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Open Access Publications from the University of California

Scholarly Works (16 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2019)

Spaceflight Associated Neuro-ocular Syndrome, bone decalcification, and muscle atrophy are among the most prevalent risks associated with long-duration spaceflight. Implementing the lower body negative pressure (LBNP) technique is a potential countermeasure for these risks. LBNP counteracts head-ward fluid shifts and generates ground-reaction forces (GRFs). GRFs are beneficial for maintaining bones and muscles by generating gravitational-like loads we experience on Earth. Currently, LBNP devices are large/bulky and require the subject to maintain a stationary position. However, the mobile Gravity Suit I designed is relatively small, untethered, and flexible. It is hypothesized that by designing and developing a mobile Gravity Suit, we can generate greater GRFs than an LBNP chamber. Static LBNP chambers achieve only one GRF on the subject. This can be expressed as AW(LBNP) = GRF, where Aw = cross-sectional area (CSA) of waist seal. However, the mobile Gravity Suit may achieve an additional GRF using the following equation, (AF + AW)LBNP = GRF, where AF = CSA of subject’s feet. The additional force can be further expressed as F1 + F2 = AF(LBNP), where F1 = spinal loading force, F2 = waist shear force, and AF(LBNP) = the total downward foot force. While lying supine, GRF data were recorded in both devices using foot sole sensors and a weigh scale. The data show that the Gravity Suit generated a mean maximum bodyweight of 125% +/- 22% whereas the LBNP chamber generated 91% +/- 24%. The mobile Gravity Suit demonstrates higher percent of bodyweight, due to the suit's novel biomechanical design.

    Cover page: Designing and Developing the Mobile Gravity Suit for Long-Duration Spaceflight
    • Thesis
    • Peer Reviewed
    UC San Diego Electronic Theses and Dissertations (2019)

    It is well documented that MAP decreases after aerobic exercise due to factors such as reduced TPR, maintained vasodilation in previously active muscles, reduced sympathetic innervation, and resetting of baroreceptors. Even with a significant decrease in MAP, it has been shown that cerebrovascular function, middle cerebral arterial flow velocity, and CPP are well-maintained. CPP, the pressure gradient that drives cerebral blood flow, is defined as the different between MAP and ICP. Thus, it was hypothesized that ICP will concomitantly decrease with MAP to maintain CPP and allow for adequate blood supply to the cerebrum.

    To test this hypothesis, 17 healthy human subjects were recruited to complete a 30-minute period of stationary cycling. Measurements of ICP, MAP, TPR, and HR were taken before exercise to serve as a baseline; one hour of follow-up measurements were collected and compared to baseline values. It was discovered that ICP significantly decreased after stationary cycling (ANOVA P=0.0352) in conjunction with a significant decrease in MAP (ANOVA P=0.0025) and TPR (ANOVA P<0.0001). HR was elevated for the entire hour after exercise (ANOVA P<0.0001).

    Sixteen of the subjects had their ICP estimated non-invasively while one of the subjects had their ICP measured invasively. The non-invasive ICP results closely matched the invasive ICP results and confirmed that the estimated ICP was accurate. These trends support my hypothesis that ICP decreases with MAP following moderate exercise.

      Cover page: Effects of Aerobic Exercise on Intracranial Pressure in Humans
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2013)
      The human lumbar spine is uniquely adapted to the gravitational stress of upright posture. Using upright magnetic resonance imaging (uMRI), we characterized the adult lumbar spine's response to upright gravitational loading. We hypothesized that a gravitational load would compress the intervertebral discs (IVDs). Five volunteers underwent T2 weighted sagittal 0.6T uMRI scans of the lumbar spine while supine, upright, and upright with a 10% body weight (BW) backpack. While upright, the 10% BW load significantly compressed the L4‐L5 and L5‐S1 IVDs by about 10% relative to supine (p < 0.05). Upright posture and the 10% BW load significantly compressed the anterior L5‐S1 IVD relative to supine (p < 0.05). Gravitational loading of the lumbar spine plus an added 10% BW load compresses the caudal IVDs (L4‐L5 and L5‐S1), suggesting that these discs are either bearing more of the load or are more compliant than the other IVDs. This study is the first radiographic analysis to describe the adult lumbar spine in the upright position with added spinal load. This study was supported by the National Aeronautics and Space Administration Grant NNX10AM18G and the UCSD Clinical Translational Research Institute Fellowship Award.
        • Article
        • Peer Reviewed
        UC San Francisco Previously Published Works (2016)

        Purpose

        Disc herniation risk is quadrupled following spaceflight. This study tested the hypothesis that swelling-induced disc height increases (comparable to those reported in spaceflight) stiffen the spine and elevate annular strain and nuclear pressure during forward bending.

        Methods

        Eight human lumbar motion segments were secured to custom-designed testing jigs and subjected to baseline flexion and compression and pure moment flexibility tests. Discs were then free-swelled in saline to varying supraphysiologic heights consistent with prolonged weightlessness and re-tested to assess biomechanical changes.

        Results

        Swelling-induced disc height changes correlated positively with intradiscal pressure (p < 0.01) and stiffening in flexion (p < 0.01), and negatively with flexion range of motion (p < 0.05). Swelling-induced increases in disc height also led to increased annular surface strain under combined flexion with compression. Disc wedge angle decreased with swelling (p < 0.05); this loss of wedge angle correlated with decreased flexion range of motion (R (2) = 0.94, p < 0.0001) and decreased stiffness fold change in extension (p < 0.05).

        Conclusion

        Swelling-induced increases in disc height decrease flexibility and increase annular strain and nuclear pressure during forward bending. These changes, in combination with the measured loss of lordotic curvature with disc swelling, may contribute toward increased herniation risk. This is consistent with clinical observations of increased disc herniation rates after microgravity exposure and may provide the basis for future countermeasure development.
          Cover page: The effect of simulated microgravity on lumbar spine biomechanics: an in vitro study
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2013)

          Background

          We aimed to provide information on whether or not the correlation between body tilt and the pulse amplitude of transcranial ultrasonic time-of-flight waveform can be observed in the anterior-posterior skull direction. Also, we asked the question whether or not the skull pulsation can be detected since the cranial bones involved are thicker.

          Methods

          The experimental model of body tilt that alters intracranial pressure by shifting body fluid headward was employed. Transcranial ultrasound waveforms were examined in 15 healthy volunteers positioned at five tilt angles of +30 degrees, 0 degrees, -30 degrees, -60 degrees, and -90 degrees from the horizontal body position. A pulse-echo transducer was placed on the middle forehead and ultrasound waveforms were recorded. Synchronized variations in the ultrasonic time-of-flight with heartbeats were monitored using the pulsed phase locked loop technique for the output voltage of the ultrasound transducer. Simultaneous effects of body tilt on cardiovascular parameters were also evaluated.

          Results

          Pulse amplitudes of ultrasonic time-of-flight waveforms were found to vary with body tilt. Repeated-measures ANOVA and regression analysis showed a negative correlation between body tilt angle and pulse amplitude. The regression line has the equation: pulse amplitude = (1.158-0.01023 x tilt angle) x 10(-4) voltage. There was no such relationship between head-down body tilt and altered mean blood pressure or heart rate.

          Conclusion

          An increase in the pulse amplitude of the anterior-posterior transcranial ultrasonic time-of-flight waveform can be detected when the head-down body tilt angle increases.
            • Article
            • Peer Reviewed
            UC San Diego Previously Published Works (2016)
            Microgravity-induced lumbar paraspinal muscle deconditioning may contribute to back pain commonly experienced by astronauts and may increase the risk of postflight injury. We hypothesized that a combined resistive and aerobic exercise countermeasure protocol that included spinal loading would mitigate lumbar paraspinal muscle deconditioning during 60 days of bed rest in women. Sixteen women underwent 60-day, 6° head-down-tilt bed rest (BR) and were randomized into control and exercise groups. During bed rest the control group performed no exercise. The exercise group performed supine treadmill exercise within lower body negative pressure (LBNP) for 3-4 days/wk and flywheel resistive exercise for 2-3 days/wk. Paraspinal muscle cross-sectional area (CSA) was measured using a lumbar spine MRI sequence before and after BR. In addition, isokinetic spinal flexion and extension strengths were measured before and after BR. Data are presented as means ± SD. Total lumbar paraspinal muscle CSA decreased significantly more in controls (10.9 ± 3.4%) than in exercisers (4.3 ± 3.4%; P < 0.05). The erector spinae was the primary contributor (76%) to total lumbar paraspinal muscle loss. Moreover, exercise attenuated isokinetic spinal extension loss (-4.3 ± 4.5%), compared with controls (-16.6 ± 11.2%; P < 0.05). In conclusion, LBNP treadmill and flywheel resistive exercises during simulated microgravity mitigate decrements in lumbar paraspinal muscle structure and spine function. Therefore spaceflight exercise countermeasures that attempt to reproduce spinal loads experienced on Earth may mitigate spinal deconditioning during long-duration space travel.
              • Article
              • Peer Reviewed
              UC San Francisco Previously Published Works (2016)
              Intervertebral disc herniation rates are quadrupled in astronauts following spaceflight. While bending motions are main contributors to herniation, the effects of microgravity on the bending properties of spinal discs are unknown. Consequently, the goal of this study was to quantify the bending properties of tail discs from mice with or without microgravity exposure. Caudal motion segments from six mice returned from a 30-day Bion M1 mission and eight vivarium controls were loaded to failure in four-point bending. After testing, specimens were processed using histology to determine the location of failure, and adjacent motion segments were scanned with micro-computed tomography (μCT) to quantify bone properties. We observed that spaceflight significantly shortened the nonlinear toe region of the force-displacement curve by 32% and reduced the bending strength by 17%. Flight mouse spinal segments tended to fail within the growth plate and epiphyseal bone, while controls tended to fail at the disc-vertebra junction. Spaceflight significantly reduced vertebral bone volume fraction, bone mineral density, and trabecular thickness, which may explain the tendency of flight specimens to fail within the epiphyseal bone. Together, these results indicate that vertebral bone loss during spaceflight may degrade spine bending properties and contribute to increased disc herniation risk in astronauts.
                Cover page: Spaceflight‐induced bone loss alters failure mode and reduces bending strength in murine spinal segments
                • Article
                • Peer Reviewed
                UC San Diego Previously Published Works (2016)
                Leg muscle mass and strength are decreased during reduced activity and non-weight-bearing conditions such as bed rest (BR) and spaceflight. Supine treadmill exercise within lower body negative pressure (LBNPEX) provides full-body weight loading during BR and may prevent muscle deconditioning. We hypothesized that a 40-min interval exercise protocol performed against LBNPEX 6 days week(-1) would attenuate losses in leg lean mass (LLM), strength, and endurance during 6° head-down tilt BR, with similar benefits for men and women. Fifteen pairs of healthy monozygous twins (8 male and 7 female pairs) completed 30 days of BR with one sibling of each twin pair assigned randomly as the non-exercise control (CON) and the other twin as the exercise subject (EX). Before and after BR, LLM and isokinetic leg strength and endurance were measured. Mean knee and ankle extensor and flexor strength and endurance and LLM decreased from pre- to post-BR in the male CON subjects (P < 0.01), but knee extensor strength and endurance, ankle extensor strength, and LLM were maintained in the male EX subjects. In contrast, no pre- to post-BR changes were significant in the female subjects, either CON or EX, likely due to their lower pre-BR values. Importantly, the LBNPEX countermeasure prevents or attenuates declines in LLM as well as extensor leg strength and endurance. Individuals who are stronger, have higher levels of muscular endurance, and/or have greater LLM are likely to experience greater losses during BR than those who are less fit.
                  Cover page: Treadmill exercise within lower body negative pressure protects leg lean tissue mass and extensor strength and endurance during bed rest
                  • Article
                  • Peer Reviewed
                  UC San Diego Previously Published Works (2009)
                  Cardiovascular deconditioning after long duration spaceflight is especially challenging in women who have a lower orthostatic tolerance (OT) compared with men. We hypothesized that an exercise prescription, combining supine aerobic treadmill exercise in a lower body negative pressure (LBNP) chamber followed by 10 min of resting LBNP, three to four times a week, and flywheel resistive training every third day would maintain orthostatic tolerance (OT) in women during a 60-day head-down-tilt bed rest (HDBR). Sixteen women were assigned to two groups (exercise, control). Pre and post HDBR OT was assessed with a tilt/LBNP test until presyncope. OT time (mean ± SE) decreased from 17.5 ± 1.0 min to 9.1 ± 1.5 min (−50 ± 6%) in control group (P < 0.001) and from 19.3 ± 1.3 min to 13.0 ± 1.9 min (−35 ± 7%) in exercise group (P < 0.001), with no significant difference in OT time between the two groups after HDBR (P = 0.13). Nevertheless, compared with controls post HDBR, exercisers had a lower heart rate during supine rest (mean ± SE, 71 ± 3 vs. 85 ± 4, P < 0.01), a slower increase in heart rate and a slower decrease in stroke volume over the course of tilt/LBNP test (P < 0.05). Blood volume (mean ± SE) decreased in controls (−9 ± 2%, P < 0.01) but was maintained in exercisers (−4 ± 3%, P = 0.17). Our results suggest that the combined exercise countermeasure did not significantly improve OT but protected blood volume and cardiovascular response to sub tolerance levels of orthostatic stress.
                    Cover page: WISE-2005: effect of aerobic and resistive exercises on orthostatic tolerance during 60 days bed rest in women
                    • Article
                    • Peer Reviewed
                    UC San Francisco Previously Published Works (2022)

                    Purpose

                    The paraspinal muscles (PSM) are a key feature potentially related to low back pain (LBP), and their structure and composition can be quantified using MRI. Most commonly, quantifying PSM measures across individual muscles and individual spinal levels renders numerous separate metrics that are analyzed in isolation. However, comprehensive multivariate approaches would be more appropriate for analyzing the PSM within an individual. To establish and test these methods, we hypothesized that multivariate summaries of PSM MRI measures would associate with the presence of LBP symptoms (i.e., pain intensity).

                    Methods

                    We applied hierarchical multiple factor analysis (hMFA), an unsupervised integrative method, to clinical PSM MRI data from unique cohort datasets including a longitudinal cohort of astronauts with pre- and post-spaceflight data and a cohort of chronic LBP subjects and asymptomatic controls. Three specific use cases were investigated: (1) predicting longitudinal changes in pain using combinations of baseline PSM measures; (2) integrating baseline and post-spaceflight MRI to assess longitudinal change in PSM and how it relates to pain; and (3) integrating PSM quality and adjacent spinal pathology between LBP patients and controls.

                    Results

                    Overall, we found distinct complex relationships with pain intensity between particular muscles and spinal levels. Subjects with high asymmetry between left and right lean muscle composition and differences between spinal segments PSM quality and structure are more likely to increase in pain reported outcome after prolonged time in microgravity. Moreover, changes in PSM quality and structure between pre and post-spaceflight relate to increase in pain after prolonged microgravity. Finally, we show how unsupervised hMFA recapitulates previous research on the association of CEP damage and LBP diagnostic.

                    Conclusion

                    Our analysis considers the spine as a multi-segmental unit as opposed to a series of discrete and isolated spine segments. Integrative and multivariate approaches can be used to distill large and complex imaging datasets thereby improving the clinical utility of MRI-based biomarkers, and providing metrics for further analytical goals, including phenotyping.
                      Cover page: Using hierarchical unsupervised learning to integrate and reduce multi-level and multi-paraspinal muscle MRI data in relation to low back pain
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