Jesse Muir

Short applications of very low-magnitude vibrations attenuate expansion of the intervertebral disc during extended bed rest

BACKGROUND CONTEXT Loss of functional weightbearing during spaceflight or extended bed rest (BR) causes swelling of the lumbar intervertebral discs (IVDs), elongates the spine, and increases the incidence of low back pain (LBP). Effective interventions for the negative effects of unloading are critical but not yet available. PURPOSE To test the hypothesis that high-frequency, low-magnitude mechanical signals (LMMS) can attenuate the detrimental morphologic changes in the lumbar IVDs. STUDY DESIGN/SETTING Volunteers were subjected to 90d of BR and 7d of reambulation. While retaining this supine position, 18 random subjects received LMMS (30Hz) for 10min/d, at peak-to-peak acceleration magnitudes of either 0.3g (n=12) or 0.5g (n=6). The remaining subjects served as controls (CTRs). PATIENT SAMPLE Eighteen males and 11 female (33+/-7y) healthy subjects of astronaut age (35+/-7y, 18 males, 11 females) and without a history of back pain participated in this study. OUTCOME MEASURES A combination of magnetic resonance imaging and computed tomography scans of the lumbar spine of all subjects were taken at baseline, 60d, 90d, and 7d post-BR. Back pain was self-reported. METHODS IVD morphology, spine length, and back pain were compared between CTR and LMMS subjects. RESULTS Compared with untreated CTRs, LMMS attenuated mean IVD swelling by 41% (p<.05) at 60d and 30% (p<.05) at 90d. After 7 days of reambulation, disc volume of the CTR group was still 8% (p<.01) greater than at baseline, whereas that for the LMMS group returned the disc volume to baseline levels. In contrast to BR alone, LMMS also retained disc convexity at all time points and reduced the incidence of LBP by 46% (p<.05). CONCLUSIONS These data indicate that short daily bouts of LMMS can mitigate the detrimental changes in disc morphology, which arise during nonweightbearing, and provides preliminary support for a novel means of addressing spinal deterioration both on earth and in space.

Safety and severity of accelerations delivered from whole body vibration exercise devices to standing adults

OBJECTIVES Whole body vibration devices are used as a means to augment training, and their potential to treat a range of musculoskeletal diseases and injuries is now being considered. The goal of this work is to determine the degree to which acceleration delivered by whole body vibration devices at the plantar surfaces of a standing human is transmitted through the axial and appendicular skeleton, and how this mechanical challenge corresponds to the safety threshold limit values established by the International Standards Organization ISO-2631. DESIGN Non-blinded laboratory assessment of a range of whole body vibration devices as it pertains to acceleration transmission to healthy volunteers. METHODS Using skin and bite-bar mounted accelerometers, transmissibility to the tibia and cranium was determined in six healthy adults standing on a programmable whole body vibration device as a function of frequency and intensity. Measures of transmissibility were then made from three distinct types of whole body vibration platforms, which delivered a 50-fold range of peak-to-peak acceleration intensities (0.3-15.1 gp-p; where 1g is Earths gravitational field). RESULTS For a given frequency, transmissibility was independent of intensity when below 1g. Transmissibility declined non-linearly with increasing frequency. Depending on the whole body vibration device, vibration ranged from levels considered safe by ISO-2631 for up to 8h each day (0.3 gp-p @ 30 Hz), to levels that were seven times higher than what is considered a safe threshold for even 1 min of exposure each day (15.1 gp-p @ 30 Hz). Transmissibility to the cranium was markedly attenuated by the degree of flexion in the knees. CONCLUSIONS Vibration can have adverse effects on a number of physiologic systems. This work indicates that readily accessible whole body vibration devices markedly exceed ISO guidelines for safety, and extreme caution must be practiced when considering their use.

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

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.

Transmission of low-intensity vibration through the axial skeleton of persons with spinal cord injury as a potential intervention for preservation of bone quantity and quality

Abstract Background/objective: Persons with spinal cord injury (SCI) develop marked bone loss from paralysis and immobilization. Low-intensity vibration (LIV) has shown to be associated with improvement in bone mineral density in post-menopausal women and children with cerebral palsy. We investigated the transmissibility of LIV through the axial skeleton of persons with SCI as an initial approach to determine whether LIV may be used as a clinical modality to preserve skeletal integrity. Methods: Transmission of a plantar-based LIV signal (0.27 ± 0.11 g; 34 Hz) from the feet through the axial skeleton was evaluated as a function of tilt-table angle (15, 30, and 45°) in seven non-ambulatory subjects with SCI and ten able-bodied controls. Three SCI and five control subjects were also tested at 0.44 ± 0.18 g and 34 Hz. Transmission was measured using accelerometers affixed to a bite-bar to determine the percentage of LIV signal transmitted through the body. Results: The SCI group transmitted 25, 34, and 43% of the LIV signal, and the control group transmitted 28, 45, and 57% to the cranium at tilt angles of 15, 30, and 45°, respectively. No significant differences were noted between groups at any of the three angles of tilt. Conclusion: SCI and control groups demonstrated equivalent transmission of LIV, with greater signal transmission observed at steeper angles of tilt. This work supports the possibility of the utility of LIV as a means to deliver mechanical signals in a form of therapeutic intervention to prevent/reverse skeletal fragility in the SCI population.

Associations of Computed Tomography-Based Trunk Muscle Size and Density With Balance and Falls in Older Adults

BACKGROUND Deficits in balance and muscle function are important risk factors for falls in older adults. Aging is associated with significant declines in muscle size and density, but associations of trunk muscle size and density with balance and falls in older adults have not been previously examined. METHODS Trunk muscle size (cross-sectional area) and attenuation (a measure of tissue density) were measured in computed tomography scans (at the L2 lumbar level) in a cohort of older adults (mean ± SD age of 81.9±6.4) residing in independent living communities. Outcome measures were postural sway measured during quiet standing and Short Physical Performance Battery (SPPB) at baseline, and falls reported by participants for up to 3 years after baseline measurements. RESULTS Higher muscle density was associated with reduced postural sway, particularly sway velocities, in both men and women, and better Short Physical Performance Battery score in women, but was not associated with falls. Larger muscle size was associated with increased postural sway in men and women and with increased likelihood of falling in men. CONCLUSIONS The results suggest that balance depends more on muscle quality than on the size of the muscle. The unexpected finding that larger muscle size was associated with increased postural sway and increased fall risk requires further investigation, but highlights the importance of factors besides muscle size in muscle function in older adults.

Longitudinal assessment of human bone quality using scanning confocal quantitative ultrasound

of calcaneus bone loss in a 90-day bedrest. QUS scanning was performed at proximal femur (cadaver) and calcaneus (bedrest subjects) regions with QUS images of 80x80 mm2 for hip and 40x40 mm2 for calcaneus. QUS was processed to calculate the ultrasound attenuation (ATT; dB), wave ultrasound velocity (UV), and the broadband ultrasound attenuation (BUA; dB/MHz). Human cadaver proximal femurs have been measured with the SCAD, micro-CT, DXA, and mechanical strength test. Human calcaneus of bedrest subjects were measured using SCAD and DXA in day 0 (baseline), day 60 and day 90. Results demonstrated that QUS measurement has the capability to predict bone BMD, microstructure and mechanical properties in human bone, and indicated significant sensitivity to the progressive change of bone quality, particularly in the trabecular bone region with remodeling activities.

Mechanical vibrations reduce the Intervertebral Disc swelling and muscle atrophy from Bed Rest

Loss of functional weight bearing, such as experienced during space flight or bedrest, causes detrimental morphological volume changes to the intervertebral disc (IVD) beyond the normal diurnal cycle (10-13%), and muscle. Bed rest (BR) was used to test the hypothesis that, short-duration, low-magnitude, high-frequency mechanical vibrations will attenuate the IVD swelling and intrinsic back muscle atrophy induced by long-term bedrest. Control and experimental subjects underwent BR for up to 90d and were scanned by CT at Od and 90d, and by MRI at Od, 60d, 90d and 8d after completion of BR. In addition, experimental subjects received vibrations at 0.3g and 30Hz for lOmin/day. Muscle volume was measured by CT scans and IVD volume was measured by MRI scans. During BR, mechanical vibrations abated the IVD swelling at 60d by 150% and 90d by 65%. Eight days after bed rest, the control group showed a plasticity of 9%, while the experimental group showed no residual change (p>0.05). Mechanical vibrations reduced the intrinsic back muscle atrophy from long-term bed rest by 33% (p>0.05). These data demonstrate the rapid deterioration of the musculoskeletal system with BR but present vibrations as a promising non-pharmacologic countermeasure to disc degeneration and muscle atrophy.

Spatial distribution and remodeling of elastic modulus of bone in micro-regime as prediction of early stage osteoporosis

We assessed the local distribution of bone mechanical properties on a micro-nano-scale and its correlation to strain distribution. Left tibia samples were obtained from 5-month old female Sprague Dawley rats, including baseline control (n=9) and hindlimb suspended (n=9) groups. Elastic modulus was measured by nanoindentation at the dedicated locations. Three additional tibias from control rats were loaded axially to measure bone strain, with 6-10N at 1Hz on a Bose machine for strain measurements. In the control group, the difference of the elastic modulus between periosteum and endosteum was much higher at the anterior and posterior regions (2.6GPa), where higher strain differences were observed (45μɛ). Minimal elastic modulus difference between periosteum and endosteum was observed at the medial region (0.2GPa), where neutral axis of the strain distribution was oriented with lower strain difference (5μɛ). In the disuse group, however, the elastic modulus differences in the anterior posterior regions reduced to 1.2GPa from 2.6GPa in the control group, and increased in the medial region to 2.7GPa from 0.2GPa. It is suggested that the remodeling rate in a region of bone is possibly influenced by the strain gradient from periosteum to endosteum. Such pattern of moduli gradients was compromised in disuse osteopenia, suggesting that the remodeling in distribution of micro-nano-elastic moduli among different regions may serve as a predictor for early stage of osteoporosis.

Quantitative ultrasound imaging reconstruction in peripheral skeletal assessment

Disuse osteopenia affect mineral density, microstructure and integrity of bone, which lead to increased risk of osteoporosis and fracture during long term space mission. In order to provide a non-ionizing, repeatable method of skeletal imaging, a novel hand-held scanning confocal quantitative ultrasound (QUS) device has been developed. A mobile scanning ultrasound images were collected using ten sheep tibia and eight human volunteers at the wrist, forearm, elbow, and humerus with the arm submerged in water. A custom MATLAB software used a least-error algorithm to adjust adjacent lines and remove unwanted image waver from hand movement. Ultrasound attenuation (ATT) values of 42.4±0.6 dB and 41.5±1.0 dB were found in water and gel coupling, respectively. Scans of the human subject revealed detailed anatomy of the humerus, elbow, forearm, and hand. Repeat measures of the distal radius found an attenuation of 37.2±3.3 dB, with negligible changes cause by wrist rotation of ±10° (36.4 to 37.3 dB) indicating sma...

Retention of bone density and postural status with a non-invasive extremely low level mechanical signal: A ground based evaluation of efficacy

In space, rapid losses in bone mineral density (BMD) leave astronauts at an increased risk of bone fracture. Longer microgravity missions combined with the lack of efficacy of current exercise regimes in reducing this loss leads to the need of a new treatment. This study has the goal of testing a treatment in the form of a low magnitude mechanical vibration. As an analog of space flight, 18 subjects spent 90 days in continuous 6 degree head down tilt, eight of which received 10 minutes of vibration treatment a day. Measurements of bone density and balance found that there was a 30-50% nonsignificant reduction in BMD loss in the hip, as well as a significant decrease in the loss of postural control. The combined factors of stronger bones and increased balance greatly reduce the risk of bone fracture. With a proposed multi-year planetary mission to Mars being planned by NASA, the need for improved musculoskeletal health is of increasing importance, and this device may provide the needed mode of increasing astronaut safety.