**5. Applications for the balance indices**

• The dynamic postural steadiness index (DPSI): The DPSI (4) was based on the kinetic data, which was calculated for three principal directions and was reported as a sen‐ sitive measure index. The DPSI is a composite of the medio‐lateral steadiness index (MLSI; 1), anterior‐posterior steadiness index (APSI; 2), and vertical steadiness index (VSI; 3), which are mean square deviations assessing fluctuations around a zero point, rather than standard deviations assessing fluctuations around a group mean. The sta‐ bilization time was also determined as an objective postural control measure by using three indices of analysis based on the resultant GRF. The MLSI and APSI assessed the fluctuations from a zero point along the frontal and sagittal planes of the force plate, respectively. The VSI assessed fluctuation of the subject's body weight to normalize the vertical scores for standardization of the vertical GRF along the transverse plane of the force plate. This measure allowed comparison of individuals with different body

**Figure 3.** The rotation angle of the core spine computed from kinematic markers. For computing stability index, initial transition time (5 s) was excluded. Out of 25 s requested holding time, successful holding time (duration) was measured as the total duration until subject fail to stand on one leg (large arrow). The kinematic stability was measured as the square root sum of axial angle subtracted from its own mean value during successful holding time (see equation).

> \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ [ *<sup>Σ</sup>* (0 <sup>−</sup> *<sup>x</sup>*)<sup>2</sup> \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ number of data points

] (1)

weights (mass).

MLSI <sup>=</sup> <sup>√</sup>

202 Innovations in Spinal Deformities and Postural Disorders

The normalized kinematic stability index for specific portions of the body was compared with the kinetic stability index from the force plate.

As shown in **Figure 4**, a threshold was determined as 10 Newtons for the normalized kinetic stability. Although the value might not be the optimal quantity, the results would not be sig‐ nificantly different if neighboring values were selected.

**Figure 4.** The change of threshold on normalized kinetic stability for the force platform between groups based on visual input. A threshold of approximately 10 Newtons could separate the LBP group from the control group.

A recent review article indicated that there are no evidence‐based guidelines currently avail‐ able to assess spinal instability [54, 55]. Their summary indicated that both spinal stability and mobility concepts represent a new frontier in the study of the painful degenerative spine. The development of new rehabilitation strategies for LBP based on information regarding the kinematic and kinetic stability indices could help restore the normal function of spinal seg‐ ments and protect the adjacent segments. Previous studies also reported that postural steadi‐ ness, including trunk coordination, is a foundational necessity to prevent early mechanical deterioration of the entire body [8, 14, 56]. Postural steadiness has been used to character‐ ize the dynamics of the postural control system associated with maintaining balance [57]. However, the compensatory function of postural steadiness needs to be implemented within the whole body to prevent recurrence of pain and further injuries.
