Stiffness and Damping in Human Walking Across Different Directions and Walking Trials.

Human walking can be modeled using a springmass-damping (SMD) system. While most studies have focused on standard activities in healthy young to middle-aged populations, less attention has been given to participants' responses to unexpected gait perturbations. These responses may be valuable predictors of falls, particularly in older adults. Our previous study modeled walking in the vertical direction for a diverse group of participants. Since most gait perturbations occur in the medio-lateral and anteriorposterior directions, the next step involves extending the analysis to these directions and treadmill walking between perturbations.The study included 60 adults (aged 18-87 years), who walked on a perturbation treadmill while wearing an inertial measurement unit (IMU) at the lumbar region to capture body acceleration. Participants first walked at their preferred speed on the treadmill ("normal gait data"), followed by "perturbation trials" with gait perturbations. The gait data between the perturbations were analyzed as "inter-perturbation gait data". Force data was recorded using the treadmill's built-in force plates. The SMD model was applied to calculate damping and stiffness coefficients.The lowest median spring stiffness was found in medio-lateral direction and the highest in anterior-posterior direction. The lowest damping coefficient was found in medio-lateral direction and the highest in vertical direction. Compared to "normal gait data", "inter-perturbation gait data" showed higher stiffness and, for some participants, higher damping coefficients, while others exhibited decreased damping. Damping and stiffness coefficients were successfully extracted from treadmill walking data across all directions for a diverse group of participants and linked to gait dynamics. The analysis highlighted human gait adaptability under various conditions. This study provides groundwork for future research on individual responses to unexpected gait perturbations.Clinical relevance- Describing human walking with damping and stiffness coefficients in different directions could contribute to understand reactive dynamic balance, and thus give a sound estimation of a relevant risk factor for falls in older people.