NEUROMUSCULAR CONTROL IN LUMBAR DISORDERS^*

^*Doctoral dissertation presented on the 6^th of November 2003 at the the Faculty of Medicine of the University of Kuopio, Finland, by permission of the Faculty of Medicine of the University of Kuopio, Finland.

Impaired motor and sensory functions have been associated with low back pain (LBP). This includes disturbances in a wide range of sensorimotor control e.g. sensory dysfunctions, impaired postural responses and psychomotor control. However, the physiological mechanisms, clinical relevance and characteristics of these findings in different spinal pathologies require further clarification.

The purposes of this study were to investigate postural control, lumbar muscle function, movement perception and associations between these findings in healthy volunteers (n=35), patients with lumbar disc herniation (n=20) and lumbar spinal stenosis (LSS, n=26).

Paraspinal muscle responses for sudden upper limb loading and muscle activation during flexion-extension movement and the lumbar endurance test were measured by surface electromyography (EMG). Postural stability was measured on a force platform during two- and one-footed standing. Lumbar movement perception was assessed in a motorised trunk rotation unit in the seated position. In addition, measurements of motor-(MEP) and somatosensory evoked potentials (SEP) and needle EMG examination of lumbar multifidus muscles were performed in the LSS patients. Clinical and questionnaire data were also recorded.

A short latency paraspinal muscle response (~50 ms) for sudden upper limb loading was observed. The latency of the response was shortened by expectation (p=0.017). The response latency for unexpected loading was similar in healthy persons and disc herniation patients but the latency was not shortened by expectation in the patients (p = 0.014). Also impaired postural control (p < 0.05) and lumbar movement perception (p = 0.012) were observed in disc herniation patients. The impaired lumbar movement perception (p=0.054) and anticipatory muscle activation (p = 0.043) tended to be restored after successful surgery but postural control had still not recovered after 3 months of follow-up. The majority of LSS patients were unable to sense a rotational movement in the lumbar area and thus had clearly impaired lumbar movement perception (p = 0.006). Abnormal MEPs had only inconsistent and SEPs showed no associations with impaired movement perception and postural stability in LSS. Abnormal needle EMG findings and flexion-extension activation of paraspinal muscles were frequently observed in LSS patients. Lumbar paraspinal muscle endurance was better than in previously evaluated healthy subjects and chronic LBP patients (p < 0.001).

The results demonstrated clearly impaired lumbar sensory and motor function in sciatica and LSS patients. The pure reflex activation of paraspinal muscles was not affected in sciatica but a difference was found in the premotoneuronal response control. The impaired proprioceptive functions and premotoneuronal response control seem to recover at least partially but the maintenance of postural stability is a complex activity which does not seem to recover automatically in operated sciatica patients at least in three months follow-up. Paraspinal muscle denervation and dysfunction were clearly detectable in LSS but lumbar paraspinal muscle endurance was unexpectedly good.

KEY WORDS: Low back pain, intervertebral disk displacement, spinal stenosis, electromyography, evoked potentials, posture, psychomotor performance, comparative study, prospective study