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OCCUPATIONAL HEALTH/ERGONOMICS

Lumbar Spine Paraspinal Muscle and Intervertebral Disc Height Changes in Astronauts After Long-Duration Spaceflight on the International Space Station

Chang, Douglas G. MD, PhD; Healey, Robert M. BS, MBA; Snyder, Alexander J. BS; Sayson, Jojo V. PT, DMT; Macias, Brandon R. PhD; Coughlin, Dezba G. PhD; Bailey, Jeannie F. MS; Parazynski, Scott E. MD§; Lotz, Jeffrey C. PhD; Hargens, Alan R. PhD

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doi: 10.1097/BRS.0000000000001873
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Abstract

The lumbar paraspinal muscles (PSM) provide postural stability, enabling gait and supporting upper extremity movements.1,2 They are critical to function in a gravitational environment. In particular, these muscles facilitate vertebral motion, and protect articular structures, discs, and ligaments from excessive strain and injury.3 Atrophy of these muscles is evidenced by altered fat content, cross-sectional area (CSA), and higher proportions of type II fast-twitch fibers,4,5 and is strongly associated with low back pain on Earth.6,7 How these muscles function and respond during space flight is, however, not well described.

With microgravity exposure in space, several spine-related issues are observed among crewmembers.8 The torso lengthens 4 to 6 cm, approximately 2 to 3 times the normal diurnal increase (1–2 cm) on Earth.9,10 This reportedly occurs because of spinal unloading, flattening of spinal curvature, loss of paravertebral muscle tone, and vertebral disc degeneration.11,12 Flight medical data indicate that more than half of the US astronauts report spine pain during their mission.13–15 While in space, astronauts report that a lumbar flexed, “fetal tuck” position to stretch is the most effective way of alleviating back pain.14 The back pain is described with a moderate to severe level of intensity for 14% to 28% percent of the US astronauts. Shuttle crewmembers described pain lasting for 15% to 100% of their mission. The location of pain is reported most frequently in the following anatomic regions: 50% low back, 11% midback, 11% neck, and 1% chest. Even after their return to Earth, approximately 40% of crewmembers report spine pain.16 Another indication of lumbar pain is vertebral hypomobility from guarding,17 and preliminary data indicate such spinal stiffness is seen with prolonged space flight.18,19

Even with an exercise protocol in place during prolonged space missions, significantly decreased muscle size is seen at multiple sites in the body, including the lumbar paraspinals.20 The exercise protocols have evolved over time, but traditionally they have not specifically focused on core strengthening.21 LeBlanc and coworkers describe an exponential recovery of preflight muscle size after Mir missions, and the recovery is complete within 30 to 60 days. These measurements were made by manually tracing the outline of muscle cross-sections seen on 1.5 Tesla magnetic resonance images of 16 crew members. It is unknown whether fatty replacement, fluid redistribution, or actual lean muscle mass changes occur, such as that observed in patients or ground-based bed rest simulations of microgravity.22,23

Lastly, a concerning risk of intervertebral disc (IVD) herniation is seen postflight. The incidence of herniated nucleus pulposus is reported as 4.3 times higher in the US Astronaut Corps compared to matched aviator control populations on Earth.11 The highest risk period for disc herniation appears in the first year after return to Earth, with the majority reported within the first month of landing. It is unknown how medical staff surveillance of the astronauts versus control populations, and different behavioral decisions regarding medical care seeking and reporting by crewmembers before versus after a mission might play into the observations. It, however, does strongly suggest that structural changes in the spine associated with space adaptation result in deleterious effects occurring with the reintroduction of the gravity environment. Moreover, the consequence of disc herniation may affect an astronaut's ability to return to work on Earth or conduct work upon arriving at a planetary destination such as Mars after a long space flight.

The immediate purpose of this research is to evaluate lumbar PSM CSA and IVD heights following a 6-month International Space Station (ISS) mission and a 33- to 67-day postflight recovery period. The goals are to understand the factors involved in lumbar spine strength and back pain in crewmembers during a long mission and after increased g-loads of landing and readaptation to Earth. This could provide helpful physiological information to support a manned mission to Mars. On Earth, this information could help our understanding of spinal atrophy and degeneration due to inactivity, and potential issues involved with backpack use of military personnel, and first responders.

MATERIALS AND METHODS

Institutional research board approval was obtained from the National Aeronautics and Space Administration (NASA) and the University of California, San Diego. Six ISS crewmembers volunteered for the study, 1 woman and 5 men. The range of crewmember ages spanned 46 to 55 years, height 168 to 183 cm, and body mass 60 to 93 kg. The mission duration on the ISS ranged from 117 to 213 days. This project represents 4 years of active data collection, through 2016.

Supine lumbar spine magnetic resonance imaging (MRI) scans were conducted preflight, immediate postflight, and at least 30 days postflight recovery after an ISS mission (Figure 1 A, B). Imaging took about 80 minutes, and was performed in the morning, using a Siemens Magnetom Verio 3T system at a University of Texas Medical Branch facility outside Houston, TX. Preflight imaging was performed on average 214 days before launch. While on the ISS, the astronauts engaged in 2 to 3 hours of daily exercise with a treadmill, stationary cycle, and resistive strength training of the large muscle groups.21,24–26 After landing in Kazakhstan, the “immediate” postflight imaging was performed within 1 to 2 days, in Houston. Landing details are described elsewhere.27 The astronauts completed typical post-flight astronaut strength, conditioning, and rehabilitation exercise and activities,26 including a brief trip back to Russia, and return to Houston, TX where they were imaged again. These “Recovery” period images were performed an average of 46 days (range 33–67 days) after landing. The imaging time points are summarized in Table 1.

F1
Figure 1:
Characteristic pre-, postflight, and recovery lumbar spine MR images (A) L1-S1 sagittal and (B) L3/4 axial T2 sequences.
T1
TABLE 1:
Imaging Schedule of Crewmembers

Functional cross-sectional area (FCSA) measurements of the lumbar PSMs were obtained using the T2-weighted MRI scans. We elected to focus on the L3/4 vertebral level, based on the relative ease of identifying muscle boundaries as compared to lower vertebral levels. The FCSA measurements involved an image-analysis thresholding technique to estimate lean muscle mass. The technical details are reported elsewhere.1,28–31 Briefly, the lumbar PSMs (multifidus, erector spinae, quadratus lumborum, and psoas) were identified and analyzed using Fiji imaging software (National Institutes of Health,32Figure 2A). Total PSM CSA was defined as the sum total of the CSAs obtained from the eight PSMs (combining right and left). Functional PSM CSA was measured using gray-scale thresholding to analyze those regions of the muscle cross-sections corresponding to dark, lean muscle mass. The analysis was conducted by one individual (R.M.H.). Our control studies showed that repeat measurements done by an individual and by several individuals were reliable and reproducible, with an intraclass correlation coefficient of 0.99, consistent with the literature.28,29 Statistical analysis was conducted using one-way, repeated measures analysis of variance (ANOVA) to establish significance, defined as P < 0.05, followed by post-hoc testing with the Newman-Keuls multiple comparison test33 with alpha = 0.05, using GraphPad Prism (version 5.04, GraphPad Software, Inc., La Jolla, CA) software.

F2
Figure 2:
Characteristic location of (A) lumbar paraspinal muscles identified for functional cross-sectional area (FCSA) lean muscle area measurement on axial images at the L3-L4 level, and (B) intervertebral disc (IVD) height measurement on sagittal images (anterior, middle, and posterior).

Lastly, lumbar IVD heights were measured at the anterior, middle, and posterior sections from the L1-2 to L5-S1 disc levels (Figure 2B). The fast spin echo T2 images were obtained at the midsagittal plane,34 with slice thickness 4 mm, field of view 200, 192 × 320 image matrix, voxel size 1 × 0.6 × 4 mm, and NEX 2. For each subject, the disc height at a given lumbar intervertebral level was defined as the average of measurements made in the anterior, middle, and posterior locations along the disc, modified from the Dabbs method.35 Change in the average disc height was calculated at postflight (post-preflight), recovery (recovery-postflight), and overall change from preflight to recovery (recovery-preflight). This measurement has an uncertainty with inter- and intraobserver standard deviations of 0.2 and 0.3 mm, respectively.36 Our group has used the technique to measure changes in lumbar IVD heights with Earth-bound subjects in unloaded bedrest,34 and loaded backpack studies.36,37

RESULTS

Lumbar paraspinal FCSA decreased by 19% on average from a preflight value of 8737 ± 1758 mm2 (avg ± standard deviation) down to a postflight value of 7049 ± 1822 mm2. Later, there was a change in FCSA up to a recovery value of 8195 ± 1900 mm2. ANOVA testing indicates a significant difference in FCSA measured at the three time points, with F ratio 23.39, R2 0.82, and P = 0.0002. Post-hoc testing indicates the FCSA changed significantly from pre- to postflight, and from postflight to postflight recovery. The FCSA data at the recovery time point were less than the preflight values, representing a 68% recovery of the postflight loss, a difference not significantly different as determined by post-hoc testing. In comparison, the total lumbar paraspinal CSA (that encompass the unthresholded manual outlines, and therefore includes both lean muscle and nonlean muscle components) followed a similar trend at the three time points, but with nonsignificant changes (F ratio 1.44, R2 0.22, P = 0.2832, Table 2).

T2
TABLE 2:
Lumbar Paraspinal Muscle Cross-sectional Area Data

Expressed as a percentage of the total lumbar CSA, the relative proportion of lumbar lean muscle FCSA decreased from preflight to postflight by 14 percentage points from 86% ± 5% down to 72 ± 7%. The fraction of lumbar muscle FCSA recovered nine percentage points during the next 6 weeks to an average of 81% ± 4%. ANOVA testing indicates a significant difference in percentage FCSA measured at the three time points, with F ratio 22.25, R2 0.82, and P = 0.0002. Post-hoc testing indicates the FCSA changed significantly from pre- to postflight, and from post-flight to postflight recovery. This resulted in a significantly lower lean muscle fractional content at recovery compared with the preflight values (Figure 3).

F3
Figure 3:
Functional cross-sectional area (FCSA) as a percentage of total cross-sectional area (CSA) in the lumbar paraspinal muscles, n = 6 crewmembers.

Among the six crewmembers studied, average disc height did not change in the lumbar spine. There was no consistent pattern before and after the mission (Table 3). There was considerable disc height variability from crewmember to crewmember, over various lumbar spine levels, and along anterior-middle-posterior locations of the disc.

T3
TABLE 3:
Change in Lumbar Disc Heights (Average Change ± Standard Deviation), in mm

DISCUSSION

The present study showed reductions in total CSA with long-duration space flight, but even more dramatic reductions in functional CSA, a proxy for lean muscle mass. At 6 weeks postmission, the FCSA and CSA trended toward preflight levels. After the mission, the lumbar paraspinal extensors recovered 68% of the loss after approximately 46 days back on Earth. These ISS data are comparable to previous long-duration Mir data obtained approximately 20 years ago,20 where intrinsic back muscle total CSA decreased to 84% of preflight values, and psoas CSA decreased to 96%. Direct comparisons to that study are, however, difficult to make due to several factors.

We had six crewmembers, whereas LeBlanc and coworkers report on 16 crewmembers. We used one 3 Tesla MRI scanner in Houston operated by a single team of technicians, whereas LeBlanc et al used three 1.5 Tesla scanners at two centers (Moscow, Russia, and Houston, TX). During the missions, different exercise countermeasures were used on board more recent ISS compared with previous Mir flights.38 On Mir specifically, there were no significant resistance exercises for strength. LeBlanc and coworkers report slightly more temporal variability for scan times after landing. For example, five of the six crewmembers were scanned between day 1 and 2 after landing in the present study, whereas their first postflight measurements occurred on landing day itself or up to 4 days after landing. We focused on the L3/4 lumbar level, whereas LeBlanc and coworkers made muscle volume calculations using an unspecified region of the lumbar spine. We elected not to measure the lower lumbar levels due to the greater difficulty in identifying clear muscle boundaries in a region that typically has a greater degree of fatty atrophy/intermuscular fascial connections (e.g., lumbar intermuscular aponeurosis, lumbosacral ligaments) in the multifidi/erector spinae muscles, and a fanning/thinning of the psoas and erector spinae muscles as they traverse normally away from the lumbar spine.39 Lastly, we evaluated both total and functional CSA measurements. This provides insight into lean-muscle mass changes separated from the effects of water retention or fatty replacement.

In contrast to PSM data, individual disc height changes in the lumbar spine were small and demonstrated no consistent changes across time points. Specifically, disc height increases were not seen in a significant or consistent fashion postflight. We continue to review these data in several additional ways, including total lumbar disc height (measured by summing disc heights from every level) and total lumbar length between the L1 and the L5 vertebral bodies,40 and also by making comparisons with lumbar lordosis measurements, MRI T2 water mapping techniques41 in the discs,19 and a separate data set we collected on the subjects using upright standing MRI data.36 So far, our data are compatible to previous lumbar disc height and lumbar length measurements after short-duration space flight,40 and preliminary data from in-flight ultrasound studies of cervical and lumbar disc heights, which also do not indicate significant disc height increases or swelling.42

These measurements run counter to previous hypotheses about the effects of microgravity on disc swelling,11,43 and suggest that the torso lengthening observed in crewmembers 12,44 may be due to factors other than swelling of the IVDs. Specifically, postural straightening (i.e., a flattening of spinal lumbar lordosis and thoracic kyphosis into a “neutral body posture” in microgravity) is an important factor.12,19 Our sample size is, however, presently small for the study of IVD heights, and we have no in-flight images. Further spine analysis with additional crewmembers and in-flight ultrasound imaging will be forthcoming.

Back pain is a part of life. Approximately two-thirds of the adult population will experience low back pain and a specific pathologic anatomical diagnosis is made in only approximately 15% of cases.45 Given that, what are the implications of lowered PSM functional CSA? Back pain patients do demonstrate reduced PSM CSA.7 The positive predictive value of CSA on the development of future low back pain is, however, controversial, and it has not yet been established as a strong independent risk factor.46,47 This may be similar to other reported low back pain risk factors (such as physical demands at work, job satisfaction, bodily vibration, smoking, alcohol consumption, lumbar flexibility, etc.), where reliable predictive conclusions from the literature are difficult to make for any one person due to the many intercorrelated and confounding parameters, and the fact low back pain is common even in people without such risks. Back weakness is one known risk factor for low back pain45,48 and our laboratory is analyzing Biering-Sorensen back extension endurance data to help characterize a structure-function relationship among the crewmembers. Even so, muscle endurance and strength depend not only on CSA, but also on many other factors such as muscle contractility, metabolism, and fiber-type atrophy,49,50 and neuromuscular recruitment, coordination, fatigue mechanisms,51,52 pain, and psychosocial factors.53

Astronaut exercise programs currently emphasize the maintenance of bone mineral density, aerobic/anaerobic capacity, and muscle strength/power (focused on the large muscles of the proximal hips and shoulders) and endurance.21 Preflight, the exercise program involves a mix of cardio aerobic training, functional training for activities performed in daily life, resistive weight-training (e.g., squats and deadlifts), and familiarization of in-flight exercises. In mission there is treadmill training, cycle ergometer, and resistive training (squats, deadlifts, bench/shoulder press, rows). Postflight there are cardio workouts, resistive weight training, and functional exercise, focused on balance, proprioception, agility, coordination, and power.26 These routinized exercise programs are closely monitored by NASA Astronaut Strength, Conditioning and Rehabilitation and medical staff. With such a steady-state, maintenance program in place preflight, we do not believe that significant lumbar deconditioning or strengthening occurs between the preflight images and the flight itself. We are, however, unable to substantiate this belief because mission logistics preclude testing close to the actual launch date.

Our lumbar spine data identify a specific departure from the terrestrial, baseline anatomy of astronauts. It further suggests that an exercise countermeasure is needed to focus on the lumbar PSMs. Low load, lumbar core stabilization exercises are efficacious for back pain patients,54 deconditioned1,55 and healthy56 adults on Earth, specifically improving PSM CSA atrophy and strength,4,57–59 and acute60,61 and chronic57,62,63 low back pain. Such core-strengthening exercises specifically involve isometric exercises or lumbar extensor training. Another promising exercise countermeasure for low back pain is yoga,64 which might be particularly effective in addressing spaceflight-associated lumbar stiffness and hypomobility.15,19,65 Existing exercise interventions in microgravity that target other muscle groups are effective in addressing atrophy.38 Whether new exercise countermeasures can prevent in-flight PSM atrophy, improve spinal pain and function, shorten recovery time, and how such exercise might be performed in a micro-gravity environment with available exercise equipment need further study.

Key Points

  • Lumbar spine MRI data were obtained from six astronauts before, after, and approximately 46 days after a 6-month mission on board the ISS.
  • Functional CSA percent of the lumbar PSMs decreased significantly by 14 percentage points during long-duration spaceflight (P = 0.002) and recovered 68% of the loss by postflight day 46.
  • Lumbar IVD heights were essentially unchanged after space flight.
  • Such results give insight into back pain and IVD risks, suggesting possible countermeasures targeted to the lumbar PSMs while in-flight and during the preflight and postlanding periods.

Acknowledgments

The authors thank Marilyn Johnson, Stephanie Miller, Laura Sarmiento, and Stephanie Vallarino, and Drs. S. Chiang, D. Hughes, L. Minkoff and R. Riascos-Castaneda for their work toward the success of this project. The authors also thank the International Space Station crew members for their research participation.

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Keywords:

aerospace medicine; atrophy; back pain; immobilization; intervertebral disc; magnetic resonance imaging; muscles; paraspinal muscles; spine; weightlessness

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