## 4 Discussion
The regularity index indicates the interlimb coordination of experimental animals when normal step sequence patterns are used during uninterrupted locomotion. In the SCI group of our current experiment, the regularity index dropped to 0.99% at the timepoint N2 and reached the highest recovery with 13.94% at the timepoint N5. At the same time, the highest BBB open field score after SCI, measured at the end of the experiment, was 9.611 points. According to the BBB open field scale, a performance of 12 points equals occasional (\(\leq\)50%) interlimb coordination which, in our study, was never reached by any animal during the six weeks after SCI. However, the highest result of the regularity index in the CatWalk \(\text{XT}^{\text{\tiny\textregistered}}\) analysis 5 weeks after the injury (N5) indicated a significant recovery compared to the first postinjury value (N2). Moreover, we found a good correlation (\(r=0.89\)) of the regularity index with the percentage of preserved tissue, indicating a high validity of this parameter for the usage in our injury model. Therefore, the supplement of the CatWalk \(\text{XT}^{\text{\tiny\textregistered}}\) regularity index might result in an overall more reliable and sensitive assessment of locomotor function for animals with BBB open field scores less than 12 points, similar to what has previously been reported for BBB open field scores from 11 to 14 points [19; 41]. Furthermore, although major variation from different SCI levels, injury models and severities can occur, the regularity index is one of the most popular CatWalk \(\text{XT}^{\text{\tiny\textregistered}}\) parameters used for behavioral testing after rodent SCI due to its high sensitivity [19; 41; 42; 43; 44; 45]. Stride length and base of support are two CatWalk \(\text{XT}^{\text{\tiny\textregistered}}\) parameters that indicate the level of trunk stability and gross muscle strength. We observed significant recovery of the stride length in the SCI group since the timepoint N5, while the BOS had already significantly recovered since the timepoint N3. As for the correlation with the percentage of preserved tissue, we found relatively high \(r\) values for the stride length and the BOS parameters in both, the forelimbs (0.78 and \(-0.89\), respectively) and the hindlimbs (0.89 and \(-0.91\), respectively). However, there was a relatively lower correlation of the BBB open field score with the BOS than with the stride length of the hindlimbs (\(r=0.67\) and \(r=-0.26\), respectively). An explanation for this lower correlation could be that some of the animals in our study, especially at the later timepoints, had already reached complete weight-bearing in the BBB locomotor rating, while the percentage of preserved tissue did not have this ceiling effect. Although the stride length and the BOS are two well-recognized CatWalk \(\text{XT}^{\text{\tiny\textregistered}}\) parameters, some researchers have called for their more specified and precise use, outside of general locomotion assessment [19; 44; 46; 47; 48; 49; 1]. As an example, [PERSON] et al. elaborated that stride length and BOS could alter depending on the velocity of the animal and offered a body coordinate frame to optimize the evaluation of those parameters with the context of the body [50]. Judging from our current results, the impairment and recovery of the CatWalk \(\text{XT}^{\text{\tiny\textregistered}}\) parameter average speed after SCI followed a similar tendency of spontaneous recovery as the other parameters. However, as can be observed in Figure 2B, the variation of the average speed measurements over the course of the experiment was relatively high, even in the Sham group, leading to a relatively poor correlation with the BBB open field score (\(r=0.57\)) as well as the percentage of preserved tissue (\(r=0.34\)).