The force constraint strategy for stance is independent of prior experience.

The purpose of this study was to examine the effect of prior experience concerning direction of a postural perturbation on the balance response of cats to translations of their support surface. Previous work has shown that, when cats are translated in many directions in the horizontal plane, they respond by exerting active forces with each paw in only two directions, termed the force constraint strategy. This study examined whether the force constraint strategy could be modified based on predictability of the direction of translation and whether this strategy is used by the naive animal with no prior experience of platform translation. Four cats were trained to stand quietly on the force platform using positive reinforcement, and then were implanted with chronically indwelling electrodes for recording electromyographic (EMG) activity. The first experiment concerned the response of the naive cats to their first exposure to platform translation and consisted of translations presented randomly in four different directions in the horizontal plane. The second experiment consisted of two complete sets of 16 directions of translation (15 trials per direction), with the direction of translation randomized in one set and serially ordered in the other, to make the direction of translation unpredictable or predictable, respectively. Forces exerted by the cat, EMG activity, and platform position were recorded during the 1-s trials. The use of the force constraint strategy was independent of prior experience with direction of translation, as was the amplitude of the response.(ABSTRACT TRUNCATED AT 250 WORDS)