Repeated spinal flexion modulates the flexion–relaxation phenomenon
Introduction
Epidemiologic studies illustrate that repeating bending and lifting activities are associated with increased risk of developing low-back disorders (Marras et al., 1995; Xu et al., 1997). Trunk flexion generates large bending moments in the passive spinal tissues (Dolan et al., 1994a; Adams and Dolan, 1991) and large compressive loads due to muscular forces (Dolan et al., 1994a; Schultz et al., 1985). In vitro studies show that repetitive flexion with concurrent compression can cause prolapsed disc injuries (Callaghan and McGill, 2001). Repeated flexion causes muscular fatigue (Sparto et al., 1997, Dolan and Adams, 1998), creep of the intervertebral disc and ligaments (Adams and Dolan, 1996, Wilke et al., 1998) and diminished protective muscular reflexes (Solomonow et al., 1999). Repeated bending has been hypothesized to increase the risk of injury to the intervertebral disc as the viscoelastic changes in the ligaments are thought to occur more rapidly than the disc and accordingly the disc is thought to bear a greater proportion of the flexion moment (Dolan and Adams, 1998).
The erector spinae are derecruited as subjects approach full lumbar flexion (Floyd and Silver, 1951). This phenomenon is called flexion–relaxation, and is particularly noteworthy as the external moment is borne by the passive spinal tissues (Golding, 1952; Kippers and Parker, 1984; Andersson et al., 1996; McGill and Kippers, 1994). Many studies report flexion–relaxation in static trunk flexion postures (Floyd and Silver, 1951; Golding, 1952; Kippers and Parker, 1984; Andersson et al., 1996; McGill and Kippers, 1994; Sarti et al., 2001; Wolf et al., 1979; Floyd and Silver, 1955). Recent studies report that the flexion–relaxation phenomenon also occurs during lifting (Mathieu and Fortin, 2000; Toussaint et al., 1995; Dolan et al., 1994b), lateral bending (Raftopoulos et al., 1988) and during sitting (Callaghan and Dunk, 2002). The flexion–relaxation phenomenon is modulated by the amount of load lifted (Schultz et al., 1985; Kippers and Parker, 1984; Gupta, 2001), lifting rate (Sarti et al., 2001) and differences have been reported between low-back pain and healthy subjects (Golding, 1952; Sihvonen et al., 1991; Ahern et al., 1988; Shirado et al., 1995). Previous studies have not evaluated the effect of repeated flexion on the mechanics of the flexion–relaxation phenomenon, which may be a factor in the increased risk of injury with repeated flexion.
The purpose to the current study was to assess the effects of repetitive bending on the flexion–relaxation phenomenon in order to evaluate whether changes to the tissue (passive and muscle) mechanical properties and/or neuromuscular control system occur due to repetitive bending. Our hypotheses were that repetitive bending would result in increased peak flexion during the lift and parallel increases in the angle for onset of the flexion–relaxation phenomenon. Additionally we hypothesized that adding a mass will result in greater peak flexion during the lift and greater angle for onset of the flexion–relaxation phenomenon.
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Participants
Thirty healthy and active young adults (15 males and 15 females) were recruited from a university student population (Table 1). Written consent to participate in the investigation was obtained from all subjects, and an Institutional Review Board approved the procedure for this project.
Instrumentation
Disposable Medi-Trace surface electromyography (EMG) electrodes (Ag–AgCl) were applied bilaterally to the skin 3 cm from the midline at the level of T9 (thoracic erector spinae; TES) and L4/L5 (lumbar erector
Results
Flexion–relaxation was demonstrated throughout the experiment in 21 of the 30 subjects (10 males and 11 females). The remaining subjects showed marked reductions in lumbar muscle activation near full spinal flexion, though the reductions were not sufficient to meet our stringent criterion defining flexion–relaxation.
There were no significant effects of gender observed in the study. We observed significant changes in the flexion–relaxation response with repeated spinal flexion. For maximum
Discussion
We observed significant changes in the flexion–relaxation response with repeated spinal flexion. Both the maximum flexion angle and the flexion–relaxation angle parameters showed increases due to repeated spinal flexion, although the change in maximum flexion was not significant. There was also a significant interaction between time and load as the changes due to repeated spinal flexion were larger for the no load compared to the loaded condition. We observed a significant increase in the
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