Spastic movement disorder: should we forget hyperexcitable stretch reflexes and start talking about inappropriate prediction of sensory consequences of movement?

Spastic movement disorder is characterized by reduced ability to selectively activate muscles with significant co-activation of antagonist muscles. It has traditionally been thought that hyperexcitable stretch reflexes have a central role in the pathophysiology and the clinical manifestations of the disorder. Here we argue that the main functional challenges for persons with spastic movement disorder are related to contractures, paresis, weak muscles and inappropriate central motor commands, whereas hyperexcitable reflexes play no or only an insignificant functional role. Co-activation of antagonist muscles and stiff posture and gait may rather be adaptations that aim to ensure joint and postural stability due to insufficient muscle strength. Aberrant (involuntary) muscle activity is likely related to an inadequate prediction of the sensory consequences of movement and a resulting impairment of muscle coordination. We argue that improvement of functional muscle strength and muscle coordination following central motor lesions may be achieved by optimizing integration of somatosensory information into central feedforward motor programs, whereas anti-spastic therapy that aims to reduce reflex activity may be less efficient. This opens for novel investigations into new treatment strategies that may improve functional control of movement and prevent reduced joint mobility in people with brain lesions.