A9 Neurological Motor Disorders

Topic

Movement control is physically altered when the central or peripheral nervous system sustains damage. Such damage does not always alter movement control in the same way; it can manifest as four distinct physical impairments.

The first impairment is spasticity: damage to the central or peripheral nervous system causes the motor controller's proportional term—which corrects movement based on the distance from the intended position—to increase in a velocity-dependent manner. The faster one attempts to move or stretch the affected muscle, the greater this increase becomes, and the greater the resistance the muscle offers against the movement.

The second impairment is paresis or plegia: the same damage to the central or peripheral nervous system reduces the muscle activation reaching the muscle—that is, the signal the nervous system sends to trigger muscle contraction. Partial reduction of this activation results in paresis, while complete reduction results in plegia.

The third impairment is ataxia: damage to the central or peripheral nervous system increases movement error within the motor controller, causing the actual movement to deviate increasingly from the intended trajectory, resulting in uncoordinated and imprecise movements.

The fourth impairment is dystonia: damage to the central or peripheral nervous system causes abnormal co-activation—specifically, simultaneous and inappropriate activation of opposing muscles acting on the same joint. Consequently, both muscles contract at once rather than alternating, and this abnormal co-activation produces sustained abnormal postures or movements.

The model distinguishes between these four impairments—spasticity, paresis or plegia, ataxia, and dystonia—because each affects a different aspect of movement control: the motor controller's proportional term, muscle activation, movement error within the motor controller, or the co-activation pattern between opposing muscles. This distinction allows the motor deficit to be quantified in physical terms, based on the extent to which each of these aspects deviates from normal functioning.

Quantifying the motor deficit in this way helps guide neuro-rehabilitation interventions: an intervention aimed at reducing spasticity does not correct paresis or plegia, and neither corrects ataxia or dystonia when these are the impairments actually affecting movement control.

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