Fig 1. Ankle stiffness and spinal feedback.

A. Spinal feedback is probed by electrically stimulating the sensory fibres of a muscle and measuring the resulting change in muscle contraction, called the H-reflex. B. Decrease in soleus H-reflex in challenging balance conditions: when standing facing a cliff (left) [4], when standing on a narrow support (middle)[5] and standing with the eyes closed (right)[6]. Co-contraction of antagonist ankle muscles is observed in each of these three cases. C. Decrease in soleus H-reflex in older adults in normal stance [7–9]. Co-contraction of antagonist ankle muscles during quiet standing is observed in older but not in young subjects [7,12].