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Experiment Shows Cyclic Loading Can Be A Substantial Risk

Ligament creep (progressive deformation of soft tissue due to loading over an extended period of time) produced through the application of low, moderate and high cyclical loads to feline in vivo lumbar supraspinous ligament did not recover after 7 hours of rest in a laboratory experiment.  Delayed hyperexcitability was detected in small lumbar paraspinal muscles following cyclical moderate and high loads suggestive of a local acute inflammation, according to authors Le et al.


The experiment implies that, despite the magnitude of a lumbar load, 7 hours of rest may be an insufficient recovery duration for 60 minutes of cyclic work.  With a lack of recovery, Le et al. suggest that further application of loads to these tissues may lead to chronic inflammation and cumulative trauma disorder.

Experiment Design

The dorsolumbar fascia of 19 anesthetized cats was exposed from the thoracic to the sacral spine level.  With the cat in a prone position, an external structure was applied to stabilize the thoracic and sacral regions relative to the lumbar spine. An S-shaped device was hooked around the L4-L5 supraspinous ligament and connected to a vertical actuator.  Three pairs of fine wire electrodes were inserted into the right L3-L4, L4-L5, and L5-L6 multifidus muscles.  The 19 cats were divided into three groups.

In one group (n=6), a vertical 20 N force was generated by the actuator and transmitted through the S-shaped device to the L4-L5 supraspinous ligament at a frequency of 0.25 Hz for a 10 minute duration.  A 10 minute rest period followed this activity.  Six episodes of cyclic work (20 N applied at 0.25 Hz for 10 minutes) and rest (no load for 10 minutes) were applied over a 2 hour duration.

Upon the completion of the 2 hour activity, a 7 hour rest/recovery period occurred.  During this 7 hours, the supraspinous ligament was tested at 10, 20, 30 minutes of rest and at each subsequent hour.  A vertical 20 N tension was applied for 8 seconds to assess the vertical displacement, residual creep and EMG recovery.

The second and third groups followed the same protocol with a 40 N (n=7) and 60 N (n=6) actuator force. 

During the first 10 minute load cycle for each group, cyclic loading, vertical displacement and EMG response were recorded every 1.5 seconds.  In the following load cycles and for each subsequent test during the recovery period, samples were taken at 20 second intervals.

Other Findings

After the 2 hour load/rest sequence, lumbar spine creep was found to be 139%, 83% and 66% (percent greater length than the initial mean displacement) for 20 N, 40 N, and 60 N, respectively.  After the 7 hour rest/recovery period, lumbar spine creep was found to be 108%, 55% and 44% for 20 N, 40 N, and 60 N, respectively.  Less creep was found with the higher loads due to the increased activity of the lumbar paraspinal muscles which stiffened the spine.

The authors noted that less force was required to induce lumbar spine physiologic changes with cyclic loading as opposed to static loading (identified in an earlier similar experiment).  They speculated that repeat stretching of viscolelastic fibers within collagen tissues that occurs with cyclic activity induces microdamage and impairs the ability of those structures to sustain loads.

Study Limitations

Although performed on felines, the authors felt experimental findings can apply to humans due to biomechanical and physiologic similarity between the two groups.

No comparison was made with the level of load commonly sustained by the human L4-L5 supraspinous ligament during movement and the forces applied in this experiment.

Article Title: Cyclic Load Magnitude is a Risk Factor for a Cumulative Lower Back Disorder

Publication: Journal of Occupational and Environmental Medicine 49(4): 375-387, 2007

Authors: P Le, M Solomonow, B-H Zhou, Y Lu, and V Patel

This article originally appeared in The Ergonomics Report™ on 2007-05-11.