Effects of static flexion–relaxation on paraspinal reflex behavior

Abstract

Background.-Static trunk flexion working postures and disturbed trunk muscle reflexes are related to increased risk of low-back pain. Animal studies conclude that these factors may be related; passive tissue strain in spinal ligaments causes subsequent short-term changes in reflex. Although studies have documented changes in the myoelectric onset angle of flexion-relaxation following prolonged static flexion and cyclic flexion we could find no published evidence related to the human reflex response of the trunk extensor muscles following a period of static flexion-relaxation loading.Methods.-Eighteen subjects maintained static lumbar flexion for 15 min. Paraspinal muscle reflexes were elicited both before and after the flexion-relaxation protocol using pseudorandom stochastic force disturbances while recording EMG. Reflex gain was computed from the peak value of the impulse response function relating input force perturbation to EMG response using timedomain deconvolution analyses.Findings.-Reflexes showed a trend toward increased gain after the period of flexion-relaxation (P < 0.055) and were increased with trunk extension exertion (P < 0.021). Significant gender differences in reflex gain were observed (P < 0.01).Interpretations.-Occupational activities requiring extended periods of trunk flexion contribute to changes in reflex behavior of the paraspinal muscles. Results suggest potential mechanisms by which flexed posture work may contribute to low-back pain. Significant gender differences indicate risk analyses should consider personal factors when considering neuromuscular behavior.

Disturbed Paraspinal Reflex Following Prolonged Flexion-Relaxation and Recovery

“…26 However, the results of this investigation were inconsistent with our previous results where reflex behavior was recorded from subjects following 15 minutes of continuous static flexionrelaxation. 6 In that study, we found a trend toward increased reflex gain immediately following prolonged static flexion-relaxation, which we concluded was consistent with the hyperexcitable EMG response found in animal research. 26 There are several possible explanations as to why our previous results differ from what we found here.…”

“…A servomotor (Pacific Scientific, Rockford, IL) was used to elicit paraspinal reflexes according to protocol described in our previous work. 6 The motor provided a constant isotonic preload of 100 N that the subjects were instructed to resist by maintaining their upright seated posture. Pseudorandom stochastic force perturbations of ±75 N were superimposed on the preload (Figure 2A).…”

“…Measurements on human beings similarly showed hyper-excitable paraspinal reflex response immediately following static flexion. 6 Whether this behavior in human beings is transient and followed by reduced reflex response remains to be shown. Other joints have shown reduced reflex behavior in human beings following passive flexion.…”

Spine stability: The six blind men and the elephant

“…Clearly, there are a host of possible risk factors that can lead to impaired feedback control such as prolong flexion (Solomonow et al, 2003;Granata et al, 2005;Rogers and Granata, 2006), whole body vibration (Roll et al, 1980;Gauthier et al, 1981); and muscle wasting (Hides et al, 1996) to name a few. Delayed reflex responses are another source for controller impairment that has been associated with LBP (Radebold et al, 2000;Radebold et al, 2001;Reeves et al, 2005), and more recently has been shown to increase the risk of injury .…”

A 3-D Stability-Based Dynamic Computational Model of Human Trunk Movement: Towards the Development of a Paradigm for Forensic Spinal Injury Biomechanical Analysis

“…Granata et al investigated the influence of trunk posture [14], exertion direction [17,18], and preload [19] on spinal stability during flexion and extension exertions. Shirazi-Adl et al [20,21] developed a multi-degree of freedom finite element model to quantify the role of passive and active tissues in spinal stability.…”