The 2nd Berlin BedRest Study has provided a unique set of circumstances to investigate the effects of two different rehabilitation programs. The advantage of this study was that a homogenous group of healthy male individuals was exposed to a controlled environment known to induce predictable changes in lumbo-pelvic muscles. Both rehabilitation programs used in this study are currently widely used in the community for people with and without LBP [45
Both rehabilitation programs adopted were successful in restoring the CSA of the multifidus muscle. This is important as decreases in the CSA of the multifidus muscle have been documented not only in those who have undergone bed rest studies [4
], and these changes have been linked to LBP incidence after bed rest [4
]. Data from astronauts [36
] suggest that similar changes in multifidus may occur in microgravity as well as in individuals with acute and chronic LBP [14
]. While the mechanism of muscle atrophy may not be the same in these different populations, findings were promising in that they showed that rehabilitation of the multifidus muscle was successful for both groups by R+14. Changes were greatest during the period of intensive rehabilitation between R+2 and R+14, with no significant improvements seen between R+14 and R+90 (when subjects were seen less regularly and were performing predominantly home exercise programs). As the TFS training targeted the trunk flexor muscles, the improvement in CSA of the multifidus muscle in this group is most likely due to co-contraction of the abdominal muscles with the multifidus and erector spinae muscles during the exercises performed. This phenomenon has been shown in EMG studies of exercises such as the sit-up [57
], and underlies the common belief that doing exercises such as sit-ups will simultaneously strengthen the back muscles. A possible explanation for the greater increase in multifidus CSA at the L3 vertebral level in the TFS group compared to the SMC group may relate to the location of the vertebral level (middle of the lumbar spine) being loaded by exercises which lift the trunk (maximal effect upper to mid-lumbar spine) and lift the lower limbs (maximal effect lower lumbar spine to mid-lumbar spine), though the overall higher loads developed during TFS group training may have also played a role. With regard to the SMC training, similar improvements in multifidus muscle CSA have been seen in subjects with acute LBP [24
] and athletes with LBP [26
] who underwent a similar training program.
For the psoas muscle, which increased in size during bed rest, there was a different result for the two rehabilitation groups. The TFS training resulted in an initial increase in the CSA of the psoas muscles from the end of bed rest up until R+14, with decreases after that period. This was not seen in the SMC group.
The increase in the psoas muscle size which was demonstrated in the TFS group was likely due to its action as a powerful hip and lumbar spine flexor. In this capacity, the psoas is capable of exerting substantial loads on the lumbar spine due to its attachment to the vertebral column [11
]. For this reason, researchers have advised that increasing psoas action by spine and hip flexion could have detrimental effects on the lumbar spine by increasing compressive forces. It has, therefore, been suggested that, when performing abdominal exercises in the supine position, exercises should focus on spine flexion alone [2
] rather than spine and hip flexion [2
]. Researchers have further advocated performing trunk flexion exercises with the hips and knees bent to (1) reduce tension in the psoas muscle [29
], (2) reduce involvement of the hip flexors [18
] and (3) to reduce the torque produced [48
]. However, others have shown that using this position decreased compressive forces [29
], while another study showed that it made no difference to the induced compressive forces on the spine [2
When considering the effects of the psoas and multifidus muscles working together in normal function, Quint et al. [53
] proposed that theses muscles co-contract to increase the overall stiffness of the lumbar spinal segments. Using fixed levels of simulated muscle forces in the multifidus and psoas muscles, these researchers demonstrated this to be the case for axial torque and lateral bending, but such co-contraction acted to destabilise
the lumbar segments in flexion. Thus, the situation of an imbalance between the hypertrophied psoas muscle, which induces a flexion force on the spine, and the atrophied multifidus muscle may be detrimental to the lumbar spine. Interestingly, increased psoas muscle size has also been documented in athletes with LBP when compared with athletes without LBP [43
This result would support the theory that it may be preferable to use SMC and functional retraining rather than floor-based programs following bed rest. The results of increasing the size of the multifidus muscle without concurrent increase in psoas muscle size may be due to the focus on obtaining and maintaining an “s” curve (thoracic kyphosis and lumbar lordosis) in SMC training with axial loading of the spine. This principle of exercise which has been demonstrated to be effective in the rehabilitation of LBP, and now for bed rest subjects, may also apply to training programs used for astronauts.
The other main consideration of the exercises used is the potential effect on the IVD and vertebrae. Exercises in the supine position which involve lifting the trunk or lifting the legs are often prescribed with caution in those with LBP due to the potential for increased intradiscal pressure [2
]. Evidence of this concern has been confirmed in patients with osteoporosis where a greater percentage of further wedge or compression fractures have occurred in subjects performing flexion exercises such as sit-ups [56
]. The results from the current study showed that the forces induced by the TFS program were large enough to induce changes in the anterior disc height and disc volume. While this may seem a desirable effect, it happened rapidly (by R+14), and other works [4
] have found a link between IVD changes during bed rest and LBP subsequent to bed rest. The incidence of LBP after bed rest was marginally (non-significantly) higher in the TFS group and hence a gradual return to more normal disc morphology may be preferable from a safety perspective.
While the tissues of the lumbar spine may be more susceptible to damage following bed rest [4
] and spaceflight [30
], this does not mean that these subjects should never perform higher load exercises, as the spine is exposed to high loads in everyday activities such as lifting [42
], and even coughing and sneezing [49
], due to forces created by the trunk muscles themselves. It may be sensible to commence training for bed rest subjects and astronauts using an approach such as SMC, with progression later on to higher load activities, when the spinal morphology has returned to normal. Another feature of appropriate exercise may be the use of weight bearing (closed chain) exercises which activate antigravity extensor muscles and apply axial loading to the skeleton and soft tissues [54
The main limitation of this study was the small subject sample size, which is common to all bed rest studies due to their complex nature and expense. Despite the small sample size, significant changes were seen in the muscles of the lumbo-pelvic region in terms of muscle atrophy, muscle hypertrophy and response to rehabilitation. Although it was not possible to include a control group for the rehabilitation phase of the study due to small numbers, results from the 1st Berlin BedRest Study [6
] (where subjects did not receive rehabilitation after bed rest) showed that changes in the multifidus muscle were long-lasting without rehabilitation.