The Existence of Shared Muscle Synergies Underlying Perturbed and Unperturbed Gait Depends on Walking Speed  
[PERSON]  
1 Department of Rehabilitation, Radboud University Medical Center, 6525 GA Nijmegen, The Netherlands 12 Department of Biomechanical Engineering, University of Twente, 7522 NB Enschede, The Netherlands; [EMAIL_ADDRESS]  
[PERSON]  
3 Research and Development Unit, University Rehabilitation Institute Republic of Slovenia, 1000 Ljubljana, Slovenia; [EMAIL_ADDRESS] (M.Z. ); [EMAIL_ADDRESS] (A.O.) [PERSON]  
3 Research and Development Unit, University Rehabilitation Institute Republic of Slovenia, 1000 Ljubljana, Slovenia; [EMAIL_ADDRESS] (M.Z. ); [EMAIL_ADDRESS] (A.O.) [PERSON]  
2 Department of Biomechanical Engineering, University of Twente, 7522 NB Enschede, The Netherlands; [EMAIL_ADDRESS]  
[PERSON]  
3 Research and Development Unit, University Rehabilitation Institute Republic of Slovenia, 1000 Ljubljana, Slovenia; [EMAIL_ADDRESS] (M.Z. ); [EMAIL_ADDRESS] (A.O.) Footnote 38: email: {[PERSON]. counteracting balance perturbations during walking. They demonstrated that a common set of synergies underlies both perturbed and unperturbed walking for intermediate and high walking speeds. Various types of perturbations can disturb the cyclical way of walking. Different factors, including perturbation modality, perturbation onset, amplitude, direction, and walking speed can induce different responses to restore balance [4; 5; 6]. An important strategy to restore balance is the ankle strategy, in which the center-of-pressure under the stance leg is displaced in the direction of the perturbation [4]. The ankle strategy is regularly followed by the main dynamic balancing response known as the stepping strategy, in which the foot of the leg in swing is placed at an adjusted location to re-establish balance [7]. Another strategy is the hip strategy, which is an inertial strategy in which limb segments are rotated to change the body's angular momentum [4; 8]. [PERSON] et al. [8] investigated the influence of walking speed on balance strategy selection after perturbations were applied in the mediolateral direction. Results indicated that the stepping strategy was induced for inward-directed perturbations, independent of walking speed (see Figure 1 for the definition of perturbation directions). Contradictorily, walking speed did significantly affect the balance strategy for outward-directed perturbations. At the highest walking speed examined (0.8 m/s), the stepping strategy was mainly used. However, at the lowest walking speed examined (0.4 m/s), the hip strategy, instead of the stepping strategy, dominated the balance response. For the intermediate walking speed (0.6 m/s), the contribution of the hip strategy decreased while the contribution of the stepping strategy increased. Whether these differences in the recruitment of balance strategies at different speeds were also accompanied by changes in the recruitment of muscle synergies was not assessed. The aim of the current study was to investigate the existence of a shared set of muscle synergies underlying balance responses assessed in a group of healthy subjects, following inward- and outward- directed perturbations in the mediolateral direction at various perturbation onsets and walking speeds. A study by [PERSON] et al. [3], examining muscle synergies underlying perturbation induced dynamic balance responses during gait, has demonstrated that individuals use a common set of synergies in perturbed and unperturbed walking. However, this study was performed for intermediate and high walking speeds (around 1.2 m/s). To the best of our knowledge, no study has examined muscle synergies underlying unperturbed and perturbed gait at walking speeds between 0.4 and 0.8 m/s. Figure 1: The experimental setup and perturbation directions. (**a**) A graphical illustration of the experimental setup. Subjects walked on an instrumented treadmill while receiving pelvic perturbations induced by the BART device. (**b**) A top-view illustration of the perturbation directions.