## 4 Discussion
In previous research focused on'regular' forward walking, a range of four to eight synergies that accounted for >80% of the variance had been reported by eleven studies [1]. It had been observed that for undisturbed walking, muscle synergies ranged from 5-8 muscle synergies for roughly 91% VAF [14]. When examining the whole gait cycle, seven synergies were observed for >80% VAF (the major muscles involved were the external obliques (EOBs)) [22] and five synergies were observed for >90% VAF; (the major muscles involved were the medial gastrocnemius medialis (MG) and the biceps femoris (BF)) [23]. In our study, we too observed for the healthy group that the right BF and the right MG showed significant increases in activation for FW and BW, and changes were observed post- compared to pre-training. For the stroke group, the right TA showed significant increases post-training for BT-W. Also, for the stroke group, the right BF (FT-W) and left MG (BW) also showed significant increases. Further, for the healthy group the right RF (quad muscle) showed significant increases post- compared to pre-training. It is important to note that in our study we used a tighter threshold than the aforementioned studies, with VAF set at 98%. This combined with measurements of only four muscles per leg (eight total) may have led to our differences in terms of number of muscle synergies observed. Further, the populations we examined here differed from previous studies in that we researched older adults including those that had suffered a stroke. A previous study was arranged for 6 months and found that the muscle synergy did not change but the muscle definition stability improved [24]. Our study was for an approximately 6-week training cycle per participant. We were able to observe changes in the numbers of synergies, on average, pre- versus post-training for some of the walking conditions, and we did observe differences in synergies across the two groups. More specifically, we found a significant difference between the number of synergies post-BW and post-BW-T for both the healthy and the stroke groups, which highlights the challenges of completing these tasks as the number of synergies increase post- to pre-training. Muscle analysis, in particular synergies, may thus offer the clinician a viewpoint of neural structure underlying motor behaviors [25]. The possibility of diagnosis (i.e., use of synergies to allow quantification of changes in motor deficits and while undergoing rehabilitation) shows promise. Moreover, such information could inform diagnostic tools and evidence-based interventions for the aging population. Conceptualization, J.B., A.B., N.A. and L.A.T. ; methodology, J.B., A.B., N.A., Y.E.H. and L.A.T. ; software, J.B., A.B., N.A. and Y.E.H. ; validation, A.B. and N.A. ; formal analysis, J.B., A.B., N.A., Y.E.H. and L.A.T. ; investigation, J.B., A.B., N.A., Y.E.H. and L.A.T. ; resources, L.A.T. ; writing--original draft preparation, J.B. and L.A.T. ; writing--review and editing, J.B., A.B., N.A., Y.E.H. and L.A.T. ; visualization, J.B., A.B., N.A., Y.E.H. and L.A.T. ; supervision, L.A.T. ; project administration, L.A.T. ; funding acquisition, L.A.T. All authors have read and agreed to the published version of the manuscript. This research was funded by the NSF Waterman Award Grant (2229575). The NIH NIA MSTEM (1R25 AG067896) provided funding support for some of the scholars on this project. The study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Board for studies involving humans (protocol no. 979744-1). Informed Consent Statement: Informed consent was obtained from all participants involved in the present study. The raw data supporting the conclusions of this article will be made available by the authors on request. We would like to acknowledge all the CBRE researchers who contributed to the data collection and training tied to this research. The authors declare no conflicts of interest.