Muscle synergies in patients with low back pain : A statistical gait analysis study pre- and post-rehabilitation

Low back pain (LBP) can be caused by abnormal motor control strategies during locomotion. However, appropriate physiotherapy rehabilitation programs may prevent the onset of LBP. The aim of this study is to demonstrate that muscle synergies provide a useful quantitative evaluation of motor control strategies before and after a rehabilitation program. Statistical gait analysis was performed on 3 chronic LBP sufferers at baseline and after 1-month physiotherapy. Subjects walked overground for 5 minutes, at self-selected speed, while surface electromyography (EMG) signals were recorded from 12 lower limb and trunk muscles. Non-Negative Matrix Factorization (NNMF) was used to extract muscle synergies for each group of 10 concatenated gait cycles. The average Variance- Accounted-For (VAF) curves were obtained for a number of synergies ranging from 3 to 8. For each patient, the VAF curves before and after rehabilitation were compared to those obtained from healthy subjects. The results obtained were in good agreement with the Numeric Pain Rating Scale (NPRS) assessed by clinicians. Moreover, for each specific subject, it was possible to quantitatively assess the changes in motor control after rehabilitation, in terms of modifications of muscle weights and activation coefficients of the muscle synergies. Furthermore, since muscle synergies were calculated on a high number of valid gait cycles (143 ± 28), separately for each 10-cycle group, it was possible to accurately estimate the intra-subject repeatability, i.e. the consistency of the muscle synergies during a physiological walk. In conclusion, this study demonstrated that statistical gait analysis combined with the extraction of muscle synergies allow for a robust description of motor control changes after a rehabilitation program.