Continuum of reflex excitability in hemiplegia: influence of muscle lengthand muscular transformation after heel-cord lengthening and immobilizationon the pathophysiology of spasticity and clonus

The electromyographic (EMG) responses to tendon taps at the ankle and ensui ng muscular twitch forces and temporal parameters were studied at varying a ngles across the joint range in 18 children, aged 3 to 14.9 years, with con genital hemiparetic cerebral palsy and 22 healthy (control) children, aged 3 to 13.6 years. Those subjects with hemiparesis were community ambulators without assistance. In all subjects, passive muscle stretch caused a waxing of the reflex EMG and twitch force near neutral (with the sole of the foot at right angles to the tibia) and a diminution of these with further dorsi flexion. Twitch times increased with each dorsiflexing increment, being slo west at maximum dorsiflexion and fastest at the resting plantarflexion angl e. Heterogeneity of the hemiparetic-limb data is evident when compared with data of non-paretic and unaffected limbs, with clear differences in the cl onic (fast twitch) as opposed to non-clonic (slow twitch) muscles. In four cases with clinical clonus, clonus frequency was reduced by passive dorsifl exion. Plaster immobilization for 1 month produced clonus which was previou sly absent in one subject, and caused a fast-twitch phenotype to emerge in two subjects. Follow-up after heel-cord lengthening in one subject showed t hat clonus frequency diminished from 9 to 3 Hz with slowing and strengtheni ng of muscle-twitch phenotype. Short- and long-term peripheral manipulation s appear to regulate neuromuscular excitability according to whether muscle s are loaded or unloaded. Although damage to the nervous system provides th e setting for reflex excitability, the data suggest that the muscle length (which specifies the joint angle) and the muscle-twitch phenotype of any gi ven limb for any given case appear to dictate the actual speed and strength of reflex muscle-twitch and clonus profiles. This study illustrates how pe ripheral manipulations of muscles and tendons may alter the expression of w hat have hitherto been considered as exclusively central phenomena.