Head repositioning errors in normal student volunteers: a possible tool to assess the neck's neuromuscular system

Abstract

Background: A challenge for practitioners using spinal manipulation is identifying when an intervention is required. It has been recognized that joint pain can interfere with the ability to position body parts accurately and that the recent history of muscle contraction can play a part in that interference. In this study, we tested whether repositioning errors could be induced in a normal population by contraction or shortening of the neck muscles.

Methods: In the experimental protocol, volunteers free of neck problems first found a comfortable neutral head posture with eyes closed. They deconditioned their cervical muscles by moving their heads 5 times in either flexion/extension or lateral flexion and then attempted to return to the same starting position. Two conditioning sequences were interspersed within the task: hold the head in an extended or laterally flexed position for 10 seconds; or hold a 70% maximum voluntary contraction in the same position for 10 seconds. A computer-interfaced electrogoniometer was used to measure head position while a force transducer coupled to an auditory alarm signaled the force of isometric contraction. The difference between the initial and final head orientation was calculated in 3 orthogonal planes. Analysis of variance (1-way ANOVA) with a blocking factor (participants) was used to detect differences in proprioceptive error among the conditioning sequences while controlling for variation between participants.

Results: Forty-eight chiropractic students participated: 36 males and 12 females, aged 28.2 +/- 4.8 yrs. During the neck extension test, actively contracting the posterior neck muscles evoked an undershoot of the target position by 2.1 degrees (p <0.001). No differences in repositioning were found during the lateral flexion test.

Conclusion: The results suggest that the recent history of cervical paraspinal muscle contraction can influence head repositioning in flexion/extension. To our knowledge this is the first time that muscle mechanical history has been shown to influence proprioceptive accuracy in the necks of humans. This finding may be used to elucidate the mechanism behind repositioning errors seen in people with neck pain and could guide development of a clinical test for involvement of paraspinal muscles in cervical pain and dysfunction.

Figure 1

Photographs of the experimental equipment. A) a participant in the neutral position in preparation for an Extension test. B) a participant in the extended position. The CA-6000 linkage measures head position relative to the base affixed at the first thoracic vertebra. Matching Lucite blocks, one attached to the headband of the CA-6000, and the other attached to the load cell, provide for alignment during the neck extension. In this position the patient can exert force against the load cell for measuring maximum voluntary contraction during the "Active Hold" conditioning.

Figure 2

A plot of raw AP-Flexion motion in the Extension test for one participant. The sequence of activities is evident: 10 seconds of static neutral posture at the initial target position, followed by 5 deconditioning repetitions of neck extension. In the "No Hold" condition, the patient attempts to retarget to neutral immediately. In both "Passive Hold" and "Active Hold," there is a 10-second delay in the extended position. Vertical lines indicate the 5-second intervals over which average values were obtained for initial and final head orientation. In this particular case, both the No Hold and Passive Hold conditions produced an overshoot of the target position by 2.5 degrees. The Active Hold condition actually produced more accurate repositioning.