Figure 4. Effects of age and contextual uncertainty on the timing of step initiation .

The loading–unloading forces registered by a force platform beneath the stepping limb provided timing information about the instant of first step lift‐off under four different experimental conditions: voluntary–certain (instructed reaction time (RT) stepping without perturbations), induced–certain (perturbation‐induced stepping without instruction), voluntary–uncertain (instructed RT stepping with threat of random perturbations), induced–uncertain (instructed RT stepping with actual random perturbations). In the induced–uncertain trials, the perturbation was presented at different times prior to the imperative RT light cue. When older subjects initiate a voluntary step in response to a light cue (voluntary–certain), step initiation is slower than for younger adults. Paradoxically, older subjects are as fast as the young to initiate a step in response to the postural perturbation (induced–certain). When a voluntary step was initiated under the threat of a perturbation (induced–uncertain), step initiation timing was delayed, particularly in the old, who remained slower than the young. When subjects performed RT stepping but were perturbed prior to the imperative go‐cue (induced–uncertain), both groups delayed the onset of stepping. This indicated that the initiation timing for triggering induced stepping did not reflect entirely an immediate necessity or a last resort response to balance instability (values represent group means and SDs for 32 older subjects and 14 young; adapted from Rogers et al. 2003 a).