Trunk muscle and motor control impairments in patients with lumbar instability

Sheri P. Silfies

doi:10.17918/00009880

While a majority of back pain patients recover within a short period of time, some develop chronic low back pain (CLBP). One plausible cause of CLBP is lumbar instability, which is presumably associated with osteoligamentous laxity or damage. Given Panjabi's model of spinal stabilization and supporting evidence that remaining subsystems can compensate for attenuated passive structures, the assessment of trunk muscle performance and motor control warrants exploration. The purposes of this study were to examine the trunk active and motor control subsystems for differences between CLBP subgroups with and without evidence of spinal instability and asymptomatic subjects by assessing: (1) fatigue; (2) activation levels; and (3) onset timing and sequence of activation. To examine localized trunk extensor muscle fatigue, spectral electromyography (EMG) changes were analyzed during a sustained isometric contraction at 80% of maximum voluntary activation. Subjects with CLBP attributed to lumbar instability demonstrated less fatigability than the healthy control group. This may be the result of adaptation in the musculature that allows improved ability to provide spinal stability during short periods of submaximal activation. There were no significant differences in between subgroups of CLBP. To examine trunk muscle activation patterns and synergist coactivation levels, normalized EMG amplitude was compared across groups during a functional reaching task. The CLBP group with instability demonstrated higher levels of muscle activity (rectus abdominus and external oblique) and altered abdominal synergistic coactivation patterns around a neutral standing position. These alterations in muscle recruitment may indicate motor control dysfunction of the local trunk stabilizing muscles. Again, no differences were found between the two CBLP subgroups. To examine trunk muscle onset timing and sequence of activation, a model of self-generated center of mass perturbation was achieved by having the subject perform a unilateral arm raise. All three groups demonstrated feedforward activation of the lumbar multifidus presumably to control anterior displacement of the body's center of mass. These data also suggest a lack of preparatory activation of the external oblique muscles in the CLBP with evidence of lumbar instability, which may indicate poor frontal and transverse plane stability of the lumbar spine during performance of asymmetrical movements.

Trunk muscle and motor control impairments in patients with lumbar instability

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Trunk muscle and motor control impairments in patients with lumbar instability

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