TRUNK MUSCLE ACTIVATION PATTERNS ADAPT TO DEFICITS IN INDIVIDUAL SPINAL SYSTEMS
Quirk, David Adam
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In the early 1990’s Panjabi proposed a theoretical model suggesting one risk of low back pain is a spinal instability event theorizing a deficit to any spinal system (active, passive and neural) could represent a risk. However, the interplay between systems allows for compensation in the event of a deficit. Despite this theory being used for the treatment of low back pain, there is limited supporting empirical evidence. The overarching purpose of this dissertation was to investigate this compensation theory. Study one compared two populations (older adults and those recovered from a low back injury) suspected of having deficits in all three spinal systems with a young asymptomatic control group. The objective was to determine whether both groups had different muscle activation patterns compared to controls. Both deficit groups had higher agonist and antagonist activation amplitudes and evidence of reduced responsiveness of trunk muscles to changing external moments. While the direction of adaptations observed in both deficit groups, suggests that these adaptations were in response to common spinal system deficits, the magnitude differed. Thus, it was unknown if adaptations were unique to a deficit in any singular spinal system or a result of recent pain. To investigate these limitations, three studies compared those with high or low function of the active, passive, and neural systems and whether recent low back pain modified these adaptations. These comparisons identified that lower function within any single spinal system had different trunk muscle activation patterns that interacted with recent pain. Adaptations included increased agonist and antagonist amplitudes and different (increased or decreased) responsiveness to changing external moments. Muscle adaptations were specific to the measured spinal system to produce higher relative muscle force (active) or increase the general stiffness of the spine (passive and neural). Collectively these findings were consistent with Panjabi’s theoretical model that trunk muscle activation patterns adapt to dysfunction within spinal systems. While cross-sectional data cannot infer causation, a rigorous methodology was employed, contributing to a broader understanding of the interrelationships between individual spinal system function, neuromuscular control during dynamic tasks and the modifying effect of recent pain.