Changes in the Achilles Tendon Reflexes Following Skylab Missions

A generalized hyperreflexia was reported in the crewmembers of the first Skylab mission during the immediate postflight clinical evaluations (ref. 1). This finding supports earlier reports of an increase in reflex amplitude by Soviet researchers (ref. 2). To document possible neuromuscular changes, a decision was made to conduct measurements of the Achilles tendon reflex during the subsequent Skylab missions.

Various devices have been utilized to measure the Achilles reflex duration. Some of these devices are a light beam and photocell arrangement; capacitance changes and various types of mechanical transducers. Reported normal duration time for the Achilles reflex varies widely and depends heavily on the transducer type and interpretation of the data by the investigator. Nordyke, et al.,(ref 3) report normal reflex durations from 250 to 410 milliseconds while Bowley, et al., (ref. 4) report a normal range of 160 to 280 milliseconds. Such wide variation in reflex durations dictated that each crewman serve as his own control in the experiment.

The present study is an attempt to quantitate in time

1. any changes in the duration of the Achilles reflex relative to extended space flight, and

2. the duration of the muscle potential associated with the reflex.

Methods and Materials

One preflight test (F-5) and four postflight tests were obtained on Skylab 3 crewmen. The postflight tests were done immediately on recovery day and thereafter on a regular basis at the end of Lower Body Negative Pressure (ch. 29) experiments. Each one of the Skylab 4 crew-members participated in three preflight and six postflight tests. The schedules for Skylab 3 and 4 tests were as follows:

Skylab mission                  Preflight (day)                                  Postflight day)

       4          F-30, F-15, F-5    R+0, R+1, R+5, R+11, R+17, R+31

In a typical test session, the erect crewmember positioned his right knee on a firm support, with additional support as necessary, to achieve relaxation of the gastrocnemius muscle. A relative displacement transducer was firmly attached to the plantar bearing surface. Three electrode sites were prepared on the midsection of the gastrocnemius muscle to obtain muscle potentials, and silver electrodes of 2 centimeter area were fixed to these sites with a conducting gel. The Achilles tendon was struck several times as a warm up and to check the gain setting of the recorders. The signals from both the displacement transducer and the electrodes were amplified and recorded simultaneously on strip chart and FM magnetic tape. To elicit reproducible and well inscribed tendon reflexes, the Achilles tendon was struck every 2 seconds for 30 seconds with a percussion hammer. No reinforcement maneuver was used to augment the reflex.

Data Analysis.—For each experiment, an average number of 12 complexes on strip chart were analyzed. The Achilles reflex duration was measured from the initial stroking of the tendon until all movement had ceased. This method of determining duration should detect change occurring in the contraction and/or relaxation phases of the reflex. The muscle potential interval for each reflex was measured from the beginning of the mechanical upstroke to the point of greatest amplitude of the muscle potential spike (fig. 15-1).

For the Skylab 3 mission the mean and standard deviation were computed for both the Achilles reflex duration and the muscle potential interval. Student’s t-test was used to determine if any postflight data was significantly different from the preflight data.

For the Skylab 4 mission fiducial limits for the normal were calculated since a preflight baseline consisting of three separate test values was available.

Results

The results of the single preflight and the four postflight tests for the Skylab 3 mission are presented in figure 15-2. The duration of the Achilles reflex immediately postflight showed a significant (P<0.01) shortening for all three crewmen. The reflex duration exhibited further significant shortening on the fourth day after recovery. At the 16^th postflight day there was a significant (P<0.01) lengthening of the reflex for the Scientist Pilot and Pilot while the Commander showed lengthening which was not quite statistically significant. By 29 days post recovery, the reflex duration of the Commander had essentially returned to its preflight value. However, the Scientist Pilot and Pilot continued to show a significant lengthening of the reflex duration with a suggested trend toward their preflight values.

The results for the Skylab 4 mission are presented in figure 15-3. The Commander showed an initial shortening of his reflex time that was within his preflight baseline. By the 5-day post-flight test there was a significant lengthening of his reflexes well outside the fiducial limits of his baseline testing. In subsequent tests the Commander’s reflex time decreased slowly until he was well within his baseline values by the 31-day postflight test.

The Scientist Pilot presented reflex times shorter than his baseline limits on recovery day. The reflex time lengthened on day 1 and by day 5 postflight it had increased to the point of being greater than his baseline limits. Subsequent testing showed an oscillating reflex time which by the 31-day postflight test had returned to within baseline limits.

The Pilot showed an immediate decrease in reflex time on recovery day. This condition lasted through the day 5 test. At the postflight day 11 test there was a significant lengthening of reflex times. In subsequent tests the Pilot’s reflex times decreased until he was within his baseline limits by day 31 postflight.

The muscle electrical component of the reflex for the Skylab 3 mission proved to be difficult to obtain on the Scientist Pilot and Pilot. However, a full set of data was obtained on the Commander (fig. 15-4). The course of the postflight muscle potential interval paralleled the Commander’s reflex duration, i.e., the first two tests showed a shortening of the time interval while the last two tests showed a slowly increasing time interval not quite reaching his preflight value. Despite spotty data, the other two crewmembers also showed a similar response in muscle potential intervals.

The electrical component of the reflex still proved difficult to obtain on the crewmembers of the Skylab 4 mission. No data were collected on the Pilot but fairly complete data were obtained on the Commander and Scientist Pilot. Inspection of the muscle potential intervals shows good agreement, i.e., when the reflex time increased the muscle potential interval increased and vice versa (fig. 15-5).

Discussion

The six Skylab crewmen tested and some of the Cosmonauts have exhibited similar findings. These findings include a peculiar gait upon return to the one-g field; muscular soreness and weakness; overcompensation in movements while in a vertical position and changes in the reflexes. While a fully satisfactory explanation for these neuromuscular findings is still lacking, several factors have been implicated.

Dealing specifically with the alterations in reflex times it might be postulated that an imbalance is present between the postural muscle groups. These muscle groups specifically support the body against the pull of gravity and if not adequately exercised might be expected to undergo selective relative disuse atrophy in a weightless environment. After return to the gravitational field of Earth following partial or total acclimatization to weightlessness, the sudden burden imposed on these muscles could possibly result in a state of disequilibrium between the flexor and extensor groups. This gravity stressor may be implicated in the altered reflex durations seen in the Skylab 3 and 4 crewmen. Interestingly, the reflex duration had approximately returned to preflight values at the time muscular soreness disappeared and the gait had returned to normal.

Another causal possibility is that of interaction between biochemical and hormonal factors and reflex duration. In the second manned Skylab mission the postflight thyroxin and epinephrine values were reported to be slightly elevated (ch. 23) which conceivably could account for some of the changes seen in reflex duration. An increase in both calcium and potassium was noted the first 2 days postflight and there was a transient increase in the concentration of ionized serum calcium (ref. 5). It seems unlikely, however, that these biochemical and humoral changes would affect the reflex durations for the time periods seen here.

It is interesting to observe that the crews of the Skylab 4 mission seemed perhaps slightly less affected, as regards reflex durations, by their increased time in space. There is also the added factor of progressively increased exercise regimens, both on the bicycle ergometer and the other devices (minigym Skylab 3 and 4, "Treadmill" Skylab 4).

Conclusion

The best explanation for the changes in reflex duration is perhaps related to the servofeedback system of the postural muscles themselves. These muscles, after weeks of inactivity and a loss of mass, must suddenly resume upright support of the body in a one-g environment, with an attendant strain and stretch in these muscles resulting in an over stimulation of the neuromuscular system causing the initial decrease in reflex duration. As the muscles regain strength and mass (refs. 6,7) there occurs an over compensation reflected by the increased reflex duration. Finally, when a normal neuromuscular state is reached the reflex duration returns to baseline value. In general the available data seem to support this proposed hypothesis.

Acknowledgments

The authors wish to express their appreciation to John Donaldson for his aid in electronics and to Mary Taylor for manuscript preparation.

References

1. Ross, C.E., and J.R. HORDINSKY. Crew health; skylab ½ R + 21 day interim report, pp. 26-59. NASA, JSC 08706, 1973.

2. CHEREPAKHIN, M.A., and V.I. PERUUSHIN. Medical support and principal results of examination of the "Soyuz-9" spaceship crew. Kosmicheskaya Biologiya i Meditsina, 4:34-41, 1970. (Russian)

3. NORDYKE, R.A. Long term follow up of patients treated for thyrotoxicosis using Achilles reflex times. Hormone, 1:36-45, 1970.

4. BOWLEY, A.R. A new simple detector for Achilles reflex measurements. Medical and Biological Engineering, 9:351-357, 1971.

5. LEACH, C.S., P.C. JOHNSON, and P. RAMBAUT. Biochemistry Fluid Electrolyte Results: Second Skylab Mission (SLIII). Presented at the Aerospace Medical Association, Washington, D.C., May 1974.

6. THORNTON, W., G.W. HOFFLER, and J. RUMMEL. Anthropometric and Functional Change: Skylab 3 R + 21 Day Interim Report, pp. 119-155, NASA, JSC 08504, 1973.

7. JOHNSON, R.L., and G.W. HOFFLER. Experiment M092. Lower Body Negative Pressure; Skylab 3 R + 21 Day Interim Report, pp. 312-351. NASA, JSC 08504, 1973.

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