Doctor of Philosophy (PhD)



Document Type



Analysis of the mechanical and neural regulatory mechanisms of the flexion-relaxation phenomenon (FRP), observed in deep trunk flexion, was performed since it is believed these mechanisms provide insight into the causes of low back injury and pain. Three methods were used to analyze the behavior of the lumbar tissues during trunk flexion-extension exercises: 1) active continuous cyclic movement, 2) acute cyclic movement at different orientations, and 3) passive continuous cyclic movement. All activities were performed at a rate of 0.1 Hz (6 cycles•min-1) while monitoring the surface electromyography (EMG) of the lumbar paraspinal muscles. Abdominal, hamstring, and quadriceps muscle activities were also monitored during acute cyclic movement. Trunk inclination and lumbar flexion angles were time synchronized with the recorded EMG signals. Increases in the myoelectric silent period with respect to inclination and flexion angles were apparent over time (p < 0.001) during the active continuous cyclic activity. Muscular fatigue and constant gravitational loading of the system were thought to influence the observations. Acute cyclic movements of trunk flexion-extension were performed in standing and supine positions. In standing, abdominal EMG activity increased when silent periods were present in lumbar paraspinal and hamstring muscles. Performance of flexion-extension from the supine position resulted in sustained silence of the paraspianl muscles once gravitational load was presented as the trunk flexed beyond vertical into deep flexion. EMG activity within the paraspinal muscles increased while extension was executed. During passive continuous cyclic movement the trunk motion was controlled by an external mechanical system to isolate the movement from fatigue. A significant decrease in the torque supplied by the posterior passive tissues was observed. EMG amplitudes remained relatively low during the passive session. A significant increase in the EMG amplitude and frequency was observed during active flexion movements performed after the passive session. Evidence suggests the primary control mechanism of the FRP to be mechanical in origin for acute loading. Decrease stability of the spine may occur with prolonged repetitive trunk flexion-extension. The increased duration of the FRP during continuous cyclic flexion-extension suggests neural mechanisms may supersede mechanical mechanisms during repetitive lifting activities.



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Committee Chair

Li Li

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Kinesiology Commons