History And Definitions

Walking is a product of three interrelated nervous system functions: locomotion, balance, and adaptation. Locomotion comprises the stereotyped patterns of muscle activation (synergies) of limbs and trunk that produce repetitive stepping. The nervous system must be able both to initiate and arrest stepping and to alter stepping patterns for turns, different speeds, and different support surfaces. Balance or equilibrium synergies include a number of postural responses that enable an individual to arise and remain erect during standing and locomotion. Standing is an active process (static postural response) in which the sway of the body is kept within the limits of the base of support provided by the feet. Anticipatory postural responses are changes in postural muscle groups that precede voluntary movements made to offset disturbances in balance that would result from the voluntary movement. For example, paraspinal and leg muscle activation precedes voluntary movements of the arm in a standing person to protect the person's upright stability. Anticipatory postural responses are feed-forward responses. They anticipate the perturbation of balance caused by the voluntary action. Reactive postural responses are feedback responses. They protect against unexpected external perturbations and are triggered by sensory cues indicating that the body is outside the limits of stability.

Adaptation refers to the adjustment of locomotor and balance synergies to the constraints produced by the environment, the body, and the ongoing voluntary activities. The gait pattern of normal individuals at any point in time depends on the person's perception of the environment, the condition of the body (clothes, shoes, disease), and the individual's personal goals. In addition to locomotor and balance synergies, a secure gait depends on the following: (1) information about the environment and the position of the body in the environment as relayed by proprioceptive, vestibular, and visual pathways; (2) ability to interpret and integrate the afferent information; (3) ability to produce force through the bones, joints, and muscles; (4) ability to modulate force for optimum performance; and (5) ability to select and adapt locomotor and balance synergies to the environmental requirements and the individual's capabilities.

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