Central to any analysis of a child's goal directed actions is the interaction between the child, the task and the environment.
Successful performance of a task such as stepping up and down, depends on adapting the postural responses and patterns of movement to the structure of the environment, provided by visual information pickup.
The most obvious information needed for stepping up and down is the height and stability of the step.
This video clip of Will (33 months) and Roan (4 years) stepping up onto and down from a small plastic step beautifully illustrates how young children adaot their actions to suit the physical environment.
When stepping up the ROM of hip flexion is adjusted to allow the foot to be lifted up onto the step.
Will needs to add lateral pelvic tilt to allow the foot to clear the step. In this sequence he is using the wall for support so does not need to pay as much attention the balance as he does when stepping up without hand support.
Step height also affects the amount of momentum that is needed to supplement to hip and knee extension as the COM is raised when stepping up.
Roan is able to step down by flexing the hip and knee in a controlled manner, with the trailing foot only leaving the step once the weight is transferred to the leading foot.
For Will the step height is such that hip and knee flexion do not allow the foot to be lowered all the way to the floor, so the stepping down action includes a flight phase.
However, Will has adequate control to absorb the momentum and maintain balance he lands on the leading foot.
One aspect of the stepping up and down actions of both children is how they position their feet in readiness for stepping up and down.
When stepping down they both carefully position their feet on the edge of the step.
Both children are able to walk towards the step and step up without hesitation. This involves planning the preceding steps to allow the foot of the stance LE to be positioned close to the step.
Video: Roan and Will stepping up and down