Kinematic analyses show trunk flexion paired with ankle extension. The hip strategy entails early ventral trunk and thigh muscle activation associated with a relative increase in shear forces at the support surface and little phasic activation in ankle muscles. Kinematic analyses therefore show body movement predominantly at the ankle joint, and only small movements at the hip. These muscle activations produce torque at the support surface. The ankle strategy involves early dorsal ankle muscle activation followed by dorsal thigh and trunk muscle activation (for responses to backward translations). Conversely, the hip strategy moves the body as a double-link inverted pendulum with counter-phase motion at the ankle and hip, all other standing joints (knee, vertebrae and neck) remaining rigid. The ankle strategy repositions the center of mass by moving the whole body as a single-link inverted pendulum by producing torque at the ankle, other standing joints (knee, hip, vertebrae, neck) remaining rigid. This model assumes two strategies (ankle and hip centered), that can either be used by the central nervous system to produce adaptable control over the horizontal position for the center of mass in the sagittal plane.
Human postural control can be studied with the single- or multi-link inverted pendulum model.
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