## 4 Discussion
However, this information can be determined through visual inspection of phase-portraits. Figure 4 reveals that the velocity of all the measured joints for the SRD participants is reduced throughout the entire stride and again resulting in smaller P-P areas relative to the NT participants. As P-Ps reflect joint control features, Figure 4 and Table 7 confirm a significant difference in lower-limb control between the two groups throughout the stride that cannot be discerned from observing peak velocity values alone. This is especially true for the ankle, where the areas of the ankle P-P reflect a significant difference, but no difference in the peak velocities (Figure 4 and Table 2). In addition to the necessity of intra-segmental coordination, inter-limb coordination is also necessary for effective walking. SIs are a common technique to assess inter-limb coordination by comparing the degree of equality of the behavior between joints of the two legs. Table 4 reflects large differences in the SIs for the ROM for all joints, with the SRD group being less symmetrical than the NT group. Likewise, the symmetry between the legs for each joint is less in the SRD participants than the NT participants. In all cases, the variability (i.e., SD) is much greater in the SRD compared to the NT group for both the ROM and peak velocities. The only exception is the hip peak velocity SDs. In combination, these measures reflect the increased difficulty of the SRD participants to finely coordinate and control their two limbs to produce symmetrical walking patterns. Given that the treadmill belt was moving at a fixed speed, and there were still difficulties in producing symmetrical gait, it is expected that further decreases in symmetry occur during overground walking. It should also be noted that the CVs of the SRD group for all measures are much greater, reflecting more relative variability in this group compared to the NT group. For both groups the relative variability of the ankle always exceeds that of the hip and knee. This indicates that across the sample, both groups displayed a wider range of ankle joint behaviors, in terms of both joint angular position and velocity, than the two more proximal joints. Although the pattern of variability across the joints is the same between the two groups, the magnitude of relative variability is much greater in the SRD group. Table 8 displays CV comparisons between the two groups, with eight of those revealing the SRD values were 100 or more percent greater than those of the NT group. This also indicates a wider range of joint behaviors, for all joints, exhibited by the SRD group. The amount of relative variability is not a function of the age of the participants, as the NT group always displayed less than the SRD group despite each participant being aged-matched with a participant from the other group. This suggests that there is not a stereotypical gait pattern that can be labeled as 'SRD gait', in the sense that stereotypical gait patterns of individuals with Parkinson's disease produce 'Parkinsonian gait'. These findings also indicate that individuals with SRD are more likely to produce individualized gait patterns regardless of age. This contrasts with NT participants who from a very young age produce very a stereotypical gait pattern that is often labeled as 'healthy gait'. Overall, the patterns of differences between the SRD and NT groups are consistent with the findings of the [PERSON] et al., (2022) report [15] which used some of the same analysis techniques as the current report. However, those authors discussed a single case study of one individual with SRD whose gait measures were compared to that of a single age-matched NT individual. The current study expands on that previous work through a significant increase in the number of participants and a measure of intra-segmental coordination (i.e., A-A areas). Like the previous work, the use of both linear and non-linear measures allows for a greater characterization of gait patterns than using a single category of measures.