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Depression, Mood State, and Back Pain During Microgravity Simulated by Bed Rest

From the Life Science Division (J.R.S., K.H., A.R.H.), NASA Ames Research Center, Moffet Field, CA; and Institute of Psychology (S.G.C.), University of Göteborg, Göteborg, Sweden.

Address reprint requests to: Jorma R. Styf, MD, Department of Orthopaedics, Sahlgrenska University Hospital, Sahlgrenska, Guldhedsg 19, 413 46 Göteborg, Sweden. Email: jorma.styf{at}orthop.gu.se

	ABSTRACT

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OBJECTIVE: The objective of this study was to develop a ground-based model for spinal adaption to microgravity and to study the effects of spinal adaptation on depression, mood state, and pain intensity.

METHODS: We investigated back pain, mood state, and depression in six subjects, all of whom were exposed to microgravity, simulated by two forms of bed rest, for 3 days. One form consisted of bed rest with 6 degrees of head-down tilt and balanced traction, and the other consisted of horizontal bed rest. Subjects had a 2-week period of recovery between the studies. The effects of bed rest on pain intensity in the lower back, depression, and mood state were investigated.

RESULTS: Subjects experienced significantly more intense lower back pain, lower hemisphere abdominal pain, headache, and leg pain during head-down tilt bed rest. They had higher scores on the Beck Depression Inventory (ie, were more depressed) and significantly lower scores on the activity scale of the Bond-Lader questionnaire.

CONCLUSIONS: Bed rest with 6 degrees of head-down tilt may be a better experimental model than horizontal bed rest for inducing the pain and psychosomatic reactions experienced in microgravity. Head-down tilt with balanced traction may be a useful method to induce low back pain, mood changes, and altered self-rated activity level in bed rest studies.

Key Words: depression, • mood, • lower back pain, • lower abdominal pain, • leg pain, • bed rest.

Abbreviations: HBR = horizontal bed rest;; HDT = head-down tilt;; NASA = National Aeronautics and Space Administration.

	INTRODUCTION

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Back pain is one of the most frequently occurring medical problems during space flight (1, 2). It has been reported by 68% of astronauts (3). The duration of pain varies from 14% to 100% of the flight, which may seriously jeopardize a crew member’s performance on orbit. Many microgravity-induced responses in humans, including total body height increase and back pain, have been studied in simulation using 6 degrees of head-down tilt (HDT) (4–7).

Bed rest exposes humans to restricted mobility. It has been shown that HDT with balanced traction is a better method than horizontal bed rest (HBR) to induce back pain in healthy subjects (7). The purpose of this study was to evaluate the effects of 3 days of 6 degrees HDT with balanced traction and HBR on depression, mood state, and pain intensity in the lower back, lower abdomen, and lower extremities.

	MATERIALS AND METHODS

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Subjects
Six healthy male subjects (mean age = 38 years, range = 33–43 years; height = 181 cm, range = 175–188 cm; weight = 84 kg, range = 70–99 kg) volunteered to participate in the study, which was conducted at the Human Research Facility at NASA Ames Research Center. Informed, written consent was obtained from each subject. Screening medical examinations documented that the subjects were healthy. Subjects were randomly assigned to two groups (three in each group). Each group underwent two forms of bed rest for 3 days. Group I underwent HBR followed by 6 degrees of HDT in balanced traction. At the same time, group II underwent 6 degrees of HDT in balanced traction followed by HBR. Subjects in both groups had a 2-week period of recovery. The study was approved by the Human Research Institutional Review Board at NASA Ames Research Center.

Experimental Setup
Two kinds of bed rest were investigated, HDT with balanced traction (7) and HBR. The traction load applied to the subject’s legs was similar to the arrangement that may be used in clinical settings for acute treatment of patients with hip fractures. Each subject had boots attached to both legs. Balanced traction along the spine was obtained by applying a load corresponding to 5% of the body weight on each leg. The load on the cables, which were attached to the boots, counteracted the compressive forces that act on the spine in this position and prevented the subject from sliding down the bed. The subjects were supine but could turn on their side or assume a prone position. The second group of subjects were placed in HBR.

Pain Analyses
The location of back pain, leg pain, abdominal pain, and headache were measured using pain drawings (8), and intensity of pain was measured on a 10-cm visual analog scale between 2:00 and 3:00 PM daily (9, 10). Subjects were allowed to take painkilling medication, which was supplied by the nursing staff.

Mood State
A set of analog scales was used to rate subjective feelings (11). This set of scales consisted of 16 items grouped into three factors: alertness, contentedness, and calmness.

Beck Depression Inventory
The Beck Depression Inventory is a 21-item instrument with emphasis on cognitive symptoms of depression (12). The response format is from 0 to 3, giving a theoretical range of 0 to 63 points. Its reliability and validity have been studied, and strong support for the psychometric quality of the questionnaire has been provided (12, 13). The item about sexual activity was judged irrelevant for this study and was therefore excluded.

Statistical Analysis
Results are given as mean ± 1 SD. Significant differences between mean values were tested with analyses of variance and paired t tests. The Wilcoxon signed-rank test was used for nonparametric tests.

	RESULTS

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Back pain, abdominal pain, and headaches were significantly more intense during HDT with traction than during HBR (Table 1). Spinal pain was located in the low back, and abdominal pain was located in the lower part of the abdomen. Location of headaches was not analyzed. By clinical investigation (analysis of abdominal percussion sounds), subjects had more intestinal gas formation during HDT than during HBR.

	DISCUSSION

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Our subjects experienced significantly greater intensity of lower back pain during HDT with traction than during HBR. The pain intensity during HDT with balanced traction was similar to that reported by others in astronauts (3) and in subjects exposed to HDT without balanced traction (6). For these reasons HDT with balanced traction may be a better method than HBR to simulate low back pain and abdominal pain in microgravity. One possible explanation for the increased pain intensity during HDT in balanced traction may be the significantly increased elongation of the spinal column, which may induce stretching of the lumbar nerve roots (7).

Subjects exposed to HDT with balanced traction experienced significantly more lower abdominal pain (p < .05). The pain was correlated with formation of intestinal gas, which was documented by the occurrence of hypersonoric abdominal percussion sounds. Subjects often complain of gastrointestinal discomfort during HDT (D. O’Hara, personal communication, 1994), and reduced gastrointestinal motility during HDT has been documented (14). The pain was located mainly in the lower abdomen. During transabdominal Doppler investigation of lumbar discs, it was obvious that the gas was located in the distal part of the intestines (7). Patients in HDT were less mobile than those in HBR. We think this may be a reason for the greater intensity of lower abdominal pain.

This study showed that HDT with balanced traction was associated with a trend for increased depression scores during bed rest. Depression scores may have been higher because the subjects experienced significantly more lower back pain, lower abdominal pain, headache, and leg pain. Location of headaches and leg pain was not further analyzed. However, no subject had any radiating pain in the lower limbs.

In summary, as compared with HBR, HDT has the advantages of simulating fluid shift as reported in other studies (4, 5). Moreover, HDT with balanced traction also presumably eliminates the axial compressive forces of HDT without traction. We therefore suggest that HDT with balanced traction may combine the advantages of HDT and HBR to simulate the fluid shift, total body and spinal lengthening, and back pain associated with microgravity. Finally, HDT with balanced traction may be a better experimental model than HBR to induce the psychosomatic reactions seen in microgravity.

	ACKNOWLEDGMENTS

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We express our gratitude and appreciation to the test subjects; to Dee O’Hara, manager of the Human Research Facility; and to her staff members, Gita Murthy and Wenona Wercoutere. This work was supported by NASA grants 199-26-12-38, an LäKarutbildningsavtalet (LUA) grant from the University of Göteborg, by a National Research Council Senior Fellowship, and by a stipend from the Swedish Orthopaedic Society (to J.R.S.).

Received for publication September 26, 2000.

	REFERENCES

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