## 4 Discussion  
The purpose of this study was to investigate the effects of age and task challenge on neuromuscular control and resultant complexity of neuromuscular activation patterns of women during step ascent by examining muscle synergy organization during stepping tasks with incremental step heights in both forward and lateral directions. Our results show that complexity of motor control is quite robust across step heights and age groups for stepping in forward and lateral directions. We found no differences in the number of synergies between age groups and step heights for either age group. Further analyses of VAF by four synergies revealed main effects, but no interaction effect, of age and step height on complexity of motor control during forward stepping, indicating subtle differences that could not be detected by analyses of the number of extracted synergies. These analyses revealed that older women actually exhibited more complex motor control strategies, indicated by lower VAF [29], for each step height. Additionally, forward step heights of 20 and 30 cm yielded a significantly lower VAF at \(n=4\) synergies compared to 10 cm. Comparisons of muscle synergy organization revealed that muscle contributions to individual synergies (e.g., muscle weightings) during forward stepping were quite robust between step heights for both age groups. Finally, age-related differences in synergy organization were characterized by a notably lower similarity index between step heights for lateral stepping in young women and increased between-subject variability of the temporal activation patterns in older women. The extraction of four synergies from step ascent is in agreement with previous studies of locomotion in healthy adults that included a maximum of ten lower limb muscles [24; 40]. An important aspect of this study is the inclusion of EMG signals obtained from 10 lower limb muscles, including the  
Figure 5: Between-subject variability (time-averaged standard deviation) of temporal activation patterns across time-normalized trials, expressed as a percent of 100 (in arbitrary units, \(n\) synergies = 4). Data are displayed separately for forward (Fstep) and lateral (Lstep) step directions with step heights of 10, 20 and 30 cm. * indicates a significant difference at \(p=0.05\), ** indicates a significant difference at \(p=0.01\). gluteus medius, gluteus maximus and the erector spinae. Our results were in line with a previous study by [PERSON] et al., who also found four synergies were sufficient to reconstruct the EMG signals of ten lower limb muscles. However, they also proposed that the addition of EMG measurements from hip extensors and abductors during gait would likely increase dimensionality [40]. Our data show that this is not necessarily true for step ascent. For example, during forward stepping, gluteus medius activation coincided with activation of the triple extensors (gluteus maximus, plantar flexors and rectus femoris). During lateral stepping, inclusion of the gluteus medius and erector spinae did not increase dimensionality in the form of an additional synergy as their activation coincided mainly with tibialis anterior activation during lift-off of the trailing foot and trunk stabilization prior to the double support phase after ascent. In line with previous studies including healthy older adults, our results indicate that age did not affect the number of synergies required to reconstruct the muscle activation, which implies that the complexity of motor control (or motor repertoire) of our healthy older cohort was not reduced [6; 8; 17; 45]. However, we were surprised to find that the VAF at a fixed number of synergies, which can be used as an alternative way to assess complexity of motor control [29], was decreased in older women and with step height, indicating increased rather than decreased complexity. We propose that this may be due to the increased challenge posed by step ascent for older adults, forcing some to adopt different motor strategies to compensate for decreased physical capacity [15; 46].