Simulated microgravity and the lumbo-pelvic musculature Development and application of novel analysis techniques and implications for lumbo-pelvic pain ætiology

Abstract

The presence of gravity throughout evolution has shaped the human body. The European Space Agencys Berlin Bed-Rest Study (BBR) offered a unique opportunity to researchers from The University of Queensland to study the effects of a lack of weightbearing (inactivity, unloading and/or sedentarism) on the lumbo-pelvic (LP) region. LP pain has been linked to a lack of muscular stabilisation and some authors have suggested a link between LP pain and inactivity. Furthermore, studies in bed-rest and microgravity have largely ignored the LP region, hampering the development of effective countermeasures. The muscular systems of the LP region are differentially involved in postural control and joint stabilisation. By its effect on the body and environment, gravity generates continual challenge to the central nervous system (CNS) for controlling the muscles of the LP region. As part of coping with this challenge, the CNS selectively recruits the deep (local) muscles to stabilise the joints with more task-oriented activation in the superficial (global) muscle systems. Concomitantly, the CNS exploits a number of motor control strategies: modulation ongoing (tonic) muscle activity and overall activation levels as well as anticipatory muscle function and co-contraction. Studies where the gravitational challenge is removed (bed-rest, spaceflight and related unloaded environments) show significant effects on the neuromuscular system. Greater atrophy of deep anti-gravity extensor muscles occurs. A down-regulation of anticipatory muscle activity occurs. Underactivity of the deep muscles with little change or overactivity in the superficial and flexor systems is observed. Tonic muscle contraction is inhibited and there is an increase in superficial muscle co-contraction. These findings bear parallels to changes observed in LP pain. Selective atrophy of the deep extensor muscles occurs. A delay of anticipatory muscle activity, especially in the deep muscles, is also observed. Superficial LP muscle overactivity and increased co-contraction has also been shown. Recent studies have also found a reduction in tonic muscle contraction in the deep systems. Importantly, however, there are distinct limitations in the quantification methods used to measure tonic and phasic muscle activity. The goals of this work were firstly to develop novel electromyographic quantification algorithms and secondly to examine the effect of bed-rest on the LP musculature; specifically in terms of muscle size (cross-sectional area, CSA), tonic and phasic muscle contraction, timing of muscle activity, activation levels and co-contraction. As part of the BBR, twenty subjects underwent 8-weeks bed-rest with 1-year follow-up. Ten subjects received a vibration exercise countermeasure, whilst the remaining ten were completely inactive. Non-invasive magnetic resonance imaging was used to study CSA changes in the deep and superficial muscles and superficial electromyography during a repetitive-movement task provided data on the motor control parameters. In addition to the development of novel analysis techniques, the key outcomes of this thesis were findings on the effect of bed-rest: (a) selective atrophy of the deep multifidus muscle (b) increased CSA of the LP flexor muscles, which appears to persist up to 6-months after bed-rest in the oblique muscles, (c) a shift from tonic to phasic muscle contraction in the superficial LP muscles, (d) a delay of LP muscle activation, (e) superficial LP muscle overactivity, (f) decreased co-contraction and, critically, (g) that a number of the changes persist up to the end of the 1-year follow-up period. Another important finding is that the vibration exercise countermeasure employed in the BBR prevented an overwhelming number of the changes observed in the inactive subjects. However, whilst reducing multifidus atrophy, it did not prevent it, and did not reduce lumbar erector spinae muscle atrophy. The significance of these results are that, firstly, it suggests the pattern of LP muscle and motor control change occurring in bed-rest is indeed similar to that observed in LP pain. This could suggest that long-term inactivity results in neuromuscular changes that reduce LP stabilization and increase risk of LP pain. Importantly, also, this thesis is the first detailed study of the LP musculature in bed-rest and provides information to space agencies necessary for the development of countermeasures. Furthermore, the vibration exercise countermeasure was in part effective in ameliorating the effects of bed-rest, and further steps can be taken for its optimization