Postural instability caused by extended bed rest is alleviated by brief daily exposure to low magnitude mechanical signals

Abstract

Loss of postural stability, as exacerbated by chronic bed rest, aging, neuromuscular injury or disease, results in a marked increase in the risk of falls, potentiating severe injury and even death. To investigate the capacity of low magnitude mechanical signals (LMMS) to retain postural stability under conditions conducive to its decline, 29 healthy adult subjects underwent 90 days of 6-degree head down tilt bed-rest. Treated subjects underwent a daily 10 min regimen of 30 Hz LMMS at either a 0.3g-force (n=12) or a 0.5g-force (n=5), introduced by Low Intensity Vibration (LIV). Control subjects (n=13) received no LMMS treatment. Postural stability, quantified by dispersions of the plantar-based center of pressure, deteriorated significantly from baseline in control subjects, with displacement and velocity at 60 days increasing 98.7% and 193%, respectively, while the LMMS group increased only 26.7% and 6.4%, reflecting a 73% and 97% relative retention in stability as compared to control. Increasing LMMS magnitude from 0.3 to 0.5 g had no significant influence on outcomes. LMMS failed to spare loss of muscle extension strength, but helped to retain flexion strength (e.g., 46.2% improved retention of baseline concentric flexion strength vs. untreated controls; p=0.01). These data suggest the potential of extremely small mechanical signals as a non-invasive means of preserving postural control under the challenge of chronic bed rest, and may ultimately represent non-pharmacologic means of reducing the risk of debilitating falls in elderly and infirm.

Conflict of interest statement

Conflict of Interest

CTR is a co-founder of Marodyne Medical, Inc. This potential conflict of interest was disclosed to the Internal Review Board of Johnson Space Center, the University of Texas Medical Branch, and Stony Brook University, and was included in the Informed Consent provided to each subject considering entering the trial. To minimize any potential conflict, the trial was designed such that CTR had no interaction with the subjects during recruitment, preadmission evaluation, bed-rest or recovery. No other authors have any conflict of interest.

Copyright © 2010 Elsevier B.V. All rights reserved.

Figures

Figure 1

Figure 1a. Setup showing LMMS treatment. Coupling spring attached to bottom surface of vibration platform provides a load of 60% of the subjects’ pre-bed rest body weight via a shoulder harness. The subject pushes on the plate with straight legs, in a “relaxed stance,” for 10 minutes while the platform provides a 30 Hz, 0.3g sinusoidal acceleration along the their load bearing axis. Figure 1b. Surface acceleration of the top platen of the vibration device during subject treatment with 60% body weight, showing 0.3g peak to peak acceleration. A closed loop feedback control system built into the platform uses a build in accelerometer to automatically adjust the electrical drive signal to the actuator to provide a consistent, high-fidelity sinusoidal acceleration/deceleration for subject of various heights and weights.

Figure 2

Stabilogram of a typical subject at baseline (A) and after bed rest (B), with anterior-posterior and medio-lateral COP displacement expressed in mm. At baseline, the subject’s COP remains near the center of the plate, with occasional perturbations away from the stable region, which become exaggerated after chronic bed rest.

Figure 3

As compared to baseline, control subjects (n = 13) realized a large increase in peak AP (A) and ML (B) COP displacement and velocity (C) as well as root-mean-square (D) of velocity. During upright stance after bed rest, subjects were unable to maintain a constant upright stance and experience COP dispersions at higher magnitudes and velocities, as shown in AP Velocity, and in variability, as seen in the AP RMS Velocity. In contrast, LMMS subjects (n = 17), in both the eyes closed and open conditions (light gray), showed significantly improved retention of baseline postural control measures. † p<0.05, ‡ p<0.1, ° indicates outliers from the box-plot. Eyes closed data shown.

Figure 4

Frequency analysis of stabilogram showing frequency changes in AP shear forces during quiet stance in (A) low, (B) mid, and (C) high frequency ranges. Control subjects displayed an increase in frequency in all three frequency groups. The LMMS subjects showed the greatest increase relative to baseline in low frequency and the smallest in high frequency; however these changes were not significant. The LMMS subjects showed significantly better retention of baseline mid and high frequencies as compared to the control group. † p<0.05 ‡ p<0.1, ° indicates outliers from the box-plot. Eyes closed data shown.

Figure 5

An increase in stabilogram diffusion analysis parameters occurred after 90 days of bed rest, further indicating the deterioration of postural control. The increase in short term coefficient represents a decrease in stability in short time intervals during quiet stance. When the postural control system is compromised, the open loop control system becomes more unstable, and a higher degree of displacement occurs before the body switches to a closed loop system to maintain upright stance. The LMMS subjects showed significantly better retention of baseline measures as compared to the control group. † p