Does frontal-plane asymmetry in compensatory postural responses represent preparation for stepping?

The bilateral symmetry of feet-in-place responses to postural perturbations in the anterior-posterior direction has not been well studied. This paper presents evidence that right- and left-leg responses that appear to be approximately symmetrical in the sagittal plane may actually involve an asymmetry in the frontal plane, namely, a lateral weight shift. Comparison with trials where subjects stepped suggests that these lateral weight shifts represent early preparations for stepping responses that are aborted before the foot is actually lifted. Thus, it would seem that compensatory stepping involves a sequence of discrete modifiable stages, rather than a single immutable motor program. Moreover, postural responses that appear to be similar in the sagittal plane may actually be seen to involve quite different postural strategies, i.e. in terms of preparation for stepping, when viewed in the frontal plane.