In an earlier series of gait studies showed that dual task interference in people with Parkinson's disease was directly proportionalto the complexity of the secondary task.
When people were required to recite a simple sentence 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 valuesfor 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.