Adaptation of multi-joint coordination during standing

25 Standing balance requires multi-joint coordination between the ankles and hips. We investigated 26 how humans adapt their multi-joint coordination to adjust to various conditions and whether the 27 adaptation differed between healthy young participants and healthy elderly. Balance was disturbed 28 by push/pull rods, applying two continuous and independent force disturbances at the level of the 29 hip and between the shoulder blades. In addition, external force fields were applied, represented by 30 an external stiffness at the hip, either stabilizing or destabilizing the participants’ balance. 31 Multivariate closed-loop system identification techniques were used to describe the neuromuscular 32 control mechanisms by quantifying the corrective joint torques as a response to body sway, 33 represented by Frequency Response Functions (FRFs). Model fits on the FRFs resulted in an 34 estimation of time delays, intrinsic stiffness, reflexive stiffness, and reflexive damping of both the 35 ankle and hip joint. The elderly generated similar corrective joint torques but had reduced body sway 36 compared to the young participants, corresponded to the increased FRF magnitude with age. When a 37 stabilizing or destabilizing external force field was applied at the hip, both young and elderly 38 participants adapted their multi-joint coordination by lowering or respectively increasing their 39 neuromuscular control actions around the ankles, expressed in a change of FRF magnitude. However, 40 the elderly adapted less compared to the young participants. Model fits on the FRFs showed that 41 elderly had higher intrinsic and reflexive stiffness of the ankle, together with higher time delays of 42 the hip. Furthermore, the elderly adapted their reflexive stiffness around the ankle joint less, 43 compared to young participants. These results imply that elderly were stiffer and were less able to 44 adapt to external force fields. 45