HOW DOES CREEP OF SPINAL TISSUES AFFECT REFLEX ACTIVATION OF THE BACK MUSCLES?

Abstract

During forward bending activities, the collagenous tissues of the spine are protected from injury by reflex contractions of the back muscles which prevent excessive spinal flexion. Animal experiments have shown that this reflex response is diminished when spinal ligaments are subjected to creep ( 1). This study examined the effects of creep on the latency and amplitude of reflex activation of the back muscles in living people.

Ten healthy volunteers (4M/6F) consented to participate in the study. Subjects underwent two flexion treatments: i) prolonged sitting in a low chair for 2 hours, ii) 100 toe-touching exercises, each on a separate day. Before and after each treatment, subjects performed a standardised forward bending task during which simultaneous measurements were made of lumbar flexion, using the 3-Space Fastrak, and surface EMG activity of the erector spinae muscles at T10 and L3 ( 2). The latency of the reflex response was measured by recording the amount of lumbar flexion that occurred prior to the onset of muscle activation at each site. The amplitude of the reflex was measured by determining the peak EMG activity during the bending task. Each subject’s range of lumbar flexion was also measured independently before and after each treatment to determine the extent of any creep.

Both treatments caused creep, as indicated by a significant increase in the range of lumbar flexion. The treatments also brought about a significant delay in the reflex activation of the back muscles in the standardised bending task: after prolonged sitting, lumbar flexion during the bending task increased by 9.2 ± 7.4° and 5.7 ± 4.6° before the onset of EMG activity at T10 and L3 respectively; following the toe-touches, the equivalent increases in lumbar flexion were 5.4 ± 3.9° and 3.1 ± 4.4°. The amplitude of the reflex response was unchanged following prolonged sitting, but after the toe-touches, a 50% increase in peak EMG activity was observed at L3.

Creep in spinal tissues as a result of prolonged or repetitive flexion was associated with delayed reflex activation of the back muscles. There was no associated reduction in the amplitude of the reflex. The increase in peak EMG activity following the toe touches may reflect increased activation as a result of muscle fatigue. These results suggest that creep in spinal tissues may allow increased lumbar flexion and hence increased bending stresses to be applied to the intervertebral disc.