## 3 Results  
Time-normalized kinematic data from the heel and pelvic markers showed high similarity (\(r>0.9\)) in averaged vertical displacement over time between young and older women for all step heights. An example of the averaged vertical displacement patterns and standard deviations at 30 cm step height is provided in Figure 1.  
Two-way ANOVA (age \(\times\) synergy number) of VAF revealed significant main effects of synergy number for all step directions and heights (\(p<0.001\)), but no interaction effects with age (\(p\geq 0.05\)). A significant main effect of age (\(p=0.028\)) was detected only for lateral stepping at 30 cm. For the group-averaged VAF, four muscle synergies were required to achieve a threshold level of 85% VAF for reconstructed signals across both age groups, step directions and step heights (Figure 2). This indicates that age, step direction and step height did not affect the number of synergies needed to reconstruct the EMG data. Consequently, the following results were obtained assuming \(n=4\) synergies. For forward stepping, four synergies accounted for 90.5%, 89.8% and 91.8% of variance in young women and 88.5%, 87.3% and 87.4% in older women for step heights of 10, 20 and 30 cm respectively. In lateral stepping, VAF by four synergies was 90.3%, 90.0% and 91.7% in young women and 88.2%, 88.0% and 87.4% in older women for step heights of 10, 20 and 30 cm respectively. Two-way ANOVA (age \(\times\) step height) on VAF obtained from \(n=4\) synergies revealed a significant main effect of step height (\(p=0.002\)) and age (\(p=0.026\)) in forward direction, but not in lateral direction (\(p=0.187\) and \(p=0.138\) respectively). No significant age \(\times\) step height interaction effect was found for either step direction (\(p>0.05\)). For forward stepping, related-samples \(t\)-tests revealed a significant difference between step heights of 10 cm versus 20 and 30 cm (\(p=0.009\) and 0.014 respectively), but not between 20 and 30 cm (\(p>0.05\)). Independent samples \(t\)-tests revealed a significant difference between age groups for each step height (\(p=0.005\), \(p=0.041\) and \(p=0.019\) for 10, 20 and 30 cm respectively). Figure 1: Averaged vertical displacement of the heel and sacrum in young (left column) and older women (right column) at 30 cm step height in forward direction (Fstep) and lateral direction (Lstep). Vertical lines indicate average point of transition from foot clearance to pull-up phase. Comparisons between muscle weightings (Table 1, Figures 3 and 4) showed that synergy 2 and 4 had high inter-step height similarity for both age groups and step directions. Synergy 4 also appeared to be highly similar between age groups. In synergy 2, a lower similarity between age groups was found, probably due to a difference in quadriceps/hamstring co-activation, as characterized by a significantly decreased contribution of the quadriceps and increased contribution of the hamstrings for most stepping conditions in older women. The composition of synergy 3 appeared to be the most variable. In contrast with forward stepping, which showed a robust synergy organization within and between age groups, lateral stepping resulted in lower correlations between step heights for the young group when compared to the older group.