A fundamental human motor ability is placing or "targeting" the feet optimally relative to obstructions. Foot placement is required to be stable and allow the obstacle or surface (such as a roadside kerb) to be negotiated at the right time and place within the step cycle. The authors have shown, for example, that on approaching obstacles step length is regulated in order to cross at 80% of stride length independent of obstacle height (Sparrow, 1996). We have also demonstrated that when stepping over obstacles there is a complex modulation of lead and trail foot impulses with little evidence of a direct perception-action coupling of visual information to a single gait parameter, such as vertical impulse (Begg et al., 1998).

In another study we examined speed-accuracy characteristics of step length regulation when individuals were required to walk at normal speed and place either one foot or both feet between parallel lines (targets) either 3, 5, or 7 cm wider than foot length (Halliday et al., 1999). In this foot-targeting task individuals reduced step velocity and stance duration consistent with the increased spatial constraints. In addition, elderly subjects showed increased trail foot braking impulses when only the lead foot was constrained to a restricted target area. Thus, increased risk of slipping in the elderly may be associated with the non-constrained foot when the lead foot is constrained in its accuracy demands by obstacles.

A recent study of foot placement effects when negotiating a raised surface showed that elderly individuals compromised their safety by placing the trail foot a long way from the raised surface edge (Begg et al., in press). This strategy provided the lead foot with little horizontal clearance over the step edge (about 7 cm) and little recovery time in the event of loss of balance.

In conclusion, it is proposed that there may be processes of step length regulation that are common to all these gait tasks.