Influence of knee angle and individual flexibility on the flexion-relaxation response of the low back musculature.

In many occupational settings (e.g. agriculture and construction) workers are asked to maintain static flexed postures of the low back for extended periods of time. Recent research indicates that the resulting strain in the viscoelastic, ligamentous tissues may have a deleterious effect on the stability of the spine and the normal reflex response of spinal tissues. The purpose of this study was to evaluate the previously described flexion-relaxation response in terms of the interactive effect of trunk flexion angle (30 degrees, 50 degrees, 70 degrees, 90 degrees ), knee flexion angle (0 degrees (straight knees), 20 degrees, 40 degrees ) and individual flexibiliteky (low, medium, and high). These conditions were tested under two levels of loading: no load (just supporting the weight of the torso) and trunk extension moment equal to 50% of the subject's posture-specific maximum voluntary trunk extension capacity. Surface electromyographic (EMG) data were collected from the multifidus, the longissimus, the iliocostalis, the vastus medialis, the rectus femoris, the vastus lateralis, the biceps femoris, and the gastrocnemius-soleus group from a sample of eight male participants as they performed isometric weight holding tasks in the postures defined by the combinations of trunk angle and knee angle. The results of this study showed that knee angle did have a significant effect on the lumbar extensor muscle activity but only consistently at the 90 degrees trunk angle. Participant flexibility showed a consistent trend of decreasing lumbar extensor muscle activity with decreased flexibility across all trunk angle values. Most interesting was the interactive response of flexibility and knee angle, wherein the flexibility of the participant influenced the trunk angles at which the knee flexion angle affected the flexion-relaxation response. Highly flexible subjects showed an effect of knee angle on the flexion-relaxation response only at the 90 degrees trunk angle; subjects in the medium flexibility category showed a similar response in both the 70 degrees and 90 degrees trunk angles; subject in the low flexibility group showed no knee angle effect on the flexion-relaxation response. Overall the results confirm previous results with regard to the contribution of the passive tissues to the overall trunk extension moment but also show that the tension in the bi-articular biceps femoris, which was influenced by knee flexion angle and flexibility, affects the ratio of active extensor moment contributions of the lumbar extensor musculature to passive extensor moment contributions from the muscular and ligamentous tissues. The results of this study provide empirical data describing this complicated, interactive response.