J Appl Physiol 109: 1801–1811, 2010. First published September 23, 2010; doi:10.1152/japplphysiol.00707.2010.

Countermeasures against lumbar spine deconditioning in prolonged bed rest: resistive exercise with and without whole body vibration Daniel L. Belavý,1 Gabriele Armbrecht,1 Ulf Gast,1 Carolyn A. Richardson,2 Julie A. Hides,2,3 and Dieter Felsenberg1 1

Centre for Muscle and Bone Research, Charité Universitätsmedizin Berlin, Berlin, Germany; and 2School of Health and Rehabilitation Sciences, The University of Queensland, Brisbane; and 3Mater/UQ Back Stability Clinic, Mater Health Services Brisbane Limited, South Brisbane, Queensland, Australia Submitted 23 June 2010; accepted in final form 20 September 2010

magnetic resonance imaging; microgravity; spaceflight; countermeasures; intervertebral disk

around the world have set the long-term goal of manned missions to Mars. As part of this, it is imperative to optimize countermeasure exercise for such long-term flights to prevent musculoskeletal deterioration. It is relevant to develop countermeasures against changes at the lumbar spine due to the incidence of low back pain (LBP) as part of adaptation to spaceflight (46), and also recent data have shown that astronauts have a higher incidence of intervertebral disk herniation, particularly after spaceflight (22). Prevention of injury to passive structures of the lumbar spine, such as the intervertebral disk, is dependent on the muscular system and its guidance via the central nervous system (31). As such, in considering the effect of countermeasures against lumbar spine

SPACE AGENCIES AND GOVERNMENTS

Address for reprint requests and other correspondence: D. L. Belavý, Zentrum für Muskel-und Knochenforschung, Charité Campus Benjamin Franklin, Hindenburgdamm 30, D-12200 Berlin, Germany (e-mail: [email protected]). http://www.jap.org

changes, it is relevant to consider both the passive soft tissue, as well as active muscular elements of the spine. Prolonged bed rest is used as a model by space agencies to simulate the effects of spaceflight on the human body (30), but bed rest is also a model to help to understand the effects of inactivity and immobilization on the human body. Works on the lumbar spine in bed rest have provided evidence for a relationship between the incidence of LBP after bed rest and the preceding muscle atrophy (7, 19), particularly of the lumbar multifidus (MF) muscle (7), as well as morphological changes in the intervertebral disks occurring during bed rest (7, 19). Similar to spaceflight (46), LBP has also been noted to be particularly prevalent in the first week(s) of bed rest (21). In assessing the effectiveness of countermeasures against lumbar spine changes in bed rest, it would, therefore, be relevant to assess muscle size, alterations in the intervertebral disks, as well as LBP incidence. We have identified four studies (10, 14, 19, 29) that have, to date, considered countermeasure exercise for the lumbar spine in prolonged bed rest. One conclusion that is possible to draw from these works is that, to prevent or reduce atrophy of the spinal extensors (particularly of the MF muscle; Refs. 7, 10, 18), low-load exercise (typically at 100% of body weight or less) does little to prevent atrophy of the spinal extensors during prolonged bed rest (14, 19), but that higher load resistive exercise (typically above 1.5 times body weight) reduces atrophy in this musculature (10). This pattern is similar to studies on the lower limb musculature, which have found that aerobic (42) or low-load exercise (12, 47) is less effective in maintaining muscle size during bed rest than high-load exercise, although this also depends on the kinds of exercises performed (1– 4, 11, 25, 41). Furthermore, the countermeasures for the lumbar spine investigated to date have been effective in either reducing or preventing some of the changes in spinal morphology (such as increases in disk shape/size, increases in spinal length, changes in the lumbar lordosis; Refs. 10, 14, 19, 29). There are, of course, a number of aspects of countermeasure optimization to be considered. Recent work has considered the effect of high-load resistive exercise with whole body vibration as a countermeasure for the lumbar spine (10) and showed that, with the training performed 11 times a week, reduced changes in extensor and psoas (Ps) muscle cross-sectional area (CSA) were seen compared with the control group and increases of disk size and spinal length were impeded. Whole body vibration is thought to increase muscle activation (16, 36) via stimulation of the muscle spindle system (16, 33, 38), and prior work has shown it can be transmitted from the feet to the

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Belavý DL, Armbrecht G, Gast U, Richardson CA, Hides JA, Felsenberg D. Countermeasures against lumbar spine deconditioning in prolonged bed rest: resistive exercise with and without whole body vibration. J Appl Physiol 109: 1801–1811, 2010. First published September 23, 2010; doi:10.1152/japplphysiol.00707.2010.—To evaluate the effect of short-duration, high-load resistive exercise, with and without whole body vibration on lumbar muscle size, intervertebral disk and spinal morphology changes, and low back pain (LBP) incidence during prolonged bed rest, 24 subjects underwent 60 days of head-down tilt bed rest and performed either resistive vibration exercise (n ⫽ 7), resistive exercise only (n ⫽ 8), or no exercise (n ⫽ 9; 2nd Berlin Bed-Rest Study). Discal and spinal shape was measured from sagittal plane magnetic resonance images. Cross-sectional areas (CSAs) of the multifidus, erector spinae, quadratus lumborum, and psoas were measured on para-axial magnetic resonance images. LBP incidence was assessed with questionnaires at regular intervals. The countermeasures reduced CSA loss in the multifidus, lumbar erector spinae and quadratus lumborum muscles, with greater increases in psoas muscle CSA seen in the countermeasure groups (P ⱕ 0.004). There was little statistical evidence for an additional effect of whole body vibration above resistive exercise alone on these muscle changes. Exercise subjects reported LBP more frequently in the first week of bed rest, but this was only significant in resistive exercise only (P ⫽ 0.011 vs. control, resistive vibration exercise vs. control: P ⫽ 0.56). No effect of the countermeasures on changes in spinal morphology was seen (P ⱖ 0.22). The results suggest that high-load resistive exercise, with or without whole body vibration, performed 3 days/wk can reduce lumbar muscle atrophy, but further countermeasure optimization is required.

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Table 1. Group characteristics Subject Group Parameter

CTR

RE

RVE

N Age, yr Weight, kg Height, cm Total physical activity score BDC R ⫹ 180

9 33.1 ⫾ 7.8 80.6 ⫾ 5.2 181.3 ⫾ 6.0

8* 31.1 ⫾ 5.1 75.0 ⫾ 12.8 179.3 ⫾ 7.7

7 32.2 ⫾ 10.4 81.5 ⫾ 6.2 179.6 ⫾ 5.8

9.0 ⫾ 1.3 8.9 ⫾ 1.3

9.6 ⫾ 1.2 9.4 ⫾ 1.4

9.0 ⫾ 1.3 9.0 ⫾ 1.0

lumbar spine (24, 39). One issue that remains, however, is the extent to which whole body vibration has an additional effect above that of high-load resistive exercise alone. In the present work, we consider this issue in relation to lumbar muscle CSA changes and spinal morphology, as measured with magnetic resonance (MR) imaging, as well as LBP incidence during prolonged bed rest. MATERIALS AND METHODS

Bed-rest protocol, countermeasure exercise protocol, and subject characteristics. Twenty-four medically and psychologically healthy men participated in the 2nd Berlin Bed-Rest Study (BBR2–2). The study protocol is discussed in detail elsewhere (9). In brief, however, subjects attended the facility for the baseline data collection (BDC) from 9 days before a 60-day 6° head-down-tilt (HDT) bed-rest period and remained in the facility for a 7-day post-bed-rest observation period. During the HDT phase, subjects performed all hygiene activities in the HDT position. Exclusion criteria specifically relevant to the present study were a history of chronic LBP, a current episode of LBP, or any history of spinal operation. The study was approved by the ethical committee of the Charité Universitätsmedizin Berlin. All subjects gave their informed, written consent before participation in the study. Subjects were randomized to three different groups: one that performed resistive exercises with whole body vibration during bed rest (RVE; n ⫽ 7), one that performed resistive exercise only (RE; n ⫽ 8), and one that performed no exercise and served as a control group (CTR; n ⫽ 9). The sample size of the BBR2–2 was based on bone parameters and assumed a power of 0.8, an ␣-level of 0.05, and standard deviation of 8% for between-group differences in change in MF muscle CSA after bed rest (based on prior data from our group; Ref. 10). An effect size of 13% of whole body vibration, in addition to resistive exercise on the change in MF muscle CSA from the start to end of bed rest between the RVE and RE groups, should be able to be detected in the present study. Table 1 lists the baseline characteristics of the subjects, including data on habitual physical activity (5) before and 180 days after bed rest. This physical activity questionnaire has been validated previously (5) and assesses occupational (e.g., time spent standing, sitting, lifting heavy items, and so forth, at work), sport-related (i.e., types of sports played, how frequently, how long), and leisure-time (e.g., extent of television viewing) physical activity. The countermeasure exercise protocol is discussed in detail elsewhere (9). In brief, however, exercise maneuvers were chosen to J Appl Physiol • VOL

Fig. 1. Countermeasure exercise training. Both the resistance exercise-only (RE) and resistance exercise with whole body vibration (RVE) groups performed their exercises on the specially designed Galileo Space exercise device. Subjects were positioned in head-down tilt (HDT) on a moveable platform with shoulder pads and handgrips preventing downward movement and permitting application of force via the platform. A pneumatic system (not shown) generated the force, applied through the moveable platform, against which the subject needed to resist and move (via the shoulder pads and handgrips). The feet were positioned either side of a platform (left side), which could be set to vibrate in the RVE group. Subjects were given visual feedback of their actual and target position in the exercise via a monitor placed in the subjects’ field of view (not shown). Here the subject is performing single-leg heel raises to train the calf musculature.

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Values are means ⫾ SD; N, no. of subjects. There were no differences between groups for subject anthropometric or physical activity variables (F all ⱕ 1.8, P ⱖ 0.20). CTR, inactive control group; RE, resistive exercise-only group; RVE, resistive exercise with whole body vibration group. Total physical activity score refers to data from the Baecke et al. habitual physical activity questionnaire (Ref. 5; no units) completed before bed rest [baseline data collection (BDC)] and 6 mo after bed rest [recovery plus 180 days (R ⫹ 180)]. One RE and one CTR subject did not complete the questionnaire at this latter time point. *One subject in the RE group dropped out at head-down tilt (HDT) day 30; thus n ⫽ 7 after this time point. There were no differences before or after bed rest or between groups: P ⱖ 0.54.

target those load-bearing regions of the body that are most affected by bed rest (i.e., lower quadrant and lumbar region). Training was performed 3 days/wk during the HDT phase. After a short warm-up, the following exercises were performed on the Galileo Space exercise device (Novotec Medical, Pforzheim, Germany; Fig. 1): bilateral squats (⬃75– 80% of pre-bed-rest maximum voluntary contraction; in RVE group vibration frequency 24 Hz, amplitude 3.5– 4 mm, peak acceleration ⬃8.7 g, where g ⫽ 9.81 ms⫺2); single leg heel raises (⬃1.3 times body weight; in RVE group vibration frequency 26 Hz, amplitude 3.5– 4 mm, peak acceleration ⬃10.2 g); double leg heel raises (⬃1.8 times body weight; in RVE group vibration frequency 26 Hz, amplitude 3.5– 4 mm, peak acceleration ⬃10.2 g); and back and heel raise (performing hip and lumbar spine extension against gravity with ankle dorsiflexion, but with ⬃1.5 times body weight applied at the shoulders; in RVE group vibration frequency 16 Hz, amplitude 3.5– 4 mm, acceleration ⬃3.9 g). The RVE group performed the same exercises as the RE group, except that whole body vibration was applied (see Ref. 9. for more details on countermeasure protocol). Note that the acceleration parameters stated refer to the acceleration of the platform itself: effective accelerations on the subject are much lower. The maximum resulting ground reaction forces transmitted to the feet of the subjects result in effective acceleration at the feet in the order of 0.7 g (unpublished observations). MR imaging protocol and image measurements. MR imaging was conducted with the subjects supine 9 or 8 days before bed rest (BDC), on day 27 or 28 (HDT27/28) of bed rest, and then on day 55 or 56 (HDT55/56) using a 1.5-T Siemens Avanto scanner (Erlangen, Germany). Twenty-nine sagittal images (thickness: 3 mm; interslice distance: 0.3 mm; repetition time: 5,240 ms; echo time: 101 ms; field of view: 380 ⫻ 380 mm interpolated to 320 ⫻ 320 pixels) were taken to encompass the entire vertebral body and include the transverse processes from the lower thoracic spine (typically T10) to the sacrum (Fig. 2). Five groups of three slices each (slice thickness: 4 mm; interslice distance: 4 mm; repetition time: 7,560 ms; echo time: 97 ms; field of view: 260 ⫻ 234 mm interpolated to 320 ⫻ 288 pixels) were positioned over the transverse process of each vertebral body from L1 to L5 and to then angulated to be parallel to the superior vertebral endplate

LUMBAR SPINE COUNTERMEASURES DURING PROLONGED BED REST

of each vertebra (Fig. 3). Images were stored for further offline processing and then later, each data set was assigned a random number (www. random.org) to ensure operator blinding in image measurement. The same operator (D. L. Belavý) then used ImageJ 1.38x (http:// rsb.info.nih.gov/ij/) to measure the following parameters from the sagittal MR images (Fig. 2): 1) spinal length, sagittal distance between the dorsorostral corner of S1 and L1; 2) disk volume, via interpolation of sagittal plane CSA measures of each disk from L1/2 to L5S1; 3) anterior and posterior disk height from L1/2 to L5S1; and 4) lumbar lordosis between the superior endplates of L1 and S1. A positive angle denoted a “lordosis”. Bilateral CSA measurements of the lumbar MF, erector spinae (ES), quadratus lumborum (QL), and Ps muscles were conducted on the paraxial MR images (Fig. 3), and the average CSA from each of the three images at each vertebral level was calculated. CSA measures were then averaged between left and right sides. To accurately delineate MF and the more laterally placed longissimus muscle, the fascial border (13) separating these two muscles was used as an anatomic landmark. The CSA measures for the three MR images at each vertebral level were averaged, and then left and right sides were averaged before further analysis.

LBP questionnaires. At baseline MR scanning (BDC-8/-9), 2 days before bed rest (BDC-2), every day during the first 2 wk of bed rest (HDT1 to HDT14), thence at weekly intervals (HDT18, HDT25, HDT32, HDT39, HDT46, HDT53, HDT57), and in the 7 days of ambulant post-bed-rest recovery (R⫹1 to R⫹7), subjects were asked to fill out a LBP questionnaire. Subjects were asked to report whether LBP was present and mark its location on a body chart and its intensity (1–100) on a visual analog scale (VAS) (17). Incidence of LBP was defined as any report of pain or discomfort between the first lumbar vertebrae and the coccyx. Due to a number of subjects reporting “no pain” (i.e., VAS ⫽ 0), it became untenable to perform parametric (i.e., relying on normal distribution) or nonparametric (i.e., using a ranking procedure) procedures directly on the VAS data, as the underlying statistical assumptions for such procedures could not be held. Therefore, the presence or absence of pain and/or number of days with pain was used in further analysis. Statistical analyses. ␹2 analyses were used to evaluate differences between groups in the number of subjects reporting LBP. MannWhitney U-tests were used to evaluate the number of days of reported LBP: the number of days of reported pain for each subject was used as the independent variable and “group” as dependent variable (either CTR vs. RE, CTR vs. RVE, or RE vs. RVE) for separate two-group comparisons. Separate analyses were performed for the bed-rest and recovery phases. The analyses from during the bed-rest phase focused on the first week of bed rest (the results were similar when the entire bed-rest phase was evaluated). Linear mixed-effects models (32) were used to assess each of the spinal morphology variables with factors of group (RVE, RE, CTR), study-date (BDC, HDT27/28, HDT55/56), a group ⫻ study-date interaction, and linear covariates of baseline (BDC): subject age, height, and weight. Random effects for each subject and, where necessary, allowances for heterogeneity of variance (such as due to group or study-date) were permitted. For disk volume and height, the additional factor of vertebral level (L1/2, L2/3, L3/4, L4/5, L5/S1) and all appropriate interactions and random effects were also used. An ␣ of 0.05 was taken for statistical significance. For the muscle CSA data, to evaluate the effect of the countermeasures, the main focus of analyses was on the average CSA from all five vertebral levels, although analyses on data from each vertebral level were also subsequently evaluated. Unpublished data from our group on the lumbar muscles considered in the present study show such average lumbar muscle CSA measurements to be highly correlated (r ⱖ 0.96) with individual muscle volume. Separate models were constructed for each muscle, and a similar approach was used as per spinal morphology. Where the group ⫻ study-date interaction was significant on ANOVA, further two-group (i.e., CTR vs. RE, CTR vs. RVE, and RE vs. RVE) linear mixed-effects models were built using the same approach to examine which countermeasures had an effect and whether the response of the RVE and RE groups differed.

Fig. 3. Muscle cross-sectional area (CSA) measurements. Left: CSA measurements were made of the psoas (PS), erector spinae (ES), and multifidus (MF) muscles from L1 to L5. Quadratus lumborum (QL) was measured from L1 to L4, as it was typically absent at L5. Arrows indicate the fascial border between MF and ES, which aided delineation of these two muscles. Right: positioning of images at each vertebral level. Three images were obtained at each vertebral level, and the CSA for each muscle was averaged between these three images.

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Fig. 2. Measurements of spinal morphology. Left: before bed rest; right: at end of bed rest in same subject. Disk volume was interpolated from sagittal plane disk area measurements of each lumbar intervertebral disk (shown at L3/4 on left side of image). Anterior and posterior disk height was also measured (shown between L2/3 at left). The lumbar lordosis angle was calculated between lines drawn at the superior endplate of L1 and S1. Spinal length (right) was measured between the dorsorostral corner of S1 and L1 and was measured parallel to the plane of the magnetic resonance bed. Note the lengthening of the spine at the end of bed rest, increase in disk size, and flattening of the spinal curvature.

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To examine the potential influence of changes in spinal shape on muscle CSA, partial correlation analyses (controlling for study-date, group, and their interaction) were performed using data on percent change (compared with baseline) of each of the average (L1-L5) muscle CSA and spinal morphology parameters. The “R” statistical environment (version 2.6.1, http://www.r-project.org/) was used for all analyses. Unless otherwise specified, results are presented as means ⫾ SD. RESULTS

Fig. 4. Number of subjects reporting low back pain on each day of questionnaire completion. As similar numbers of subjects in each group reported low back pain, data have been presented pooled across all groups. Countermeasure subjects who reported low back pain did so more frequently; however, see text for further details. BDC, baseline data collection; HDT, day of HDT bed rest; R⫹: day of post-bed-rest recovery; CTR, control group. Until HDT30, n ⫽ 24; beyond this time point, n ⫽ 23.

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At baseline, there were no differences between groups at baseline for either the muscle CSA data (P all ⱖ 0.093; Tables 2 and 3) or spinal morphology parameters (P all ⱖ 0.20; Table 4). LBP questionnaires. At questioning in the BDC phase, five CTR, five RE, and four VRE subjects reported prior history of some kind of LBP in their life. These reports ranged from low-level pain after prolonged sitting or lifting activities, to four subjects reporting an episode of acute LBP once in their life. During the entire bed-rest phase, there were no significant differences in the number of reports of LBP (␹2 ⫽ 0.18, P ⫽ 0.91) in the CTR [4/9 subjects; median (minimum to maximum) reported VAS: 19 (12–33)], RE [7/8 subjects; VAS: 14.5 (3–55); note: 1 subject dropped out at HDT30], and RVE [3/7 subjects; VAS: 25.5 (8 – 46)] groups. The number of subjects reporting pain in each group over the course of the study is presented in Fig. 4. The majority of LBP reports during bed rest were within the first week, and the number of days of reported LBP during the first 7 days of bed rest was higher in the RE [mean (minimum to maximum) 2.6 (0 –5) reported days of pain of 7 possible days; U ⫽ 10, P ⫽ 0.011 vs. CTR group; VAS: 20 (4 –55)] and RVE [1.9 (0 –5) days of pain; U ⫽ 26, P ⫽ 0.56 vs. CTR group; U ⫽ 35, P ⫽ 0.44 vs. RE group; VAS: 25.5 (8 – 46)] groups than in the CTR group [0.6 (0 –2) days of pain; VAS: 19 (12–33)]. The mean days of reported pain, reported here as the median, was typically zero in the CTR and RVE groups. In the week after bed rest, similar numbers of subjects in each group [CTR: 5/9 subjects, VAS median (minimum to maximum): 12 (3– 42); RE: 4/7 subjects, VAS: 25.5 (7–35); RVE: 4/7 subjects, VAS: 30 (10 – 48); ␹2 ⫽ 0.32, P ⫽ 0.85] reported LBP. The number of reported days of LBP was also similar across all groups in the week after bed rest [CTR: 1.2

(0 – 4) reported days of pain of 7 possible days, RE: 1.0 (0 –3), RVE: 1.1 (0 –3); U ⱖ 25.5, P ⬎ 0.94]. Of the subjects reporting LBP (13 in total) after bed rest, 6 of these did not report LBP earlier during bed rest (CTR: 3, RE: 0, RVE: 3). Seven (CTR: 2, RE: 3: RVE: 2) of 14 subjects who reported LBP during bed rest did not report LBP in the week after bed rest. Muscle CSA. An overview of the average (across all vertebral levels) muscle CSA is presented in Fig. 5. For all muscles, ANOVA showed a different response in the three groups over the course of the study for these average CSA measurements (group ⫻ study-date interaction: F ⱖ 4.1, P ⱕ 0.004). The reduction in MF muscle CSA in the CTR group (⫺7.6 ⫾ 4.7% at mid-bed rest and ⫺10.1 ⫾ 4.4% at end-bed rest) was less in the RE group (⫺5.6 ⫾ 4.8% and ⫺5.6 ⫾ 4.5%; P ⫽ 0.028 vs. CTR) and RVE group (⫺4.7 ⫾ 3.5% and ⫺4.0 ⫾ 4.5%; P ⫽ 0.007 vs. CTR). A similar effect was seen for ES muscle CSA (CTR: ⫺8.3 ⫾ 4.0% at mid-bed rest and ⫺10.4 ⫾ 4.3% at end-bed rest) with significantly less loss in the RE group (⫺5.6 ⫾ 3.4% and ⫺4.0 ⫾ 3.9%; P ⫽ 0.0014 vs. CTR), with the effect in the RVE group (⫺6.1 ⫾ 1.9% and ⫺7.0 ⫾ 3.0%; P ⫽ 0.13 vs. CTR) being nonsignificant. Reductions in QL muscle CSA (CTR: ⫺6.2 ⫾ 5.5% at mid-bed rest and ⫺9.2 ⫾ 5.9% at end-bed rest) were ameliorated in the RE group (⫺2.1 ⫾ 3.2% and ⫺0.8 ⫾ 3.6%; P ⫽ 0.0005 vs. CTR) and RVE group (⫺1.7 ⫾ 2.5% and ⫺3.2 ⫾ 3.1%; P ⫽ 0.053 vs. CTR). The increases in Ps muscle CSA seen in the CTR group (⫹3.1 ⫾ 3.3% at mid-bed rest and ⫹3.6 ⫾ 3.8% at end-bed rest) were exacerbated in the RE group (⫹8.0 ⫾ 5.0% and ⫹11.0 ⫾ 5.1%; P ⫽ 0.004 vs. CTR) and the RVE group (⫹8.9 ⫾ 4.6% and ⫹10.2 ⫾ 5.0%; P ⫽ 0.030 vs. CTR). The response of the RE and RVE groups for average muscle CSA changes did not differ significantly (P ⱖ 0.12). Analysis also considered the effect of the countermeasures on muscle CSA at each intervertebral level. The losses of MF muscle CSA seen in the CTR group (Table 2) were reduced significantly by the countermeasures at L3 and L4 only (P ⬍ 0.006 otherwise at L1, L2, and L5: P ⬎ 0.33; see also Table 2). Reductions in ES CSA seen in the CTR group (Table 3) were ameliorated by one or both of the countermeasures at L1, L2, and L4 only (P ⬍ 0.036; Table 2), although the effect of RVE alone compared with control was not significant (see Table 2 legend). In the Ps muscle, CSA increases (Table 3) were

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Fig. 5. Average muscle CSA during bed rest. All values are in millimeters squared, and CSA measures were averaged across vertebral levels before analysis. Error bars at baseline (BDC) represent SD, and error bars at mid-bed rest (HDT27/28) and end-bed rest (HDT55/56) represent SD of the change compared with baseline. *P ⬍ 0.05, †P ⬍ 0.01, ‡P ⬍ 0.001: significant difference from baseline CSA. When considering CSA averaged across all vertebral levels, the effect of the countermeasures was significant for all muscles (F ⱖ 4.1, P ⱕ 0.004), with no differences between the RE and RVE groups (F ⱕ 2.2, P ⱖ 0.12). The RE group response differed from that of CTR for all muscles (F ⱖ 4.1, P ⱕ 0.022). The RVE group response differed from CTR significantly for MF (P ⫽ 0.007) and PS (P ⫽ 0.030), but not ES (P ⫽ 0.13) and QL (P ⫽ 0.053). See text and Tables 2 and 3 for results from individual vertebral levels.

otherwise P ⬎ 0.07). There was weak evidence for greater loss of ES CSA at L1 (P ⫽ 0.036), greater increases in PS at L5 (P ⫽ 0.026), and better maintenance QL at L3 (P ⫽ 0.054) in the RVE compared with the RE group.

Table 2. Multifidus and erector spinae CSA at each vertebral level Vertebral Level Study-Date

L1

L2

L3

L4

L5

472.2 ⫾ 77.9 ⫺2.3 ⫾ 6.0 ⫺7.0 ⫾ 5.1‡

753.1 ⫾ 102.4 ⫺9.8 ⫾ 5.6‡ ⫺13.5 ⫾ 6.3‡

952.1 ⫾ 96.9 ⫺10.6 ⫾ 6.2‡ ⫺12.2 ⫾ 6.4‡

Multifidus CTR group BDC, mm2 HDT27/28, % HDT55/56, %

245.5 ⫾ 39.9 ⫺3.8 ⫾ 6.8 ⫺6.0 ⫾ 6.5†

330.2 ⫾ 65.1 ⫺4.2 ⫾ 8.4 ⫺4.0 ⫾ 6.9

BDC, mm2 HDT27/28, % HDT55/56, %

278.3 ⫾ 47.3 ⫺4.3 ⫾ 5.1* ⫺3.6 ⫾ 6.6

378.7 ⫾ 78.0 ⫺3.6 ⫾ 5.9 ⫺2.6 ⫾ 4.5

Multifidus RE group 531.6 ⫾ 123.1 ⫺4.2 ⫾ 5.2* ⫺2.6 ⫾ 4.3

716.5 ⫾ 136.4 ⫺5.6 ⫾ 6.2* ⫺6.0 ⫾ 5.6†

878.4 ⫾ 118.6 ⫺7.6 ⫾ 6.6† ⫺9.1 ⫾ 7.1†

701.1 ⫾ 142.0 ⫺4.1 ⫾ 3.7† ⫺3.7 ⫾ 5.7

874.3 ⫾ 136.8 ⫺8.0 ⫾ 4.9‡ ⫺7.6 ⫾ 5.7†

1,731.9 ⫾ 271.7 ⫺7.4 ⫾ 5.3‡ ⫺8.8 ⫾ 5.8‡

1,280.4 ⫾ 262.6 ⫺9.8 ⫾ 10.0† ⫺8.4 ⫾ 8.6†

1,761.6 ⫾ 297.7 ⫺5.7 ⫾ 4.7† ⫺1.8 ⫾ 6.3

1,179.3 ⫾ 433.0 ⫺7.7 ⫾ 10.1* ⫺1.1 ⫾ 4.9

1,546.1 ⫾ 109.1 ⫺5.2 ⫾ 2.0‡ ⫺4.6 ⫾ 3.6†

1,126.9 ⫾ 206.7 ⫺6.2 ⫾ 6.2* ⫺4.5 ⫾ 6.7

Multifidus RVE group BDC, mm HDT27/28, % HDT55/56, %

263.5 ⫾ 51.1 ⫺3.4 ⫾ 3.7* ⫺1.2 ⫾ 5.1

348.6 ⫾ 47.8 ⫺4.2 ⫾ 4.4* ⫺2.9 ⫾ 6.0

BDC, mm2 HDT27/28, % HDT55/56, %

1,908.2 ⫾ 191.5 ⫺9.3 ⫾ 5.0‡ ⫺13.2 ⫾ 5.0‡

2,166.5 ⫾ 171.6 ⫺8.4 ⫾ 3.0‡ ⫺11.3 ⫾ 3.6‡

2

485.5 ⫾ 110.6 ⫺0.2 ⫾ 5.2 0.0 ⫾ 4.6

Erector spinae CTR group 2,090.5 ⫾ 189.3 ⫺7.1 ⫾ 3.8‡ ⫺9.6 ⫾ 4.5‡

Erector spinae RE group BDC, mm HDT27/28, % HDT55/56, %

1,957.4 ⫾ 188.5 ⫺7.6 ⫾ 5.5‡ ⫺6.7 ⫾ 4.9†

2,089.0 ⫾ 223.9 ⫺4.8 ⫾ 3.1‡ ⫺4.9 ⫾ 3.6†

BDC, mm2 HDT27/28, % HDT55/56, %

1,953.4 ⫾ 201.8 ⫺8.9 ⫾ 4.1‡ ⫺11.2 ⫾ 4.3‡

2,095.7 ⫾ 199.2 ⫺6.3 ⫾ 2.6‡ ⫺8.0 ⫾ 3.4‡

2

2,023.4 ⫾ 202.2 ⫺3.4 ⫾ 4.4* ⫺3.7 ⫾ 5.0

Erector spinae RVE group 1,894.6 ⫾ 111.9 ⫺4.2 ⫾ 3.1† ⫺5.9 ⫾ 3.9‡

Values are means ⫾ SD in mm2 at baseline (BDC) and in %change to baseline at bed-rest days 27/28 (HDT27/28) and 55/56 (HDT55/56). Values have been adjusted for subject age, height, and weight. *P ⬍ 0.05, †P ⬍ 0.01, ‡P ⬍ 0.001: significant difference from baseline value. A different response in the three groups (group ⫻ study-date interaction) was seen at L4, L2, and L1 in erector spinae (ES) (P ⱕ 0.033) and at L4 and L3 in multifidus (MF) (P ⱕ 0.006). At L4 and L3 in MF, both the RE and RVE differed from control (P ⱕ 0.010). In the ES, RE showed less loss of cross-sectional area (CSA) at all levels compared with control (P ⱕ 0.042), but this was not the case for RVE (P ⱖ 0.09). There was some evidence for a different response between RVE and RE in the ES at L1 only (P ⫽ 0.036; otherwise P ⱖ 0.28). J Appl Physiol • VOL

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greater in the RVE and RE groups than in the CTR group at L2, L3, and L5 (P ⱕ 0.028). In the QL muscle, the RVE and RE groups showed significantly less CSA loss of this muscle compared with the CTR group at L3 only (P ⫽ 0.0002;

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Table 3. Psoas and quadratus lumborum CSA at each vertebral level Vertebral Level Study-Date

L1

L2

L3

L4

L5

1,267.0 ⫾ 271.9 ⫹5.0 ⫾ 3.4‡ ⫹5.0 ⫾ 4.5†

1,815.4 ⫾ 354.3 ⫹2.9 ⫾ 4.3 ⫹3.0 ⫾ 4.8

1,999.4 ⫾ 303.1 ⫹0.9 ⫾ 3.5 ⫹1.2 ⫾ 4.0

1,827.6 ⫾ 300.2 ⫹6.0 ⫾ 2.6‡ ⫹8.6 ⫾ 3.5‡

1,889.0 ⫾ 465.5 ⫹2.9 ⫾ 3.0* ⫹6.4 ⫾ 2.6‡

1,704.1 ⫾ 230.2 ⫹7.8 ⫾ 4.5‡ ⫹9.7 ⫾ 5.2‡

1,785.2 ⫾ 183.4 ⫹5.5 ⫾ 5.0† ⫹6.0 ⫾ 5.3†

Psoas CTR group BDC, mm2 HDT27/28, % HDT55/56, %

182.3 ⫾ 65.2 ⫹1.4 ⫾ 17.1 ⫹13.0 ⫾ 19.3

665.5 ⫾ 162.5 ⫹7.1 ⫾ 4.0‡ ⫹7.5 ⫾ 5.3‡

Psoas RE group BDC, mm2 HDT27/28, % HDT55/56, %

311.2 ⫾ 203.6 ⫹9.6 ⫾ 9.2† ⫹9.2 ⫾ 8.0†

813.5 ⫾ 190.7 ⫹11.0 ⫾ 3.9‡ ⫹15.2 ⫾ 3.6‡

1,355.1 ⫾ 231.3 ⫹7.6 ⫾ 2.7‡ ⫹9.7 ⫾ 3.3‡ Psoas RVE group

BDC, mm HDT27/28, % HDT55/56, %

208.3 ⫾ 162.3 ⫹17.5 ⫾ 10.3‡ ⫹15.6 ⫾ 11.0‡

679.7 ⫾ 102.6 ⫹13.3 ⫾ 5.3‡ ⫹14.7 ⫾ 6.1‡

BDC, mm2 HDT27/28, % HDT55/56, %

162.3 ⫾ 61.2 ⫺3.9 ⫾ 27.3 ⫺5.0 ⫾ 12.2

394.3 ⫾ 59.7 ⫺2.0 ⫾ 5.9 ⫺6.3 ⫾ 5.9†

2

1,187.2 ⫾ 157.0 ⫹10.8 ⫾ 4.8‡ ⫹13.0 ⫾ 4.8‡

Quadratus lumborum CTR group 769.4 ⫾ 108.1 ⫺6.8 ⫾ 8.5* ⫺8.9 ⫾ 9.1†

Quadratus lumborum RE group BDC, mm HDT27/28, % HDT55/56, %

240.6 ⫾ 127.4 ⫺0.6 ⫾ 6.8 ⫹0.1 ⫾ 6.5

425.8 ⫾ 122.8 ⫺0.2 ⫾ 5.7 ⫺0.4 ⫾ 5.6

BDC, mm2 HDT27/28, % HDT55/56, %

214.1 ⫾ 177.7 ⫺2.5 ⫾ 9.2 ⫺2.2 ⫾ 10.4

391.4 ⫾ 119.7 ⫹0.7 ⫾ 3.6 ⫺0.2 ⫾ 4.9

2

614.0 ⫾ 111.3 ⫺2.6 ⫾ 4.1 ⫺0.7 ⫾ 3.7

903.6 ⫾ 219.8 ⫺3.5 ⫾ 4.8 ⫺1.2 ⫾ 5.3

Quadratus lumborum RVE group 616.2 ⫾ 104.6 ⫹0.6 ⫾ 3.2 ⫺3.3 ⫾ 4.4

825.7 ⫾ 78.8 ⫺4.2 ⫾ 3.4† ⫺4.5 ⫾ 3.6†

Values are means ⫾ SD in mm2 at baseline (BDC) and in %change to baseline at HDT27/28 and HDT55/56. Values have been adjusted for subject age, height, and weight. *P ⬍ 0.05, †P ⬍ 0.01, ‡P ⬍ 0.001: significant difference from baseline value. A different response in the three groups (group ⫻ study-date interaction) was seen at L4 in quadratus lumborum (QL) (P ⬍ 0.001) and at L5, L3, and L2 in psoas (PS) (P ⱕ 0.028). At L3 in QL, both RE and RVE showed a different response than CTR (P ⬍ 0.001), and this was also the case at L5, L3, and L2 in PS (P ⱕ 0.032). There was some evidence for a greater increase in PS CSA at L5 (P ⫽ 0.026) and QL CSA at L3 (P ⫽ 0.054) in RVE than RE (otherwise, P ⱖ 0.11).

Spinal morphology. All spine morphology variables showed significant changes during bed rest (F all ⱖ 6.7, P ⱕ 0.003). The countermeasures did not, however, influence these changes (F all ⱕ 1.4, P all ⱖ 0.22). Generally, a reduction in

the lumbar lordosis, increases in disk height anteriorly and posteriorly, lengthening of the spine, and increase in disk volume were seen (Table 4). The effect in the control and countermeasure groups was similar for all lumbar interverte-

Table 4. Changes in lumbar spine morphology during bed rest Spinal Morphological Parameter Study-Date

Lordosis (L1-S1)

Posterior disk height

BDC HDT27/28, % HDT55/56, %

46.7 ⫾ 6.4 ⫺1.9 ⫾ 4.7 ⫺2.5 ⫾ 4.7

6.5 ⫾ 1.0 ⫹10.3 ⫾ 3.5‡ ⫹8.2 ⫾ 3.5‡

BDC HDT27/28, % HDT55/56, %

48.1 ⫾ 6.9 ⫺4.8 ⫾ 6.3* ⫺1.9 ⫾ 6.2

6.6 ⫾ 1.0 ⫹8.6 ⫾ 3.0‡ ⫹7.8 ⫾ 3.0‡

BDC HDT27/28, % HDT55/56, %

50.6 ⫾ 6.5 ⫺3.0 ⫾ 4.2 ⫺3.1 ⫾ 4.2

6.3 ⫾ 1.0 ⫹8.3 ⫾ 2.9‡ ⫹7.8 ⫾ 2.8‡

CTR group

RE group

RVE group

Anterior disk height

Length (L1-S1)

Disk volume

11.6 ⫾ 1.4 ⫹4.4 ⫾ 2.6‡ ⫹4.6 ⫾ 2.6‡

177.8 ⫾ 7.0 ⫹2.4 ⫾ 1.0‡ ⫹2.8 ⫾ 1.1‡

27.4 ⫾ 1.8 ⫹7.4 ⫾ 4.8‡ ⫹6.7 ⫾ 4.8‡

11.3 ⫾ 1.4 ⫹3.2 ⫾ 2.6‡ ⫹4.2 ⫾ 2.5‡

171.3 ⫾ 7.3 ⫹2.3 ⫾ 1.1‡ ⫹2.1 ⫾ 1.1‡

26.0 ⫾ 2.0 ⫹6.9 ⫾ 5.0‡ ⫹7.0 ⫾ 4.9‡

11.3 ⫾ 1.4 ⫹5.9 ⫾ 2.7‡ ⫹5.8 ⫾ 2.7‡

177.8 ⫾ 7.1 ⫹2.4 ⫾ 1.0‡ ⫹2.5 ⫾ 1.1‡

26.7 ⫾ 2.1 ⫹4.1 ⫾ 4.5* ⫹4.9 ⫾ 4.5†

Values for baseline (BDC) are means ⫾ SD in absolute values in the following units of measure: lordosis (between L1 and S1), degrees; posterior disk height, anterior disk height, and spinal length (between L1 and S1), mm; disk volume, cm3. Values for HDT27/28 and HDT55/56 are means ⫾ SD in %difference to baseline. *P ⬍ 0.05, †P ⬍ 0.01, ‡P ⬍ 0.001: significant difference from baseline value. Analysis suggested that the volume of each of the lumbar disks responded similarly to bed rest and countermeasures (P ⬎ 0.84), therefore, data for disk volume represent averages across the five lumbar vertebral levels. Disk height represent averages of vertebral levels L1/2 to L5S1; however, ANOVA suggested a different response between vertebral levels for anterior disk height (F ⫽ 2.3, P ⫽ 0.021), but not posterior disk height (F ⫽ 0.72, P ⫽ 0.66); see Figure 6. Negative values for change in lordosis indicate a flattening of the spine (i.e., a reduction in lordosis). J Appl Physiol • VOL

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601.1 ⫾ 152.6 ⫺9.6 ⫾ 7.3‡ ⫺11.7 ⫾ 6.0‡

LUMBAR SPINE COUNTERMEASURES DURING PROLONGED BED REST

the interested reader, partial correlation analyses between the various spinal morphology parameters are shown in Table 6. DISCUSSION

The main finding of the present study was that high-load resistive exercise (with or without whole body vibration) performed for a short (5– 6 min, excluding rest periods) period of time, three days/wk, was capable of partially reducing muscle CSA losses in the MF, ES, and QL muscles, but did not prevent changes in disk volume, disk morphology, spinal length, or lordosis shape. Also, while a similar number of subjects in each group reported LBP during bed rest, the exercise subjects reported LBP more frequently, and this was significant in the RE subjects only. Furthermore, there was limited evidence for an additional effect of whole body vibration superimposed on high-load resistive exercise in ameliorating these changes in lumbar spinal muscle CSA. In considering the comparison between the RVE and RE groups, it is important to note that there were a limited number

Fig. 6. Percent change in posterior (top) and anterior (bottom) disk height. Values are means ⫾ SD of percent change in disk height compared with baseline at each vertebral level from L1/2 to L5S1. HDT27/28, scanning performed on 27th or 28th day of bed rest; HDT55/56, scanning performed on the 55th or 56th day of bed rest. Changes compared with baseline are significant (P ⬍ 0.05) when mean percent change is 5% or more. ANOVA provided little evidence of a different response between the three subject groups (P ⬎ 0.095). ANOVA suggested a different response between vertebral levels for anterior disk height (F ⫽ 2.3, P ⫽ 0.021), but not posterior disk height (F ⫽ 0.72, P ⫽ 0.66).

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bral disks (F all ⱕ 0.51, P all ⱖ 0.84); thus the data have been presented averaged across all vertebral levels in Table 4. Greater increases in anterior disk height were seen at the upper lumbar spine (F ⫽ 2.3, P ⫽ 0.021; Fig. 6), and nonsignificantly greater increases in posterior disk height at the lower lumbar spine (F ⫽ 0.72, P ⫽ 0.66; Fig. 6). Overall, posterior disk height increased more than anterior disk height. Disk volume increased significantly in all groups. Partial correlation analyses. To assess any potential influence of spinal shape changes on muscle CSA measurements, partial correlation analyses (controlling for study-date, group, and their interaction) were performed (Table 5). The results showed that increases in spinal length and disk height/volume, as well as reduction of the lumbar lordosis, could not explain the increases in Ps muscle CSA seen. Increases in spinal length showed only a weak association with reduction in QL CSA, with no correlation for the ES and MF. Increases in disk height/volume were in some instances associated with the decreases in paraspinal muscle CSA seen during bed rest, but these associations were generally weak and nonsignificant. For

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Table 5. Partial correlations between changes in spinal morphology and muscle size %Change in Average Muscle CSA (L1-L5) %Change in Spinal Morphology Parameter

MF

ES

PS

QL

Lordosis (L1-S1) Posterior disk height Anterior disk height Length (L1-S1) Disk volume (L1-S1)

0.24 ⫺0.36 ⫺0.36 ⫺0.07 ⫺0.25

0.21 ⫺0.40 ⫺0.49* ⫺0.08 ⫺0.28

0.05 ⫺0.23 ⫺0.20 0.07 ⫺0.24

0.03 ⫺0.33 ⫺0.26 ⫺0.40 ⫺0.46*

of subjects in each group. The sample size estimate of the BBR2–2 was based on bone parameters (8), and there may not be sufficient numbers of subjects for the RVE and RE comparison for the parameters considered in the present study. Furthermore, other measures of muscle function or measures of response of the vertebral disks to loading were not evaluated in the present study and may yield further insight into the effects of the countermeasures. An example of this is a limited effect of a lower body negative pressure countermeasure being seen on lumbar paraspinal CSA (14), while showing a positive effect on spine extension force (29) in the same subjects. Furthermore, while the extent of transmission of vibrations from the feet to the lumbar spine has been considered in the upright posture (24, 39), such evaluations have not been performed for the exercise setup used in the current bed rest, or in the other bed-rest studies (19, 34), where vibration has been applied. Such measurements would be important for the future. It is also important to consider some other limitations of the present work. While reductions in muscle CSA during bed rest will reflect true muscle fiber atrophy to a certain extent, other factors, such as connective tissue changes and muscle water content, will also impact upon the overall CSA changes. Although we did not measure muscle volume, other studies of lumbar muscle volume changes during bed rest (41) have shown, similar to the present work, significant increases in Ps and significant decreases in lumbar extensor muscle size. Using partial correlation analyses, an attempt was undertaken to examine whether changes in spinal morphology (i.e., flattening/lengthening of the spine) could explain some of the changes seen in muscle CSA. The results of these analyses suggested that the increases in Ps CSA seen are most likely not a result of spinal shape changes and hence more likely due to actual increase in muscle size. Also, the reductions in CSA of the MF, ES, and QL muscles are correlated with the increases in spinal length and/or disk height seen during bed rest, but, J Appl Physiol • VOL

Table 6. Partial correlations between spinal morphology variables Percentage Change in Spinal Morphology Parameter

Lordosis(L1-S1) Posterior disc height Anterior disc height Length (L1-S1)

Posterior Disc Height

⫺0.32

Anterior Disc Length Disc Volume (L1-S1) Height (L1-S1)

0.15 0.37

⫺0.68 0.67 0.13

⫺0.20 0.55 0.20 0.39

Values are Pearson’s correlation coefficient between changes in various spinal morphology parameters while controlling for study-date, subject-group, and their interaction. Data used for correlation analyses were expressed as percentage change at HDT27/28 and HDT55/56 compared to baseline (BDC) value.

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Values are Pearson’s correlation coefficient between changes in spinal morphology and muscle CSA, while controlling for study-date, subject-group, and their interaction. *Correlations 0.015 ⬍ P ⬍ 0.023; otherwise, P ⬎ 0.05. As the main interest lay upon whether changes in spinal shape could influence muscle CSA, data used for correlation analyses were expressed as %change at HDT27/28 and HDT55/56 compared with baseline (BDC) value. A positive correlation between lordosis and muscle CSA implies that reductions in lordosis (flattening of the spine) would be associated with increases in muscle CSA. A negative correlation between the remaining morphology parameters (disk height, spinal length, disk volume) and muscle CSA implies that lengthening of the spine/increases in disk volume is associated with reductions in muscle CSA. These results imply 1) that changes in spinal morphology cannot explain the increases in PS muscle CSA seen during the study; and 2) lengthening of the spine and increases in disk height/volume may explain part of the decreases seen in MF, ES, and QL CSA seen, but, given the relative weakness of these correlations, these factors cannot present the entire explanation.

given the relative weakness of these correlations, spinal morphology change cannot be the entire explanation for the changes in muscle CSA seen. Another limitation that should be considered is that the lumbar extension maneuver performed involved the effect of gravity, in addition to loading at the spine. While this exercise could not be performed in this fashion in microgravity, it would be conceivable to modify equipment to apply an appropriate resistance at the lower abdomen and pelvic region to resist “upward” lifting of the pelvis in weightlessness. Furthermore, astronauts are typically physically highly trained before spaceflight, whereas the current group of subjects was not. It could be of interest in future work to examine highly trained individuals in bed rest, although, based on our experience, subject recruitment for such a study would indeed be difficult. Finally, caution should be applied when considering the use of the high-load exercise protocols implemented in the present study when designing rehabilitation programs for deconditioned and/or elderly individuals. Based on the findings of the present study and other works, some further insights can be gained into countermeasure exercise prescription for the lumbar spine. In other investigations (10, 14, 19), where countermeasure exercise was performed on a more frequent schedule (at least once daily) and typically for a longer duration in each session, a significant amelioration of the changes in spinal morphology during prolonged bed rest was observed. This was despite some of these studies (14, 19) utilizing lower loading levels than in the present work. It is unlikely that the limited effect of the countermeasures on spinal morphology in the present study was due to the kind of the exercise maneuvers chosen: another work (19) utilized low-level (⬃60% body wt) static loading imposed onto the body via an upper body harness and vibration applied at the feet, but with no specific exercise maneuvers performed, and this countermeasure restricted increases in disk volume during bed rest. It is possible that the main issue is that a minimum level of axial loading of the spine occurs for sufficient duration rather than peak loading levels per se. This loading also likely needs to be dynamic in fashion, as opposed to continuous static loading (20, 43, 44). Nonetheless, our understanding of optimal loading levels and patterns for the intervertebral disk is still relatively limited (40). Overall, the available data suggest that an exercise regime must be conducted on either a more frequent schedule than 3 days/wk and/or of a longer duration than 6 min to ameliorate the effects of bed rest on changes in spinal morphology.

LUMBAR SPINE COUNTERMEASURES DURING PROLONGED BED REST

1 These data are only available in the German language. Hence, to summarize their findings on low back pain for the reader, in the Long-Term Bed-Rest (LTBR) study, 25 men participated in 90-day bed rest (3, 35, 45). Nine were allocated to a “fly-wheel” exercise group (3, 4), nine to control, and seven to a group that received pamindronate (a medication that reduces bone resorption but has no known effects on muscle or other soft tissues; Ref. 45) and also performed daily spinal movements with the aim to reduce back pain in bed rest. In addition to other measurements (spinal length and movement measurements, electromyographic measurements), spinal pain questionnaires were completed three time points before bed rest, daily in the bed-rest phase, and 4 days in the recovery phase up to R⫹10. Spinal pain referred to the entire spine, not just the lumbar region. The author stated that 76.5% of back pain reports were in the lumbar region, but did not give further information as to how this was divided between the groups or over time. The author found no impact of the spinal movement protocol on spinal pain during bed rest compared with control, but found a lower pain intensity (P ⬍ 0.05) on the first 2 days after bed rest in this group. In contrast to the other two groups, the fly-wheel group showed pain throughout the bed-rest phase, with a significantly (P ⬍ 0.05) higher spinal pain intensity in this group in the first week of bed rest.

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loading is applied more physiologically (i.e., not just at the shoulders), so that subjects can more easily control their spinal posture. While the subjects of the present study did perform exercise familiarization at low load before bed rest, it could be important to gradually increase loads as part of training before bed rest, similar to exercise programs in astronauts, where much training is done in the preflight phase. This may ensure more efficient loading of the musculature for maintaining muscle size and also avoid inappropriate loading of other structures so as to avoid increasing LBP incidence. We, of course, cannot make any assertions as to the actual anatomic/ neurological sources of the LBP reported in the present study. Finally, all groups showed increases in Ps muscle CSA, with this effect being the greatest in the countermeasure groups. This is in contrast with prior findings (10), where a resistive vibration exercise countermeasure prevented increases in Ps muscle CSA during bed rest, but similar to other work (41), where a greater increase in Ps volume was seen in subjects performing high-load resistive exercise. As the countermeasure subjects typically moved into a hyperlordotic position when fatigued [in contrast to prior work (10), where subjects typically moved into spinal flexion], this may have facilitated Ps muscle activity, but subject positioning during unilateral heelraise exercises (see Fig. 1) may have also contributed, or potentially even trunk elevation in the lumbar extension exercise. The finding that Ps muscle CSA (Ref. 10 and present work) and volume (41) increased during bed rest, with nonsignificant increases seen in other studies (14, 28), while reductions in size of this muscle are seen after spaceflight (26, 27) may indicate that bed rest does not necessarily represent true “inactivity” of all muscle groups in the human body. In summary, the present work showed that high-load resistive vibration exercise (with or without whole body vibration) performed for 5– 6 min, 3 days/wk, did not significantly ameliorate changes in disk volume, disk morphology, spinal length, or lordosis shape. These countermeasures did reduce paraspinal muscle atrophy, but this effect did not influence all parts of the paraspinal muscles to the same degree, and the exercise subjects reported LBP more frequently. Finally, the present study found limited evidence of an additional beneficial effect of whole body vibration during high-load resistive exercise on changes in spinal muscle CSA during prolonged bed rest, although limited subject numbers may well have been involved in this negative finding. ACKNOWLEDGMENTS The authors thank the subjects who participated in the study, the staff of the Pflegedirektion, the nurses who cared for the subjects, and the many colleagues at the Charité who all helped make the 2nd Berlin Bed-Rest Study a success. GRANTS This work was supported by Grant FE 468/5-1 from the German Research Foundation (DFG). The 2nd Berlin Bed-Rest Study was supported by Grant 14431/02/NL/SH2 from the European Space Agency and Grant 50WB0720 from the German Aerospace Center (DLR). The 2nd Berlin Bed-Rest Study was also sponsored by Novotec Medical, Charité Universitätsmedizin Berlin, Siemens, Osteomedical Group, Wyeth Pharma, Servier Deutschland, P&G, Kubivent, Seca, Astra-Zeneka, and General Electric. D. L. Belavý was supported by a postdoctoral fellowship from the Alexander von Humboldt Foundation.

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Some insights can also be gained from the data of the present study for exercise prescription for the prevention of muscle CSA changes in bed rest. In prior work (10), where 11 sessions of exercise were performed per week, a 7.6% loss of MF muscle CSA was seen at L4 in these RVE subjects. The present work showed a 5.6% loss in the RE subjects, and a 4.1% loss in the RVE subjects was seen at the same vertebral level. These findings raise the possibility, suggested also by other work (4), that high-load resistive exercise need not necessarily be performed on a daily (or more frequent) schedule to have a similar effect on muscle loss. Compared with prior work on the lumbar spine (10, 14, 19) and lower limbs (11, 12, 42, 47), the present study further supports the argument that high-load resistive exercise (be it with or without whole body vibration) is needed to prevent muscle atrophy during bed rest, although other issues (such as duration and frequency of exercise) still need further clarification. These loading levels are, however, not without risks, it appears. The exercise subjects in the present study reported LBP more frequently than the inactive subjects, although this effect was only significant in the RE group. Other work (37) using high-load resistive exercise on the fly-wheel device also showed greater incidence of LBP in their high-load countermeasure subjects.1 In another bed-rest study (19), where countermeasure subjects received low-load axial compression of the spine with whole body vibration 10 min/day, LBP incidence during bed rest was less frequent than that of their control group, although this effect was not statistically significant in the bed-rest phase (6). One caution to these self-report LBP findings is that a number of factors can influence the extent of reporting of LBP, including subject personality characteristics and also subject expectations regarding the effects of training. With this in mind, however, it could well be that high-load resistive exercise needs to be implemented with care to limit, and ideally reduce, LBP occurrence, while at the same time attempting to prevent muscle atrophy. In the current study, we noted that it was difficult for the subjects to maintain (lumbar) spinal posture due to the high loads imposed at the shoulders, with training subjects typically moving into a hyperlordotic position. Given the role of the paraspinal musculature in maintaining the spinal curves (23), and the effect of spinal posture on muscle activation (15), it could be important to adjust exercise apparatus such that

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DISCLOSURES D. Felsenberg acts as a consultant to the European Space Agency and Novotec Medical for the exploitation of the results of this study. All other authors have no conflict of interest. REFERENCES

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