Hendershot Brad, Bazrgari Babak, Muslim Khoirul, Toosizadeh Nima, Nussbaum Maury A, Madigan Michael L
Virginia Tech - Wake Forest School of Biomedical Engineering and Sciences, USA.
Clin Biomech (Bristol). 2011 Mar;26(3):250-6. doi: 10.1016/j.clinbiomech.2010.09.019. Epub 2010 Oct 20.
Experimental studies suggest that flexed working postures reduce passive support of the spine, which could represent a significant risk factor for the development of occupational low back disorders. Neuromuscular compensations to reduced passive stiffness include increases in baseline activity or reflexive activation of trunk muscles. Yet, alterations and recovery of the synergy between active and passive tissues following prolonged flexion in humans are currently unknown.
Twelve healthy participants were exposed to all combinations of two trunk flexion durations (2 and 16 min) and three flexion angles (33, 66, and 100% of individual flexion-relaxation angle). Load relaxation was recorded throughout exposures, whereas trunk stiffness and reflexive behaviors of the lumbar extensor muscles were investigated during dynamic responses to sudden perturbations.
The magnitude of load relaxation increased with increasing flexion angle. Trunk stiffness decreased and reflex gains increased following flexion exposures; for both outcomes, acute changes were larger following exposure to increasing flexion angle. Reflex gains remained elevated one hour after exposure to maximum flexion.
Exposure to prolonged trunk flexion changed trunk stiffness and reflex behavior in patterns consistent with epidemiological evidence linking such exposure with the risk of occupational low back disorders. Observed increases in reflex gains, at least among healthy individuals, may be a compensation for decreases in passive trunk stiffness following acute exposure to flexed postures. It remains to be determined whether the neuromuscular system can similarly respond to accumulated disturbances in passive structures following exposure to repeated flexion tasks.
实验研究表明，屈曲工作姿势会减少脊柱的被动支撑，这可能是职业性下背痛发展的一个重要风险因素。对被动僵硬度降低的神经肌肉补偿包括基线活动增加或躯干肌肉的反射性激活。然而，目前尚不清楚人类长时间屈曲后主动和被动组织之间协同作用的改变和恢复情况。
12名健康参与者接受了两种躯干屈曲持续时间（2分钟和16分钟）和三种屈曲角度（个体屈曲-放松角度的33%、66%和100%）的所有组合。在整个暴露过程中记录负荷松弛情况，而在对突然扰动的动态反应中研究躯干僵硬度和腰伸肌的反射行为。
负荷松弛的幅度随屈曲角度的增加而增加。屈曲暴露后躯干僵硬度降低，反射增益增加；对于这两个结果，随着屈曲角度增加，急性变化更大。暴露于最大屈曲后一小时，反射增益仍保持升高。
长时间躯干屈曲暴露改变了躯干僵硬度和反射行为，其模式与将此类暴露与职业性下背痛风险联系起来的流行病学证据一致。观察到的反射增益增加，至少在健康个体中，可能是对急性暴露于屈曲姿势后躯干被动僵硬度降低的一种补偿。在暴露于重复屈曲任务后，神经肌肉系统是否能类似地对被动结构中的累积干扰做出反应，仍有待确定。
