Human walking is the result of a complex interaction of multiple neuroanatomical systems and the relative importance of these systems depends on the locomotor task being performed. An understanding of these relative roles and the consequences of different CNS lesions on locomotion requires comprehensive profiling of human walking behaviours in health and disease.
We assessed humans walking on a treadmill under different locomotor conditions using 3D kinematic gait analysis. We revealed task-specific, age-dependent patterns of foot endpoint control in 121 healthy adults. In older adults, cognitive distraction while walking resulted in degraded endpoint control in the form of decreased mean toe clearance and an increase in the frequency of extremely low toe clearance values. This translated into a markedly increased modelled probability of tripping. Conversely, walking with the lower visual field restricted was associated with increased clearance, but in older adults control was intermittently degraded and tripping risk increased.
We also examined arm swing behaviour during the Stroop colour-word naming task in 83 healthy adults. Remarkably, increasing difficulty in this left-lateralised language task resulted in significantly increased arm swing asymmetry, driven by a reduction in the movement on the right.
We propose that these observations are due to unilateral inhibition of a corticospinal contribution to arm swing. Men of all ages and older women were strongly affected by this phenomenon but women under 60 appeared to be resistant. The reason for this is unclear but may represent oestrogen-dependent plasticity and redundancy in the prefrontal cortex, where activation underpinning cognitive control during the Stroop task is likely to interfere with gait control. This paradigm was then applied to patients with incomplete spinal cord injury (SCI). Patients with thoracic injuries showed a significantly larger, homogeneous shift in arm swing symmetry towards left-dominant patterns during the Stroop task than those with cervical injuries. Patients with thoracic SCI may have longstanding interruption of the long propriospinal connections between the dominant lumbar central pattern generator for locomotion and its subsidiary, cervical component which usually contributes to arm swing. As a result, their arm swing is more dependent on supraspinal control and is susceptible to interference from the Stroop task.
These results taken together support an important role of supraspinal centres in diverse aspects of human walking behaviour, from fine control of toe trajectory to the more automatic, rhythmic swinging of the arms.