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Research Papers

Paraspinal Muscle Vibration Alters Dynamic Motion of the Trunk

[+] Author and Article Information
M. Arashanapalli

Department of Mechanical Engineering, University of Kansas, Lawrence, KS 66045

S. E. Wilson1

Department of Mechanical Engineering, University of Kansas, Lawrence, KS 66045sewilson@ku.edu

1

Corresponding author.

J Biomech Eng 130(2), 021001 (Mar 21, 2008) (7 pages) doi:10.1115/1.2898734 History: Received August 04, 2006; Revised September 25, 2007; Published March 21, 2008

Loss in dynamic stability of the low back has been identified as a potential factor in the etiology of low back injuries. A number of factors are important in the ability of a person to maintain an upright trunk posture including the preparatory stiffness of the trunk and the magnitude and timing of the neuromotor response. A neuromotor response requires appropriate sensing of joint motion. In this research, the role of this sensory ability in dynamic performance of the trunk was examined using a simple pendulum model of the trunk with neuromotor feedback. An increased sensory threshold was found to lead to increased torso flexion and increased delay in neuromotor response. This was confirmed experimentally using paraspinal muscle vibration which is known to alter proprioception of the muscle spindle organs. Before, during and after exposure to bilateral, paraspinal muscle vibration for 20minutes, the dynamic response of subjects to an unexpected torso flexion load was examined. Subjects were found to have a 19.5% slower time to peak muscle activity and a 16.1% greater torso flexion during exposure to paraspinal muscle vibration. Torso flexion remained significantly increased after vibration exposure relative to before exposure. These results suggest that the neuromotor response plays an important role in trunk dynamics. Loss in sensitivity of the sensory system can have a detrimental effect on trunk dynamics, increasing delays in neuromotor response and increasing the motion of the trunk in response to an unexpected load.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

The model on trunk dynamics in response to a sudden load consisted of an inverted pendulum model with a neuromotor response feedback. The neuromotor response was modulated by a neuromotor gain (G), a time delay (Td), and a detection threshold (Th).

Grahic Jump Location
Figure 2

Increasing the ratio of neuromotor gain over preparatory stiffness (r=G∕K) resulted in an increase in TF and in time to peak TF. Although the effective overall trunk stiffness of a simple proportional feedback system should be the sum of neuromotor gain (G) plus intrinsic trunk stiffness (K, which was held constant), the time delay in the neuromotor gain led to a decrease in effective overall trunk stiffness and an increase in TF.

Grahic Jump Location
Figure 3

Increasing the ratio of neuromotor gain over preparatory stiffness (r=G∕K) increases the magnitude of the neuromotor response

Grahic Jump Location
Figure 4

Increasing the detection threshold of the neuromotor response (Th) was assessed at both a low neuromotor response (a)(r=0.1) and a high neuromotor response (b)(r=0.7). With a low neuromotor response, little change was observed in the TF. With a high neuromotor response, the magnitude of TF and the time to peak TF were increased with increases in the detection threshold.

Grahic Jump Location
Figure 5

Increasing the detection threshold of the neuromotor response (Th) was assessed at both a low neuromotor response (r=0.1) and a high neuromotor response (r=0.7). Increased delays in initiation of the response were observed at both response levels. An increased time to peak flexion was observed with a high neuromotor response. With the neuromotor response gain held constant, a decreased magnitude of the neuromotor response was observed.

Grahic Jump Location
Figure 6

Increasing the detection threshold of the neuromotor response (Th) was assessed both while maintaining a constant gain (as in Figs.  45) and while maintaining a constant response magnitude. With both, the time PM was found to increase with increases in the detection threshold. The ratio (r) of gain over stiffness for this analysis was maintained at 0.7.

Grahic Jump Location
Figure 7

Increasing the detection threshold of the neuromotor response (Th) was assessed both while maintaining a constant gain and while maintaining a constant response magnitude. With both, the TF was found to increase with increases in the detection threshold. The ratio (r) of gain over stiffness for this analysis was maintained at 0.7.

Grahic Jump Location
Figure 8

In the experiment, the subject stood, pelvis fixed, with a chest harness attached to a sudden loading apparatus. Vibration was applied to the low back at the L3 level.

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