The in vivo dynamic response of the spine to perturbations causing rapid flexion: effects of pre-load and step input...

To evaluate the impact of muscle pre-activation levels and load magnitude on the response of the trunk to loading conditions causing rapid flexion.

Eight male subjects were asked to maintain an upright standing posture while resisting the application of forward flexion moments produced by four different loading conditions consisting of combinations of two pre-loads (4% or 16% of the maximum extensor moment) and two added loads (12% or 24%). Pre-loading was used to develop different initial levels of trunk muscle activity prior to the application of the added loads. Of special interest were the two conditions that resulted in total final loads of 28%.

Cocontraction of the antagonistic and agonistic muscles of the trunk are required to provide stability during normal physiological loading conditions. In several in vivo studies, levels of trunk muscle cocontraction have been observed prior to the application of unexpected or sudden loads. Forces from the abdominal muscles have been proposed to provide stability when extensor moments are generated. The response of trunk muscles to rapid flexor moments would provide further insight into the dynamic stability mechanisms of the spine.

Measurements were made of the trunk extensor moments, angular displacement of the trunk and unilateral surface EMG amplitudes of three abdominal and three trunk extensor muscles. Values were recorded during the isometric pre-load and for the maximum magnitude of each variable in response to the added load.

Higher pre-loads resulted in lower flexion rotations of the spine and higher added loads caused larger rotations. With increasing magnitudes of final loads, a corresponding increase in trunk extensor moments and trunk muscle cocontraction was observed. The largest activations were observed in the lumbar erector spinae and thoracic erector spinae muscles, while smaller yet substantial EMG activity was observed in the internal oblique and external oblique. A comparison of the 28% loading conditions showed an increased response of the trunk to the [4 + 24] loading condition (with lower initial trunk stiffness) when compared to the [16 + 12] loading condition.

Pre-activation of trunk extensor muscles can serve to reduce the flexion displacements caused by rapid loading. The abdominal oblique muscles, especially external oblique, will rapidly increase their activation levels in response to rapid loading. These changes are more pronounced when pre-activation levels are low, resulting in lower initial trunk stiffness and spine compression force. It is proposed that these factors will ultimately affect spine stability and the risk of injury.

The results of this study provide insight into several mechanisms involved in the dynamic stability of the spine. Injuries can be caused by unexpected and rapid loading of the spine. A study of the mechanisms available to respond to such perturbations is important to an understanding of spine mechanics and the etiology of low back injury.