Introduction: Recent data report increased trunk stiffness in semi-sitting in people with recurrent low back pain (LBP) during remission. This is likely to be due to increased trunk muscles activity. Although this adaptation may provide a short term strategy to protect the spine from further pain/injury it may increase the potential for pain recurrence due to increased trunk loading and compromised performance of the spine in dynamic functions. An interesting observation was that trunk damping (i.e. decay in trunk velocity) was reduced. Damping is likely to be largely related to reflex control of trunk muscles. It is possible that trunk stiffness increased in this population because reflex control was inadequate. This study aimed to determine whether stiffness and damping adapt in a similar manner in healthy individuals, with presumably normal reflex function, when challenged by pain.

Methods: Fourteen males with no history of LBP were semi-seated with their pelvis fixated and a harness placed over their shoulders. Weights (~15% of body mass) were attached via an electromagnet and force transducer to a pulley system that attached to the front and rear of the trunk harness at T9. Subjects sat upright in a relaxed, neutral posture. At an unpredictable time either the front or back weight was dropped 10 times (each) in random order. Trials were repeated in three conditions; pre-pain, pain and post-pain. During the pain condition subjects were injected with a single bolus of hypertonic saline (5% NaCl, 1.5 ml) into the right erector spinae at L4. Trunk mass (M), damping (B) and stiffness (K) were estimated when the trunk was perturbed either backwards (BW) or forwards (FW) in an identical manner to our earlier study. Parameters were described by a second order linear model and the standard least squares procedure was used to solve the estimation using the equation: F(t)=M.x(acc)(t)+B. x(vel)(t)+K.x(disp)(t). Damping and stiffness were normalized to the peak. Perturbation displacement and duration were calculated from the onset to perturbation maximum. Data were compared with repeated measures ANOVA and Duncan’s multiple range test.

Results: During experimental pain, trunk stiffness decreased in both perturbation directions (both: p< 0.02). Damping increased with FW perturbations (p=0.01). Both the displacement (p=0.03) and duration (p=0.01) of the trunk perturbation were increased during experimental pain with BW perturbation. There was no change in either parameter in the FW direction. Estimated trunk mass was lower during pain and post-pain compared to pre pain (p=0.01) with BW perturbations.

Discussion: In contrast to increased stiffness and decreased damping in people with recurrent LBP, healthy individuals respond to pain by decreasing stiffness and increasing damping of the trunk. However, this was only true for the FW perturbation. In the BW direction, damping was not increased and there was a resultant increase in the displacement and duration of the perturbation. Taken together these data suggest that damping of the trunk is adaptable and is increased to protect the spine in healthy individuals. As trunk damping is associated with reflex control of the trunk muscles these data suggest although healthy individuals may be able to tune this control during pain, this is compromised in spinal pain.