Walking is a simple task for most humans. However, the act of lifting a foot
from the ground to take a step, as when walking, destabilises the body and threatens
a loss of balance. This thesis details a series of studies designed to investigate the
control and coordination of a step. In each experiment, the body was voluntarily
translated from stationary to the location of a visually-presented target. It is first shown
that the movement of the body before a step is modulated by the future location of the
foot, even when the location of a step target is made to change unexpectedly before
the stepping foot lifts. This pre-step movement provides the body with an initial
position and velocity at the start of a step from which it begins to fall under gravity. It
is then demonstrated that the movement of the body during the step is largely
determined by its initial conditions and the influence of gravity. However, the trajectory
of the body is also modified by mid-step ankle torques, which seem particularly
important in controlling forwards motion. Next, it is shown that the movement of both
the body and leg during a step is variable between steps to the same location. This
variability is organised to reduce foot placement error, demonstrating that the body
and leg are precisely coordinated to land the foot accurately on its intended target.
Surprisingly, this was still the case when visual feedback was denied during the step.
Finally, the coordination of a step is investigated in subjects with a genetically
determined and pure form of cerebellar degeneration. Foot placement error was
increased in subjects with cerebellar dysfunction, with the results suggesting that this
originated from both impaired coordination and increased variability in the body and
leg movements used during a step