In an earlier series of gait studies, Morris et al (1996) showed that dual
task interference in people with Parkinson's disease was directly proportional
to the complexity of the secondary task. When people were
required to recite a simple sentence ('Where is the child?') over and over
whilst walking, their gait speed and stride length reduced only marginally.
As the sentence complexity increased, the speed and amplitude of
footsteps diminished. For the most difficult condition (reciting the days
of the week backwards) these variables were often less than half the values
for healthy older people, resulting in marked shuffling, tiny steps
and reduced ground clearance. This is particularly dangerous, because
reduced ground clearance during gait increases the risk of tripping on
obstacles and falling.