## 4 Discussion
The patients in the current study decreased the posterior RCIA during DLS to ensure a smoother travel of the COP but this resulted in a more posterior IA. This indicates that the patients tended to keep their body mass on the trailing weight-release foot longer, allowing a more cautious weight acceptance on the contralateral limb. In the frontal plane, during DLS, it showed increased medial-lateral RCIA and ranges of RCIA during the DLS phase, which indicated a great deceleration of IA in the frontal plane during weight transfer, resulting in a contralateral lean of the body. During SLS, however, a greater medial RCIA and greater ranges of IA in the patients during SLS still reflect a greater and faster sway of the body in the frontal plane, which could also be seen in the trend of increased step width, yet it was without statistical significance (\(p=0.052\)). These results showed that the patients presented a conservative anterior-posterior balance control strategy but an unstable medial-lateral balance control. These findings seemed to match a previous finding [13], which showed that patients with lumbar degenerative spine disease may present a weaker medial-lateral balance control due to weak hip abductors during quiet standing [49], while the back muscles compensated for the imbalance in the anterior-posterior direction [50]. Hence, it might be important to establish the relationship between dynamic balance and static balance of quiet standing. Previous studies reported that patients with degenerative spine disease would present greater postural sway during static standing [12, 13]. The current study also found that while standing with their feet close together and their eyes open for 30 s, the patients showed greater COP total distance and sway area when compared to the healthy controls. The greater postural sway could result from nerve compression, as well as impaired sensory and motor function in patients with degenerative spine disease [51]. The greater postural sway is often interpreted as poor postural control or imbalance during static standing [52, 53, 54], which theoretically would also occur during locomotion [55]. Standing balance parameters failed to predict RCIA deviations in the sagittal plane, as well as the IA and RCIA deviations during SLS in the frontal plane. In the frontal plane, standing balance parameters were able to predict deviations during DLS but not during SLS. The patients showed a strong positive correlation between medial-lateral ranges of RCIA and ACOP during DLS, indicating that poor postural control would lead to a greater velocity of body sway in the medial-lateral direction during walking, and might result in an imbalance. However, standing balance parameters were unable to predict deviations in the IA and RCIA during SLS, possibly because the standing balance tests were performed only in the double-limb support posture. In the sagittal plane, moderate to strong negative correlations between IA during the entire stance phase and DCOP and ACOP indicate that the patients adopted a conservative strategy control of the COM relative to the COP in the anterior-posterior direction, which relied on walking more slowly while having a poorer postural control. The patients also showed decreased time-averaged posterior RCIA during DLS, decreased time-averaged anterior RCIA during SLS, and decreased ranges of RCIA during SLS, which had no significant correlation with standing balance parameters. Since the dynamic balance control should consider both IA and RCIA, the failure to predict RCIA deviations in the sagittal plane indicates that standing balance parameters are not able to predict dynamic balance in the sagittal plane. In the frontal plane, a strong positive correlation between the ranges of RCIA during DLS and ACOP indicates that the patients would have risks of imbalance in the medial/lateral direction during walking if they showed greater posture sway during quiet standing.