The objective of this study was to quantify muscle strength losses in the arms and legs of all Skylab program crewmen by comparing preflight and postflight force and work measurements taken on an isokinetic dynamometer. The experiment instituted an initial minimum impact muscle function test, and as the missions demanded, added exercises and exercise devices and expanded the testing. This resulted in different exercise environments on each flight, such that there were actually three experiments, with the results of each flight affecting the next. The flights and countermeasures used inflight are described chronologically. Only aspects of skeletal muscle condition are addressed by this study, as cardiovascular aspects of conditioning and use of the bicycle ergometer are addressed in another Skylab experiment.
Evaluation of the right arm and leg was done preflight and postflight on all missions for each Skylab crewman with the Cybex Isokinetic Dynamometer. This dynamometer may be rotated in either direction without resistance until an adjustable limit speed is reached. Speed cannot be increased above this limit by forces of any magnitude. Muscle forces are continuously recorded. Various arms, handles and attachments may be fitted to the dynamometer to couple any desired segment of the body to the machine.
Ten maximum-effort, full-range flexions/extensions of the elbow and hip-knee at 45 degrees per second were recorded and evaluated for each crewman before flight and between 5 and 9 days postflight. A continuous force record was made of each repetition at a rate of 25 milliliters per second and the integral of force, or under these conditions, work, is recorded on a second channel.
A plot of peak force from preflight and postflight measurements was made. The strength for a given movement is taken as the average of 10 repetitions. Anthropometric measurements allowed computation of volume changes of limb segments.
During the Skylab 2 mission (28 days in duration), a bicycle ergometer and an isometric device were used for inflight exercise. Losses of strength and muscle mass, especially in leg antigravity groups, approached 25 percent. The arms suffered much less but also showed marked losses.
Additional exercise devices were launched and exercise time was sharply increased on the Skylab 3 mission (59 days in duration). Food intake was also increased. Good arm exercises and acceptable trunk exercises were provided by the MK I and MK II Exercisers, but loads and types of leg exercise were limited. This imbalance was reflected postflight by the lack of measurable loss in arm function. Leg function and muscle mass had improved relatively over the Skylab 2 mission, but large decreases continued to be apparent.
In addition to the other exercise devices previously described, a simulated treadmill consisting of a Teflon waking surface, a harness, and elastic bungees that provided 170 pounds equivalent weight, was used daily throughout the Skylab 4 mission (84 days). The Skylab 4 crew returned in unexpectedly good condition; only slight losses in muscle functions of arms or legs were measured. These reductions in muscle loss can be attributed to the added exercise devices and exercise time. There can be little doubt that the treadmill sharply reduced loss of leg strength and mass, since there was negligible increase in leg exercise with other devices on Skylab 4. Increased food intake may have also contributed to reduced loss of muscle mass.
There was a 6.5 to 9-fold reduction in rate-of-loss of leg extensor strength, leg volume, lean body mass and total body mass from Skylab 2 to Skylab 4. It is possible to argue that this reduction simply represents some kind of equilibrium with increasing mission duration, but this is not consistent with data that show absolute losses.
In conclusion, the investigator found that muscles, properly nourished and provided with a minimum of support, in this case physical stress, can adapt to weightlessness and retain function for return to 1-G. A properly designed treadmill used for considerably less than an hour per day inflight will not only protect leg and trunk musculature, but will also provide aerobic exercise for the cardiorespiratory system. It is not difficult to also add arm exercise such that the requirements for a single total body exerciser are met.
LaFevers EV. Evaluation of localized muscle fatigue using power spectral density analysis of the electromyogram. In: Human Factors Society, Annual Meeting, 18th, Huntsville, Ala., October 15-17, 1974. (A75-19676 07-54) Santa Monica, Calif., Human Factors Society, 1974, p. 278-283.