The timer is turned on and the device is adjusted to zero. One takes a breath, holds it to avoid "jitter" and then releases the seat to oscillate by means of a trigger on the hand bar. After three cycles of timing has been completed, the period is recorded and later voice transmitted to Earth where mass is calculated, made part of the daily medical report and teletyped back to the crew.

Figure 19-4 is a record of the total uncorrected deviations of the Specimen Mass Measuring De-vice in the Head at the 50-gram calibration point. These points were taken over three missions as shown. Without going further into the engineering aspects, maximum error for food and vomitus samples, was less than 3 grams. Repeatability of body mass measurements was ±45 grams, and absolute accuracy was between +100 grams and +450 grams and probably nearer the lower figure. A number of hardware support measurements were made during the mission with excellent results: for example the 24-hour urine pools were measured to an accuracy of a few milliliters.

                   Rationale

Until Skylab, there was an unexplained loss of weight on every American astronaut except Alan Shepard on Apollo 14 and, so far as I know, in every Russian cosmonaut.

There are three common theories to account for these losses:

    Under weightlessness, fluid is shifted from the lower portions of the body to the chest area where it is sensed as an            excess and secreted by the kidneys in accord with the Gauer-Henry theory.

   At least a portion of the loss is sometimes thought to be metabolic since food quantities and opportunities to eat are          frequently minimal.

   Under certain conditions there are periods of high physical activity accompanied by heat and other stresses which can          result in rapid loss.

A comment may be in order: One often thinks of daily weights as a highly variable measurement, as indeed they are unless carefully made. But if they are carefully made under basal conditions and if the subject is on a controlled diet, losses of a fraction of a kilogram per week become not only detectable but significant. While a few grams loss or gain per week is normally of no importance, if they are continued for months, especially under conditions which can’t be altered, they become significant indeed.

Figures 19-5, 19-6, 19-7, 19-8, 19-9, 19-10, 19-11, 19-12, and 19-13 are the plots of Skylab crew body weights—preflight and postflight— from experiment M071 (ch. 18) and the in-flight equivalent weights measured with the Body Mass Measurement Device. These data have been smoothed by taking a 3-day sliding average. These plots cover the period that the crew were on the Skylab diet.

The plots shown in figures 19-5 and 19-6 are from the Commander and Pilot of Skylab 2; the Scientist Pilot (fig. 19-7) had a similar curve with a total loss between the two shown. Data for the first day were lost during vehicle repairs, and this was also a period of heat stress. One sees a loss which began with initiation of the diet and accelerated during the mission itself. The sharp dip in-flight was coincident with extravehicular activity. Immediately postflight, there was a transient increase in weight followed by a plateau. The predominant loss pattern of the first manned Skylab flight is consistent with that of a simple metabolic deficit.

While the losses were easily sustained in this short mission they could not be tolerated on missions of long duration. Even the 3.5 kilograms (7.7 pounds) loss of the Commander is significant in a small crewman who launched with a body fat of less than 10 percent.

On Skylab 3 both food and exercise were increased, and we see a different pattern. The Commander was relatively stable preflight, had a sharp loss for the first few days in-flight, and another loss near the end. On recovery, there was the usual increase and plateau or inflection point (fig. 19-8). The Pilot, had an almost identical curve (fig. 19-9). Remember, that these crewmen had nausea and were not eating properly the first few days, and that there was a period of increased activity, especially for the Pilot and Commander prior to entry. The Scientist Pilot had a sharp loss on exposure to weightlessness and a small continued loss in-flight consistent with a metabolic deficit and a typical recovery pattern (fig. 19-10). Here, I feel that we see two other loss mechanisms demonstrated.

From the time course of the losses and gains on orbital insertion and recovery, it seems reasonable to conclude that fluids are involved. At the same time, there are periods of increased stress, such as preparation for entry or extravehicular activity on Skylab 2 which temporarily exceeded caloric intake.

On Skylab 4, food and exercise was again in-creased, and we have the second American astronaut in space who lost essentially no body mass in-flight—the Commander (fig. 19-11). His profile shows a preflight gain, a small initial loss, and a postflight gain. His crewmen had losses similar to or smaller than the astronauts on Sky-lab 3 (figs. 19-12, 19-13).

We seem to have come full circle and have demonstrated that all three mechanisms originally proposed are operative. It would appear that the most significant on this mission was a simple metabolic loss. In further support of this, the average weight loss of all crewmen was plotted versus the normalized average caloric intake (fig. 19-14). The caloric data shown are the latest obtainable from the food section. Although the sample is small, the relationship seems clear, the three subjects off the "main line" relation were also the three crewmen with the smallest amount of body fat—all three well under 10 percent.

Caloric intake required for an extrapolated zero loss is extremely high indicating a surprisingly high in-flight metabolic cost.

It must be recognized that simply adding food to the diet is not the whole answer, for while this will assuage hunger and maintain mass, body muscle might be exchanged for fat. This closely related problem of exercise and conditioning is the subject of chapter 21.

The plots in figure 19-15 and Figure19-16 are 2-day sliding averages of crew mass from Skylab 3 and 4 for 10 days following insertion and recovery to demonstrate fluid losses. On Skylab 3, there was a sharp loss of 3 to 4 percent of body weight over the first 4 or 5 days following exposure to weightlessness. On return to one-g, there was an approximate reciprocal gain. On Skylab 4, we see the same pattern in one crewman; the crewman who was nauseated and not eating and drinking, just as had been the case with all three Skylab 3 crewmen. The other two crewmen showed a much less pronounced drop, and on recovery, there was a smaller reciprocal gain except for the Scientist Pilot. It is my suspicion that transient fluid losses or gains will be small, probably on the order of 1 percent in crewmen who eat and drink adequate amounts throughout the mission. This intriguing question of fluid loss and the Gauer-Henry theory will undoubtedly be further addressed by the appropriate investigators to show routes and mechanism of loss and gain.

                   Discussion

For the future; dietary standards must be revised to meet the metabolic requirements of given missions and tasks. In-flight studies of metabolic costs of realistic activities will allow better definition of overall requirements. The requirements on this mission with its tight, 14-hour day work schedule should not necessarily be considered typical of all missions.To those of you concerned with future planning; as long as man flies and make measurements in-flight, he will continue to need mass measurements. Although the present system met the requirements, they were complex, and the equipment was heavy, and expensive. I trust that they will not become the accepted standard, for in the 8 years since development of these devices, we have devised a number of other models with marked advantages over the spring/mass oscillator.

                    Summary

In summary, we have demonstrated a new instrument for in-flight space operations and research. We have also demonstrated the previously unproven mechanisms of weight losses under weightlessness. Most importantly, we have helped to prove that the human body properly fed can sustain missions of long duration without significant obligatory mass loss.

                                        Acknowledgments

Too many people have contributed to this project to list them all. A.G. Swan made the project possible by his unstinting financial and moral support especially in the early days of development. It would never have been demonstrated or flown without the superb model shop work and design contributions of the instrument shop at the USAF School of Aerospace Medicine which included Messrs. Garbich, Rosenblum, Wright, and above all McDougal. Dick Lorenz did an excellent electronic design for the prototype hardware as did William Oakey on the mechanical prototypes. Wray Fogwell suggested the flexure pivot as a simplification of the original design. Larry Dietlein, Wayland Hull, Paul LaChance, and Sherm Vinograd at NASA were instrumental in establishing mass measuring devices as experiments.

Flight hardware was constructed by Southwest Research Institute and the NASA project engineers were Vern Kerner and Ray McKinney.

 

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