Biomechanical effects of sitting with adjustable ischial and lumbar support on occupational low back pain: evaluation of sitting load and back muscle activity
From: BMC Musculoskelet Disord. 2009 Feb 5;10(1):17. [Epub ahead of print]
Occupational low back pain is alarmingly common with a 100 million workdays lost in the United States each year. As the leading cause of disability in individuals less than 50 years old, low back pain imposes a tremendous economic burden, with an estimated annual productivity lost of $28 billion in US. Although the incidence of occupational back injury has been decreasing lately, the percentage of low back pain among all the occupational back injury still is increasing and occupational low back pain remains the primary problem for US industry. A number of occupational risk factors have been cited, prolonged sitting, in combination with awkward postures, has been confirmed to increase the likelihood of having low back pain. In fact, occupations which require prolonged sitting have a 3.2 relative risk of low back pain within the first year of employment. Two frequently cited risk factors of low back pain from occupational sitting are: 1), prolonged static sitting and 2) reduced lumbar lordosis.
During sitting, upper body weight is carried mainly by the ischial tuberosities. Elevated pressures at the ischial tuberosities is intimately associated with elevated spinal loads. Animal study provided evidence that sustained static load on the ligaments of the lumbar spine may result in paraspinal muscle spasm and hyperexcitability. Furthermore, metabolite accumulation from static load may accelerate disc degeneration and herniation. A lordotic lumbar spine has been considered a load absorber in the manner of a spring and it was found that the lumbar lordosis reduces intradiscal pressures and transferring load to the posterior annulus and apophyseal joints. These findings suggested that a lordosis in lumbar spine may have a protective effect on low back pain.
Seating options, such as lumbar supports, forward tilted seat pans, and reclined seat/backrest have been assessed for the effect on reducing the spinal loading and paraspinal muscle activities during sitting. Although lumbar supports enhances lumbar lordosis, decrease intradiscal pressure, and may reduce paraspinal muscle hyperactivity, there is concern regarding the effectiveness of lumbar supports alone. While these static seating designs have a positive effect in increasing lumbar lordosis or decrease pressure under the ischial tuberosity for the seated individual, they have limited effect in preventing pressure overload from prolonged static sitting. Therefore, periodic alternation between sitting and standing has been suggested to prevent the malignant effects of prolonged static sitting. Although it has an overall improved effect in low back pain prevention than prolonged sitting, continuous spinal loading occurs in each position with minimal dynamic movement, providing minimal rest/change on muscular activation levels.
Makhsous et al. demonstrated that lumbar supports combined with an ischial release mechanism had a significant effect in decreasing ischial pressure and maintaining lumbar lordosis in asymptomatic subjects. Therefore, we expected that this mechanism addressed some of the critical risk factors of low back pain through enhanced lumbar support designed to maintain lumbar lordosis, and the reduced ischial pressure to reduce lumbar spinal load. The hypothesis of this study was that the Off-Loading posture will reduce the sitting load and lumbar muscular activity in low back pain patients.
We reported previously the beneficial biomechanical effects of sitting in the Off-Loading sitting posture in asymptomatic subjects. The purpose of this investigation was to determine whether the Off-Loading sitting posture was also beneficial to low back pain individuals, and if any differences occurred between subjects with and without low back pain. Our low back pain target population was focused on occupational related low back pain. Based on the Roland Morris and Oswestry Low Back Pain Questionnaires, our low back pain group suffered from mechanical low back pain with a moderate disability rating.
Callaghan, et al showed that the sitting posture has significantly higher low back compressive loads than standing. As elevated ischial tuberosity pressures from sitting are intimately associated with elevated spinal loads, a seating device which can decrease ischial tuberosity pressures may help decrease and/or prevent low back pain. Results of the current study demonstrate the beneficial effects of the Off-Loading sitting posture in both Asymptomatic and low back pain subjects in improving overall pressure parameters, as both groups had a significant reduction of load on the ischial tuberosities, while redistributing load anteriorly towards the thighs, over a larger supporting surface. Furthermore, backrest load was increased, with the main component in the posterior direction. The authors believe that this increased posterior load on backrest has a beneficial effect in maintaining lumbar lordosis.
Many investigators have reported the negative effect of the seated posture associated with increased paraspinal muscle activity. The seated posture results in sustained static load of lumbar viscoelastic tissues, resulting in spinal collagen micro-damage, paraspinal muscle spasm, and maybe a transient neuromuscular disorder. It was reported that significant mechanical loading of the spine is associated with low back pain resulting from trunk muscle coactivation. Thus, decreasing paraspinal muscle activity may also help minimize low back pain.
In this study, for both the Asymptomatic and low back pain groups, the Off-Loading posture significantly decreased paraspinal muscle activity at the lumbar levels. These findings are similar to our previous study, in which we found decreased lumbar EMG levels in asymptomatic subjects with Off-Loading posture. Our previous investigation has demonstrated that the Off-Loading posture helped rotate the pelvis forward, which may contribute to the decrease of the lumbar paraspinal muscular activity. As less paraspinal muscle effort is necessary to stabilize the spine, this may prevent muscle fatigue and improve overall comfort for the seated individuals. Although the Off-Loading posture has the benefit of decreased lumbar paraspinal activity, they had concern that this sitting posture may possibly cause increased muscle activity at other spinal levels. However, their data demonstrate that the Off-Loading posture had no negative effect at the recorded thoracic levels.
They observed that, in the Off-Loading posture, the Asymptomatic group had a significantly greater increase of TCA on the middle seat, and a greater decrease in paraspinal muscle activity at the L5 and L2 levels, than those seen from low back pain group. Although they are not certain as to the exact cause behind these differences, it is probably multifactorial, as it has been commonly observed that individuals with chronic low back pain have difficulty in adopting a neutral posture of the lumbar spine or back posture, and that static balance might have been disturbed. However, the mechanism underlying the differences should be further investigated.
The results from this investigation support their hypothesis that Off-Loading posture has a beneficial effect in pressure/load redistribution and lumbar paraspinal muscle activity in both asymptomatic and low back pain subjects. In their previous pilot investigations, the Off-Loading posture also improved overall spinal alignment in a group of asymptomatic subjects. In yet another part of their study, they will investigate the effect of the Off-Loading posture in both low back pain and asymptomatic subjects on lumbar lordosis. Furthermore, as prolonged static sitting is another major cause of low back pain, their further study will also investigate the effect of Alternate sitting in a real working environment, on low back pain subjects over a period of time.
The Off-Loading sitting posture, defined as a released ischial support and an enhanced lumbar support, has been found to significantly redistribute the sitting load passing through ischial tuberosities to the lumbar spine, and reducing lumbar paraspinal muscle activity in both asymptomatic and low back pain subjects. It is concluded that this Off-Loading sitting posture might be beneficial to people whose profession requires prolonged sitting.
