Solomonow M, He Zhou B, Baratta R V, Lu Y, Zhu M, Harris M
Bioengineering Laboratory, Department of Orthopaedic Surgery, Louisiana State University Medical Center, New Orleans, LA 70112, USA.
Clin Biomech (Bristol). 2000 Mar;15(3):167-75. doi: 10.1016/s0268-0033(99)00062-5.
To determine the rest duration required for full recovery of reflexive muscular activity and laxity/creep induced in the lumbar viscoelastic structures (e.g., ligaments, discs, etc.) after 50 min of cyclic loading, and to develop a model describing such recovery.
It is well established that steady, cyclic or vibratory loading of the lumbar spine induces laxity/creep in its viscoelastic structures. It was also shown that such viscoelastic creep does not fully recover when subjected to rest equal in duration to the loading period. Rest periods of 24 h, however, were more than sufficient to allow full recovery. The exact period of time allowing full recovery of viscoelastic laxity/creep, and its pattern is not known. It is also not known what is the duration required for full recovery of reflexive muscular activity lost due to the laxity/creep induced in the spine during cyclic loading.
The lumbar spine of 'in vivo' feline preparations was subjected to 50 min of 0.25 Hz cyclic loading applied v ia the L4/5 supraspinal ligament. At the end of the loading period the spine was subjected to prolonged rest, interrupted by a single cycle loading applied hourly for measurement purposes until the laxity was fully recovered (>90%). Reflexive EMG activity was recorded with wire electrodes from the L-1-L-7 multifidus muscles. A biexponential model was fitted to the load and EMG recorded in the recovery period in order to represent viscous and elastic components of structures with different architecture (e.g., disc vs. ligament).
Full recovery of the laxity induced by 50 min of cyclic loading at 0.25 Hz required 7 h and was successfully fitted with a biexponential model. Similarly, EMG activity was fully recovered in 4 hours, and often exceeded its initial value during the following 3 h.
Full recovery of laxity induced in the lumbar viscoelastic structures by a given period of cyclic loading requires rest periods, which are several folds longer than the loading duration. Similarly, reflexive muscular activity requires 4 h of rest in order to be restored. Meanwhile, significant laxity can be present in the joints, exposing the spine to potential injury and low back pain. Increased EMG activity at the end of the recovery period may indicate that pain was possibly induced in the spinal structures, inducing hyperexcitability of the muscles during passive loading.
Although the data was derived from a feline model, and its extrapolation to the human model is not straightforward, the general pattern of decreasing reflexive muscular activity with cyclic loading is expected in both species. Therefore, workers who subject their spine to periods of cyclic loading may be exposed to prolonged periods of laxity beyond the neutral zone limits, without protection from the muscles and therefore the risk of possible injury and low back pain. Pain and muscle hyperexcitability could also be a factor associated with cyclic loading, being expressed several hours after work was completed.
确定在50分钟周期性加载后，腰椎粘弹性结构（如韧带、椎间盘等）中反射性肌肉活动以及松弛/蠕变完全恢复所需的休息时间，并建立描述这种恢复的模型。
腰椎的稳定、周期性或振动性加载会在其粘弹性结构中引起松弛/蠕变，这一点已得到充分证实。研究还表明，当休息时间与加载时间相同时，这种粘弹性蠕变不会完全恢复。然而，24小时的休息时间足以实现完全恢复。粘弹性松弛/蠕变完全恢复的确切时间及其模式尚不清楚。同样未知的是，由于周期性加载过程中脊柱松弛/蠕变导致的反射性肌肉活动完全恢复所需的时间。
对“体内”猫科动物标本的腰椎通过L4/5棘上韧带施加0.25Hz的周期性加载50分钟。在加载期结束时，让脊柱长时间休息，每小时进行一次单周期加载以进行测量，直至松弛完全恢复（>90%）。用线电极记录L-1-L-7多裂肌的反射性肌电图活动。对恢复期记录的负荷和肌电图拟合双指数模型，以表示不同结构（如椎间盘与韧带）的粘性和弹性成分。
0.25Hz的50分钟周期性加载所引起的松弛完全恢复需要7小时，并成功拟合双指数模型。同样，肌电图活动在4小时内完全恢复，且在接下来的3小时内常常超过其初始值。
给定周期加载在腰椎粘弹性结构中引起的松弛完全恢复需要数倍于加载持续时间的休息时间。同样，反射性肌肉活动需要4小时休息才能恢复。同时，关节中可能存在明显的松弛，使脊柱面临潜在损伤和腰痛的风险。恢复期结束时肌电图活动增加可能表明脊柱结构可能诱发了疼痛，导致被动加载期间肌肉过度兴奋。
尽管数据来自猫科动物模型，直接外推到人类模型并不简单，但预计在两个物种中，反射性肌肉活动随周期性加载而降低的总体模式是相似的。因此，使脊柱承受周期性加载的工作者可能会在没有肌肉保护的情况下，长时间处于超过中立区限制的松弛状态，从而面临可能的损伤和腰痛风险。疼痛和肌肉过度兴奋也可能是与周期性加载相关的因素，在工作完成数小时后表现出来。
