The effects of age and physical activity status on muscle synergies when walking down slopes

Purpose: The aim of the current study was to determine whether gait control (muscle synergies) or gait stability (margin of stability (MoS)) were different between younger and older adults when walking on level or downhill slopes. Further, it sought to determine associations between either age or physical activity with muscle synergy widths. Methods: Ten healthy younger (28.1 ± 8.0 years) and ten healthy older (69.5 ± 6.3 years) adults walked at their preferred walking speed on a treadmill at different slopes (0Ëš, − 4Ëš and − 8Ëš). Muscle synergies were extracted using non-negative matrix factorisation and compared between groups and walking slopes. Correlations between the full width at half maximum (FWHM) of the synergies' activations and weekly recreational physical activity minutes and age were also determined. Results: Younger and older adults both walked with similar muscle synergies across all tested slopes, with 4 synergies accounting for > 85% variance of overall muscle activity in both groups across all tested slopes, with high scalar products (≥ 0.86) for each synergy and slope. It was also demonstrated that physical activity and age had different associations with pooled muscle synergies across slopes, as weekly minutes spent in recreational physical activity were associated with the FWHM of a synergy activated at weight acceptance, whereas age was associated with the FWHM of synergies occurring at push off and foot clearance, respectively. Conclusion: Our results suggest that healthy older and younger adults walk with similar muscle synergies on downhill slopes, and that physical activity and age influence different muscle synergies during walking.