Date of Graduation

8-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Kinesiology (PhD)

Degree Level

Graduate

Department

Health, Human Performance and Recreation

Advisor

Ro DiBrezzo

Committee Member

Inza Fort

Second Committee Member

Michelle Gray

Third Committee Member

Paul Calleja

Fourth Committee Member

Charles Stegman

Keywords

Biological sciences; Health and environmental sciences; Aviation; Flexor carpi radialis; Head-supported mass; Hoffman reflex; Nerve function

Abstract

Long-term exposure to head-supported mass (HSM) has been linked with spinal degeneration including foraminal stenosis and disc deterioration. Anecdotally, HSM has also been linked to neck and arm pain and muscle atrophy, but nerve function has not been tested specifically. The combined effect of various head positions and HSM may be sufficient to compress the nerve root in aviators and Soldiers during job performance, potentially leading to short- and long-term neuromuscular effects. The Hoffmann (H) reflex, a well-established measure of nerve function, has shown to be sensitive to changes in nerve root space which occurs with different head positions. This study assessed the validity of the H-reflex as an assessment tool of nerve function under varied HSM loading conditions in various head positions. The H-reflex was tested in the flexor carpi radialis muscle of the right arm in a healthy population with no recent history of HSM use. Participants (n = 14) were tested under three different HSM conditions: no HSM, a low weight-moment configuration, and a high weight-moment configuration. Following a 25-minute exposure period, each HSM condition was tested in neutral and at the end point of active range of motion for four different head positions: flexion, extension, and left/right rotation. Ten stimuli were averaged for each position and compared to a neutral unloaded baseline. An expected decrease in flexion was greater under the low-weight moment condition than the no HSM (d = 0.19), and in the high weight-moment condition than in the low weight-moment condition (d = 0.34). Unlike previous studies which found amplitude increases, there is evidence of an amplitude decrease in extension (d = 0.49) and right rotation (d = 0.32) when comparing the high weight-moment condition to the low weight-moment condition. Similarly, left rotation showed a decrease in amplitude that was greater in the low weigh-moment condition than the no HSM condition (d = 0.48). As expected, there was no effect of HSM or head position on latency. The results indicate that the combination of HSM and head position may contribute to a mechanical compression of the nerve root and decreased function.

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