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.