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.

Introduction: Deficits in motor control of the trunk muscles have been extensively reported in individuals with chronic low back pain (LBP). Recent evidence suggests that these deficits can be improved with motor training. However, whether these changes in motor control are mediated by changes in the motor cortex remains unclear. As deficits in postural activation of transversus abdominis (TrA) is consistently observed in individuals with LBP, the present study aimed to investigate the representation of TrA at the motor cortex in individuals with and without chronic LBP. The potential to change the cortical representation of TrA following motor training in individuals with chronic LBP was also examined.

Methods: Eleven healthy volunteers and twenty individuals with chronic LBP participated. Chronic LBP individuals were randomly allocated into two training groups: specific motor control training that involved practice of skilled activation of TrA, or walking exercise, and trained twice per day for two weeks. Recordings of electromyographic activity (EMG) of TrA were made bilaterally with intramuscular fine-wire electrodes. Motor control of TrA was assessed as the postural activation of the muscle associated with repetitions of rapid arm flexion and extension movements. To evaluate the representation of TrA, transcranial magnetic stimulation (TMS) was delivered over pre-marked scalp sites. EMG amplitude of the responses to TMS at each site was superimposed over the grid to produce a map of response amplitude relative to scalp site. All procedures were repeated two weeks post-training for the chronic LBP group. Onset of TrA EMG relative to prime mover deltoid and the location of the centre of gravity (CoG) of TMS map were compared between individuals with and without chronic LBP, and between pre- and post-training in individuals with chronic LBP.

Results: The CoG of the cortical representation of TrA was located 2 cm anterior and lateral to the vertex in healthy individuals. However, individuals with chronic LBP showed a posterior and lateral shift in the CoG. The shift in location of the CoG of the TrA representation was associated with timing of activation during rapid arm movement tasks. Following two weeks of skilled training of TrA, motor cortical representation shifted towards that observed in healthy individuals. Changes in representation were not observed for the walking exercise group.

Discussion: These findings provide evidence of reorganisation of trunk muscle representation at the motor cortex in individuals with chronic LBP, and that cortical changes are associated with deficits in motor control. Furthermore, this study provides the first evidence that training can induce plasticity of the motor cortex in this group.

Introduction Deficits in control of the trunk muscles have been demonstrated in people with recurrent low back pain (LBP) (1,2). These changes can persist despite resolution of symptoms (1) and is thought to be associated with recurrence and chronicity. One approach that has been shown to reduce symptoms and prevent recurrence involves rehabilitation of trunk muscle control, rather than training the strength and endurance of the trunk muscles. Although we have recently shown that this rehabilitation strategy induces immediate changes in coordination (3), no study has investigated whether improvement can be maintained in the long-term. Using the model of changes in control of the deep abdominal muscle, transversus abdominis (TrA), in people with LBP(1), this study aimed to investigate whether four weeks of training of repeated voluntary TrA contractions could induce long-term changes in control of the trunk muscles.

Methods Nine volunteers with recurrent LBP trained isolated voluntary TrA contractions twice per day for four weeks. Coordination of the abdominal muscles was assessed during single rapid arm movements and walking. Measures were made before and after initial training, before and after training at week two, at week four, and at six months. Recordings of electromyographic activity (EMG) were made from trunk and deltoid muscles. Feedback of contraction during training was provided during training at the initial session and at 2 weeks with real-time ultrasound. TrA EMG activity was maintained at 5% maximum root-mean-square EMG. Onsets of trunk muscle EMG was identified during arm movement and coefficient of EMG variation (CV) was calculated during walking.

Results Onset of TrA EMG was earlier during arm flexion and extension immediately after a single session of training, and was further improved with four weeks of training (p< 0.05). In addition, the CV of the TrA EMG (indicating more sustained activity) during walking were found over four weeks of training (p< 0.05). Changes in motor control were retained at six months follow-up despite cessation of training. Changes in other trunk muscles were not significant (p> 0.05).

Discussion The results suggest that four weeks specific motor control training is associated with consistent changes in motor control of the trained muscle during functional tasks. Although improvements in symptoms were also identified, future randomized clinical trials are required to confirm these changes and their association with the changes in coordination of the trunk muscles.