Gait is an important motor function, which is unconscious and automatic. It is also a distinctive attribute; we can recognize people by their walk.1 Disorders of gait are common, and may be the presenting feature of neurologic disease. Reliable estimates of prevalence are difficult to obtain, as there are no standard diagnostic criteria. In a study from Durham, North Carolina, 15% of volunteers over 60 were found to exhibit some abnormality of gait on neurologic examination.2 In the East Boston Neighborhood Health Study, a degree of shuffling or difficulty with turns was noted in 15% of the population aged 67–74, 29% of those aged 75–84, and 49% of the population aged 85 and above.3 Gait disorders are particularly important in the elderly because they compromise independence and contribute to the risk of falls and injury.4,5 Our job as a neurologist is not finished until the gait has been examined.
Humans and other bipeds have two principal gaits: walking and running. Walking is our preferred mode. The biomechanical events of the gait cycle are illustrated with respect to time in Fig. 44–1. The stance phase begins as the right-heel strikes the floor, where it remains for 60% of the gait cycle. The stance phase for the two legs overlap, such that 20% of the gait cycle is spent with both feet planted on the ground, while the center of mass continues its forward progression. Surface electromyographic (EMG) recorded from the leg muscles reveals an orderly, phasic pattern of activation: flexor muscles during the swing phase, extensor muscles during stance. There are two tasks that the nervous system must attend to initiate and maintain walking. (1) The brain and spinal cord generate a series of stepping movements; locomotor centers specify the timing, advance position, and instructions for loading and unloading of the limbs. (2) At the same time, balance must be managed to maintain the upright posture and a stable progression.
Events of the gait cycle are depicted with respect to time. From the point of right-heel strike, the right foot is in contact with the floor 60% of the time. This is the stance phase for the right leg. Roughly 20% of the gait cycle is spent in double limb support, during which forward motion of the center of mass continues. (Reproduced with permission from Ropper AH and Samuels MA. Adams & Victor's Principles of Neurology, 9th ed. McGraw-Hill, Inc., 2009. Fig. 7–1.)
Neural Networks that Support Locomotion
In quadrupedal animals, locomotion is produced by the activity of a spinal pattern generator. Cats and dogs with high spinal transection achieve a crude pattern of walking on the treadmill, provided their balance is supported. This “fictive locomotion” can be stimulated with L-dopa or clonidine, and occurs independent of sensory feedback.6 In the spinal cord, ...